Lithium Ion Batteries On Boats: 1Q23 Update

2/1/2023: Initial Post
2/2/2023: Post-publish formatting cleanup and minor edits
2/19/2023: Add comparative Spider Diagram

Executive Summary:

Much of what has been written about Lithium Ion Batteries on boats in the preceding 18 months suggests that they are “absolutely life changing.” To this observer, much of that commentary seems optimistic, written by technical people and generally to the exclusion of the needs of technical laymen. This article summarizes, but does not focus on, the technical merits of Lithium technology. I have written other articles [1] [2] that focus on the technical considerations in much greater detail, and that material hasn’t changed since its preparation. This article considers a financial analysis of the factors and costs surrounding a retrofit of boats built upon traditional lead-acid technology to lithium chemistry technology. Conclusions: there are significant potential benefits and consequential risks that appear when retrofitting lithium chemistry batteries to pre-existing lead-acid applications. The costs of retrofitting an older boat probably cannot be justified on any financial basis; that possibility is not zero, but it is “minimal.” Lifetime retrofit-to-lithium costs are less only if the resulting system is owned for many years after the initial retrofit project is completed. Justifying a Lithium retrofit project in 2023 is only possible based on “personal, purely subjective value,” and not on any economic or financial basis

Battery Risks:

Just as there are several lead-acid battery technologies:

1. flooded wet cell,
2. AGM,
3. Gel,
4. Carbon Foam, and
5. TPPL,

there are also several lithium chemistry battery technologies found in widespread use today. Lithium chemistries include:

1. Lithium Cobalt Oxide (LiCoO2) – LCO
2. Lithium Manganese Oxide (LiMn2O4) – LMO
3. Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) – NMC
4. Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) – NCA
5. Lithium Titanate (Li2TiO3) – LTO
6. Lithium Iron Phosphate (LiFePO4) – LFP

The current American Boat and Yacht Council Lithium Ion Battery safety standard, E-13[3], is silent on requiring any one specific lithium chemistry for use on boats. The E-13 standard uses the non-specific terms “Lithium Ion Battery,” or “Lithium Ion Battery System” throughout. However, the boating industry worldwide has chosen to utilize the least energy dense, safest of the lithium chemistries, the Lithium Iron Phosphate (LiFePO4) chemistry. At year-end 2022, LFP batteries have the least risk of any of the lithium chemistries for fire or explosion on boats or in RVs, and are as safe as lead-acid batteries in equivalent use. While these batteries do not spontaneously burst into flame or explode, if they are caught up in a fire of remotely-located origin, they are reported to be more difficult to extinguish than any of the lead-acid technologies.

Figure 1: Comparison Characteristics of LiFePO4

Several of the lithium battery chemistries are routinely found in residential household electrical and electronics applications, such as the batteries used in handheld consumer electronics, UPS (Uninterrupted Power Supply) equipment, computers, hobbyist drones, home fire alarm and security systems, wireless telephones, eBikes, skate boards, portable power tools, Electric Vehicles (EVs), and of course, marine navigation equipment. Each chemistry has different electrical and mechanical characteristics. In the laboratory and in industry, different chemistries are compared to one another and selected for use in different applications based on six axises of their fundamental physical and electrical properties, as shown in graphic form as a “spider diagram” as shown in Figure 1[4]

Figure 2: Comparison of Characteristics of Three Commonly Used Lithium Batteries

Figure 2[5] shows three spider diagrams overlaid on one another comparing  three different, popular Lithium chemistries.  This allows direct comparisons of the six battery selection criteria.  LiFePO4 (LFP) is the chemistry most preferred for use on boats. The three most popular lithium chemistry batteries for e-Bikes and e-Scooters are Nickel Manganese Cobalt (NMC), Lithium Cobalt Oxide (LCO), and Lithium Iron Phosphate (LFP).  Any e-Bike battery bought at or after late 2022 and onward should be certified to conform to the safety requirements of UL2849.  Two things to note on this chart:

  1. LFP has best Specific Power, Life Span and Safety (which is consistent with marine industry descriptions of this chemistry);
  2. NMC and LCO both have better Specific Energy ratings than LFP;
  3. Cost and Performance numbers are similar for all three.

System Risks:

The configuration choices for lithium batteries range significantly, and each choice carries differing proportions of risk. One of the easiest and safest ways to upgrade to lithium today [early 2023] is with commercially manufactured “drop-in replacement” batteries from respected manufacturing companies like Battle Born, Lithionics, Mastervolt, ReLion, Renology, Victron and Xantrex. Many of these “drop-in” form factor batteries come with built-in BMS controllers, but not all.  The specific disconnect device in drop-ins are not always mechanical solenoids. Many are solid state switches sensitive to all typical solid state component failure modes; in particular, surges induced from nearby external sources. In 2023, few of these BMSs comply with ABYC requirements for pre-disconnect warning alarms and integration of cross-BMS communications controls required by the current Lithium Battery Standard, ABYC E-13.

Individual LiFePO4, 3.2V cells can be bought from any number of online sources, and in recent years DIYers have been buying these 3.2V cells and “assembling” them into complete 12V, 24V and 48V LFP batteries. This project requires the inclusion of a suitable aftermarket external BMS controller and disconnect solenoid, which should be selected to comply with now-current ABYC E-13 requirements. It is well known in the professional boating industry and amongst insurers that this DIY approach has been “abused” by many DIY builders, either through lack-of-knowledge or willful disregard of safety requirements, so availability of some components have become limited. For example, to limit corporate liability, some manufacturers of BMS equipment have stopped selling their stand-alone external BMS units to DIYers.

Insurance Limitations:

As of January, 2023, some marine insurers are refusing to insure vessels with “Lithium Ion” battery platforms. We aboard Sanctuary were formerly insured by Markel America, a good option for us because Markel offers Personal Liability Insurance through a “liveaboard” endorsement to their base policy. In 2021, Markel started turning away boats with Lithium Ion battery systems. Entering 2023, Markel and Hanover are two underwriters that have adopted very strict underwriting regarding lithium Ion batteries.

Hanover[6] Insurance will not bind, or remain with an insured at renewal, for a boat that has, or adds, lithium ion batteries to the vessel.

Markel[7] – guidelines for lithium (LiFePO4) batteries are:

1. maximum hull value $150,000,
2. maximum liability limit $500,000,
3. batteries must be sourced from a known and proven USA manufacturer, and have a BMS also “made” by a US company; note: “assembled in the USA” does not mean “made in the USA,” and
4. batteries must be professionally installed.

Markel is using these very strict criteria on all new risks.

If a current Markel policyholder installs LiFePO4 batteries, the company would not know unless/until their presence was exposed at a survey or became apparent in a claim scenario. If a claim were to occur, the underwriter could interpret that as a contract violation, so read the insurance contract exclusions very carefully.

If a new construction vessel is built with a LiFePO4 system designed and installed by the OEM manufacturer, the above conditions “may be” waived as of the time of this writing.

Ownership Benefits:

LiFePO4 batteries provide more energy density per unit space and weight than lead-acid, so watt-hour for watt-hour (amp hour for amp hour), LiFePO4 batteries take up less space and are less weight than their lead-acid cousins. In the equivalent floor footprint of a lead-acid installation, a lithium installation can provide much larger usable energy storage capacity. Re-charging LiFePO4 batteries can take less time, reducing generator runtime. Sailboats, because of their relatively short periods of engine runtime, can benefit from Lithium batteries to a larger degree than power boats. Blue Water power boats on ocean transits may benefit from lithium batteries to a greater degree than intracoastal and near-coastal cruisers. For all boats, auxiliary additions like solar panels are more functional, utilitarian and affordable for most small and mid-sized cruisers.

If the desire to retrofit LiFePO4 batteries is to extend the practical limits of “off-grid living” – that is, long-term anchoring with comforts like air conditioning and space heating – LiFePO4 can provide that technology potential, albeit with significant additional electrical system engineering needed within the operating platform, and certainly, enormously more charging system capacity. Twelve volt systems aren’t practical for these larger power demand requirements, so electrical system upgrades to 24VDC and 48VDC platforms, and resulting segmentation of the boats’s electrical system, may become a retrofit expense, and greatly add to the “total cost of retrofit.”

ABYC Standards:

The ABYC Lithium Ion Battery standard, E-13, released in July, 2022, is neither stable nor comprehensive at this time. For example, the v1.0 document states (blue text are quotes from the E-13 Standard text):

13.5 General Requirements
13.5.2 Lithium Ion Battery Systems shall be installed, commissioned and maintained in accordance with the manufacturer’s recommendations.

But, not all battery manufacturer’s have provided specific instructions in the past, nor do they all do so today. The standard is out ahead of the market. Perhaps by July, 2023…

13.4.3 SOE Parameters shall be adhered to for determining the system design, installation, storage and operation of a lithium ion battery.

But, not all battery manufacturer’s provide detailed Safe Operating Envelop (SOE) specifications today, so buyers must be familiar with the ABYC standard’s content in order to be sure the batteries they buy are actually compliant.

13.5.4 Batteries or cells shall meet the testing requirements of at least one of the following standards: IEC 62133 IEC 62619 IEC 62620 SAE J2929 UL 1642 UL 1973 UL 2054

But the above is just a list of Euro and North American test standards. Their content coverage either doesn’t overlap or only partially overlaps. The value of certification to any one of them is questionable – “feels good” – but anyone who reads that list has to conclude it’s likely to change.

I expect there will be an early review cycle for E-13 rather than the normal three year or five year standards review cycle. So Lithium buyers today are exposed to a moving target upon which insurance underwriters will roost until some form of relative stability is achieved. There are no US-made batteries on the market today that have pursued UL or ISO certification requirements in the past. One safety standard that is widely promoted is UL 1973; not itself a performance standard, but a physical abuse standard that applies generically to all battery chenistries. The good news is, UL 1973 testing does confirm that LiFePO4 batteries do not burst into flame or explode when abused. The bad news is, certification to UL 1973 is very expensive for the manufacturer and is not yet widely done. Because batteries are competitive, some less expensive than others, buyers can easily overlook a testing standard while trying to contain out-of-pocket costs of a lithium retrofit solution. I consider “standards” to be a critically important step forward, but not yet a stable environment.

New Manufacture:

Some [mostly high end] boat manufacturers are installing LiFePO4 systems as optional equipment on new construction. Boat manufacturers who are offering LiFePO4 in new construction have performed system design integration engineering (in the context of what’s known today about these systems) beyond the capabilities or self-discipline of most DIYers. These systems include the best available supplementary/ancillary controls, in order to deliver an entire system design solution that minimizes ownership risks and performance disappointments to their retail buyers. The buyer of such a vessel receives the benefit of the more detailed engineering required by the system to operate reliably. And, those new-construction systems – of which the lithium battery cells are only one component part – are itemized as “optional equipment,” carrying delta price tags of up to $100K per boat build above the prices of their base lead-acid platforms.

DIY Owner Responsibility:

One must know one’s personal skills and skillset limitations. Lithium technology brings new skills and new technology issues to the workplace. To illustrate this point, one might ask themselves if they can DIY install an inverter/charger, from scratch, so it works as intended and doesn’t trip ground fault breakers ashore. To make an even easier test case, suppose I limit the task to swapping out an old, failed inverter/charger with a compatible new unit of greater power capacity? Extending that same skills self-analysis to lithium powered electrical system design is instructive. A lithium system retrofit is a far more complex and technical undertaking than simply installing or replacing an inverter/charger, especially if the retrofit involves moving from 12V to 24V or 48VDC power.


Batteries are a commodity purchase and Return-On-Investment (ROI) is a financial measurement. The only way to get LiFePO4 batteries to offer ROI is by owning the retrofit system for 10 – 12 years at a minimum. Many buyers approach major purchase decisions based on ROI. Very few buyers will achieve any financial return from retrofitting a boat built on a lead-acid battery platform. In consideration of the often “hidden” costs of retrofitting these batteries into a boat designed and built for lead-acid platforms, the actual return period is likely to be much longer than stated above, but will not be less. For those anticipating a boat sale within 12 – 24 – 36 months, a retrofit of this magnitude would be financially unwise.


A “value proposition” includes financial considerations but IS NOT purely a financial measurement. Value propositions are filled with “personal preference” choices. A succinct “value statement” I support was recently suggested as follows: “Value is not always monetary. It can be in creature comforts, safety or just the satisfaction of ownership.” My own previous “value statement” has been: “if you want it, and you can afford it, then you should have it.” These statements describe buyers who can spend $5K or more on the batteries, and on unknown additional retrofit costs ranging from an additional $5K to $20K to $50K, without caring about any kind of actual financial return for that outlay. I view that as equivalent to paying $25K to go to the Super Bowl; “if you want to and you can afford to, then you should go.” But lots of us choose to watch the game on TV. For those who cannot or do not care to treat battery replacement as a “disposable cost,” then the calculus for “value” ranges back towards typical economic measures.

There are many personal values and attitudes involved here, and it’s unfortunate that lower total-cost-of-ownership (TCO) and actual ROI is often a primary advertising and sales strategy, suggesting to buyers that retrofitting an older technology boat can justify the cost. A full and fair discussion of “financial return” would be balanced and truthful. And for those who “want it and can afford it,” also realize that two years post retrofit, not much, if any, of the upfront retrofit “investment” cost will be recoverable/recovered at boat resale time. Especially so when strict insurance limitations are factored in.

How is ROI calculated?

Following is my attempt to answer the question…

Some relevant, necessary background:

Reviewing technical detail and language behind the “cost analysis” discussion. In the lead-acid world, there are “start service” (engine starting) batteries and “deep cycle” (house) batteries. “Start service” batteries cannot generally be used in “deep cycle” applications, but “deep cycle” batteries can be used very successfully in “start service” applications. “Deep cycle” batteries make up “house,” “inverter” and “thruster” banks on boats. The US “industry standard” is that “start service” batteries are rated by their manufacturers in terms of Cranking Amps (CA, CCA, MCA) and Reserve Capacity (RC). “Deep cycle” batteries are rated by their manufacturers in “Amp Hours.[8]” Amp Hour ratings in North America are based on an assumed standard rate-of-discharge of 20 hours. Lead-acid batteries discharged faster than the 20-hour rate cannot return their rated amp hour capacity. If a battery with a 20-hour rating of 100 amp hours is discharged in 10 hours, that battery will only return ~80 amp hours. This is electrophysics science; a function of Peukert’s Law, and all lead-acid batteries display this behavior.

Charge-Discharge Cycles:

The term “charge-discharge cycle” describes the average discharge withdrawn from a battery or battery bank as a percentage of the total capacity of the bank, per discharge, before subsequent recharge. The lifetime number of “charge-discharge cycles” a given battery or battery bank can return varies with several factors, an important one being the average depth-of-discharge per charge-discharge cycle. Depth-of-discharge is dependent on the capacity of the battery/battery bank, the electrical loads placed on the battery during use, and the time duration that loads are present. Laboratory studies show that 50% depth-of-discharge is the approximate point where lead-acid batteries return their greatest lifetime total Amp Hours of stored energy. Lesser average depth-of-discharge will provide more charge-discharge cycles, but not more lifetime Amp Hours returned. This leads to a reasonable design goal for boats with lead-acid batteries: battery bank capacity should result in an average Depth-Of-Discharge, in average use, of between 40% State-Of-Discharge (60% State-of-Charge) and 50% State-Of-Discharge (50% State-of-Charge).

Careful electrical system design is necessary to maximize battery service life. Amp hours returned over the total period of ownership is the basis of financial ROI. Aboard Sanctuary, we had times when we exceeded the 20-hour discharge rate (microwave use, main engine starter motor), which I deemed acceptable in bursts of less than 3 – 4 minutes. The way we used our boat, Depth-Of-Discharge averaged ~45% of total bank capacity (55% State-Of-Charge) by the time we recharged, measured from our Magnum coulomb-counting technology (vs conductance technology) battery monitor. “Charge-discharge cycle count” is a key advertising claim used by manufacturers to “project” usable service life. There is significant boat-to-boat variability in what different individual users and specific boats might experience in actual depth-of-discharge per charge-discharge cycle, but each specific case is unique to the specific boat and the way the particular owner uses the boat. Each specific use case is definable, predictable and repeatable.

Individual Ship’s Preferences/Needs:

Peg and I didn’t routinely anchor in one place for multiple days, but we do prefer anchoring to using marinas for our overnights. Anchoring in one place for multiple days does require periodic generator runtime. Our overnight anchoring goal was to never have to run the generator for our onboard energy needs, save in cooler weather for space heating or in warmer, humid weather for air conditioning. We never ran the genset overnight while both of us slept. The way we used our boat;

1. Battery stored energy was used for:

• house infrastructure (bilge pump(s), house water pump, toilet macerator(s), propane gas safety solenoid, inverter/charger “keep alive,” electric panel indicator lights, “always on” (parasitic) loads like boat monitor, AM/FM radio memory, etc),
• refrigeration (the single largest energy energy consumer on our boat),
• microwave, for heating drinks and meal prep (mostly supper),
• crockpot meals,
• space lighting as needed,
• TV/DVR/DVD (3 – 4 hours per evening),
• “always on” computer/network router/wi-fi
• iGadget overnight battery charging,
• anchor light,
• occasional overnight instrument use (VHF, GPS, AIS, depth sounder), and
• “Mr. Coffee” for coffee while doing email, forum, weather and route planning “stuff” in the morning, prior to engine start.

2. Management variables:

• more TV and less reading, more battery energy used;
• less TV and more reading, less battery energy used;
• oil lantern used for space lighting, less battery energy used;
• electric lights used for space lighting, more battery energy used;
• LEDs for space and nav lighting purposes reduces battery energy needs;
• more propane stove use, less microwave needed, less battery energy used.

Battery Capacity Design:

To accomplish our goal of “no generator use on overnights,” in the summer months (9 hours of darkness), we generally needed ~220 Amp Hours of battery capacity per overnight. In the winter months (14 hours of darkness), our overnight energy consumption needs went up to ~300 Amp Hours. To set up our boat for our “maximum use case,” we needed at least 300 Amp Hours of usable stored energy to be available. To get that needed energy while also honoring the 50% lead-acid technology “capacity penalty,” we needed ≥600 Amp Hours of total battery capacity to maximize service life ROI on my battery investment. So the above provides context to approach the question, what “amp hour” energy storage capacity do you need, as an individual boater on your specific boat (yes, this is a personal question) in order for you to use your boat the way you want to use your boat? EVERY BOAT AND EVERY BOATER IS DIFFERENT.

Choices, Choices:

Sanctuary was fit with a 12V DC electrical system. My options for getting 300 Amp Hours of usable energy in the lead-acid world included:

    1. 12V Group 24, Group 27 and Group 31 form factor (“size”),
    2. 12V 4D and 8D form factor (“size”), and 6V
    3. Golf Cart form factor Batteries.

Available size(s), space for batteries to occupy on the boat, and unit weight that I could safely handle unassisted determined our best choice. Spec sheets of all of the various US Battery manufacturers show Amp Hour capacity by battery form factor, so it’s easy to look up and compare manufacturer to manufacturer.

The final choice is one of the three practical, usual and prevailing lead-acid technologies:

1. flooded wet cells,
2. AGM and
3. Gel.

All of these lead-acid technologies come in all standard battery form factors, so that’s not planning limitation. Across manufacturers, all of the same form factors offer about the same amp hour capacities, so that’s not a planning limitation. The only practical difference is that wet cells need periodic watering, and AGMs and Gels do not. Flooded wet cells are 1/3 the cost of AGM and Gel batteries. They also all have about the same kind of service life if used in the same average profile of charge-discharge cycles. I had space and convenient access to my batteries, so that brought me to meet my 300 Amp Hour minimum usable capacity with six, 6V flooded wet cells. The batteries I chose were “Duracell labels” from Sam’s Club, and are manufactured in the United States by East Penn. East Penn is a major US national battery manufacturer, maker of “house label” batteries for West Marine, NAPA and other large retailers. Each of those EGC2 Golf Cart batteries was rated at 230 Amp Hours. Conclusion: a bank of three paralleled sets of two 6V EGC2s in series gave me 690 total amp hours, 345 usable amp hours, for a total cost in May of 2022 of $650, DIY installed, after core deposit.

The service life of ALL BATTERIES is measured in usable charge-discharge cycle capacity. Most lead-acid manufacturers claim ~300 charge-discharge cycles for flooded wet cells and ~400 charge-discharge cycles for AGMs and Gel. These claims are based on laboratory conditions, and I have never actually reached those numbers in the real world. I have had multiple sets of AGMs and I have never gotten anywhere near the claimed 400 charge-discharge cycles while discharging to 45% – 50% SOC. And, AGMs fail dramatically; they go from “perfectly fine” to “unserviceable” in a day or two.

Six 6V GC2s became our baseline choice, at a discount club price of $650. For AGMs of that same total capacity (~700 Amp Hours), $1800, and about the same cost for Gels. Flooded wet cells are available anywhere in the developed world and all third world countries, in ANY small town or any big city anywhere, and every minimally trained mechanic everywhere can install them. Six 6V GC2s fit in the same space footprint as two 8Ds, so more energy density can be achieved in that form factor footprint. Recycling wet cells is easy. Utility is very high. And for my $650 investment, I get 5 years of useful service life from my humble 6V flooded wet cell Golf Cart batteries the way I use my boat. Never a hiccup; it just works.


Let’s consider the Lithium option. I’ve shown that I needed 300 amp hours of usable capacity, which is a function of the boat loads and our usage of stored energy, not of the batteries themselves. To get 300 amp hour capacity with lithium, there is no 50% capacity “penalty.” I only need a little more than the real 300 amp hours. A myth about lithium is that they can be routinely 100% discharged; but that is NOT true. Doing that will shorten their service life. But the “capacity cushion” penalty is much less than lead-acid, at about a 15% – 20%. So in preparing this analysis, I used Mr. Google as my cost researcher (I “looked ’em up” online).

December 13, 2022, online ad sampler:

1. Battle Born, 400 amp hour package of 4 drop-ins, $3796.00
2. Lithionics, single 320 amp hour battery, $4499.00
3. Xantrex, 240 amp hour, $2800.00 at West Marine

Clearly, a lot more raw dollars for 300 Amp Hours of LiFePO4. And these numbers do not include any cost contingency for additional retrofit costs to protect against things like accidental BMS shutdown, bigger alternators or voltage regulators, system voltage increase or system segmentation, battery monitoring equipment if not already in place, pre-disconnect alarm signaling or drop-in coordination (or an interface to CANBUS communications and other monitoring systems). Purchasers of lead-acid batteries could safely assume that the batteries from commercial sources met the ABYC E-10 Battery Safety Standard for lead-acid batteries. In order to assume an apples-to-apples comparison here, this analysis assumes that these Lithium batteries meet the current – and still emerging – ABYC Standard E-13 for LiFePO4 batteries.  Probably not a universally true assumption in 1Q2023.

So three pages later, we finally get to the question, “where is the ‘payback?’

It is very reasonable to expect LiFePO4 batteries to last longer, in charge-discharge cycles, than lead-acid batteries. My experience was that we got 5 full years from my six 6V batteries. If the batteries are supposed to deliver 300 charge-discharge cycles, 5 years is 60 nights at anchor per year. LiFePO4 batteries claim to deliver 2000 – 4000 charge-discharge cycles. So at 60 nights per year, that means LiFePO4 batteries will last between 33 and 67 years. Well, that’s the math, anyway…

IF I JUST ACCEPT THAT MATH AS TRUE, looking at $4000.00 initial cost of lithium batteries, and coming from AGMs at $1800, the AGMs would be replaced $4000/$1800=2.22 times in order to “break even” on the cost of the batteries. The payback there comes from the lifetime difference in projected charge-discharge cycles: 2000-(2.22*400)=1112, so the LiFePO4 gives at least 1112 more charge-discharge cycles, a very nice potential lifetime return on amp hours per dollar spent. Maybe. And with my 6V flooded wet cell batteries, that projected payback would be $4000/$650=6.15 times to get to “break even” on the cost of the battery. But this time, 2000-(6.15*300)=155, so LiFePO4 gives more, but not nearly as much more, benefit in charge-discharge cycles returned.

Since the lithium batteries keep on giving “far into the future,” the $4000 initial battery cost is amortized over a longer time period, and that makes the annualized cost-of-ownership less than lead-acid. In the longer term, therefore, lifetime dollars-per-amp-hour returned is more favorable for lithium than it is for lead-acid, but getting to that point on a retrofit of an existing boat is still a lengthy proposition, measured in tens of years. Assuming the above are “typical numbers” that “most boaters” are experiencing/will experience, the straight “break even” financial return with AGMs happens after year 11 or 12, and the financial return with simple flooded wet cells happens at 6.15 *5 years=30.77 years from date of installation. Yes, owners that keep the boat long enough will certainly see ROI with a Lithium battery retrofit.

In either case, though, it may be true that you may “never need to replace batteries again.”

The “Value” Argument Revisited:

Personal Value is not always monetary. It can be realized as creature comforts, safety or just the satisfaction of ownership.” I completely support that statement. If one buys into the, “I JUST PLAIN WANTS IT NOW” argument, my question becomes, “WHY?” What do you think this is going to do for you that warrants the enormous sunk cost? What else could those same discretionary dollars go into that would ALSO return “creature comforts, safety or just the satisfaction of ownership?” Nice stabilized binoculars, refurbished salon with “his” and “hers” reclining lounge chairs, bigger/brighter video displays at dual stations for viewing the chart plotter? Something that might provide even greater personal value and utility?

The battery costs in this analysis are all proportional to amp hour energy requirements, and scale up in proportion to amp hours needed in pretty much linear fashion. As capacity-of-installation goes up (in Amp Hour needs) the ratio behind my ROI calculations stays pretty constant. Which is why I say, wait one more replacement cycle (3 – 5 years). Then at least, when making the upgrade, there will be a STABLE, PLUG ‘N PLAY system that will be much less likely to disappoint.

Now one further consideration. for those who are “a little older,” as we found ourselves to be in 2022, and facing the realities of mobility limitations and health issues, the very real question of how much longer you’ll be aboard the boat becomes real. I replaced our batteries in May, 2022. As a forced accommodation to advancing age, mobility and health issues, we sold our beloved Sanctuary in August, 2022. So I ask readers: in your opinion, which made more sense for us in May of 2022: $650 or $4K, and the need at minimum to segment my electrical system? Part of this analysis has to be, how much longer will you really own the boat? If that answer is, “greater than 10 – 12 years,” then depending on your personal values, your spending calculus could be different than if the answer is, “less than two to three more years.”

SUMMARY: there is no doubt that LiFePO4 systems have specific applications today, and I don’t doubt they are going to become the mainstream design platform in the near-term future. As of today [late 2022/early 2023], for the owner operator without electrical training, and for the less-skilled DIYer, on small and intermediate sized intracoastal and near-coastal cruising boats, these systems are very much an “emerging [beta test] technology.” To achieve “success” with them, owners must know a great deal about them. Users must be able to install, maintain, troubleshoot, and repair them WITHOUT outside technical assistance or support. Systems must include the capacity to re-charge them. Owning them is definitely not yet “install, forget and enjoy;” not “Plug ‘n Play.” The demands these systems place on owners far exceed the demands of equivalent lead-acid systems fit with a battery monitor. And note that the voices who most loudly extol the benefits of these Lithium chemistry batteries are people who, themselves, personally possess more advanced electrical skills.

My advice for the period of early 2023 through 2025: to those considering a lithium upgrade today, a thoroughly self-critical personal skills assessment is appropriate:

1. Understand that you are out ahead of the most current ABYC Standard, E-13, and thus exposed to unforeseen non-compliance issues as the standards mature and evolve.
2. Understand that UL Testing and Safety Standards, and EC/US/CAN safety standards, are also ahead of manufacturers current engineering and manufacturing ability to demonstrate compliance.
3. Be able to evaluate and select control and monitoring equipment, including the batteries and the internal BMS solutions that provide user safety, from a marketplace of generally inadequate and non-standards-compliant present equipment availability.
4. Understand what the existing standards require, and be able to do your own system design work, or be prepared to hire a qualified professional consultant to create a custom design deliverable, including a full-blown financial budget and an “Errors and Omissions” warranty.
5. Personally have the skills to install, operate, maintain and troubleshoot the installation yourself, on your own, without help.
6. Understand that paid, professional diagnostic skills are in very short supply worldwide, so if a performance or safety problem arises, be able to personally diagnose and correct a failing underlying design of a failing component(s), or both together, and implement your own updates to correct the issue(s) yourself, in exotic places and under less than convenient circumstances.

For those WITHOUT advanced electrical technical skills, then my advice is, wait another 3 – 5 years while this stuff works itself out via new and updated standards, new standards-compliant equipment and new system designs. Why? Because the current environment does have the ability to disappoint.






[3] First edition, published July, 2022, effective for boat builders and boat equipment manufacturers July, 2023.


[4] References found online in many places; i.e., here:


[5] SomEV website, here:


[6] Source: Jack Martin Insurance, Annapolis, MD; 14 December, 2022.


[7] Ibid


[8] Lithium Ion batteries are often rated in “Watt Hours,” a close cousin of “amp hours,” and the two quantities are easily converted back at forth.



Power Quality

1/20/2022 – Initial post
1/25/2022 – major addition: “IS ANY OF THIS STUFF REAL?”
3/8/2023 – editorial “clean-up”

This article discusses “Power Quality.” Throughout North America, the standard AC voltage waveform is a “sine wave” at a frequency of 60hZ.  A sine wave is an S-shaped waveform defined by the mathematical function y = sin x.  “Power Quality” is a term that refers to imperfections in the shape of the AC voltage waveform in an AC utility power distribution system.  AC voltages are ideally 1) free of distortion and 2) within the rated voltage and frequency specified for the host system. When the waveform is less than “perfect,” end user electrical equipment can be negatively affected; particularly, equipment that contains digital control circuitry.

I often remind boaters that boat electrical systems are different from residential systems. Because of several differences related to commercial utility power distribution systems, the power delivered to residential premises is “cleaner” (fewer “noise” and “distortion” components) than the power that boats might receive at marinas. And, this is not about the difference between 120V/240V and 120V/208V power; this topic has to do with distortion of, and noise on, the AC waveform. Waveform distortion on boats occurs more commonly when running on generators (and inverters) than shore power.

Figure 1 shows a conceptual portrayal of some common forms of voltage events that can lead to customer premises equipment failures:

Figure 1: Some “Typical” Power Quality Events that Occur in Utility Power Distribution Systems

Fortunately, many of these situations are fairly easy to identify and observe through simple measurement means. But, those labeled “Miscellaneous Waveform Distortions” can be quite technical and very difficult to diagnose, pinpoint and confirm without expensive, professional measuring tools. Two of those are:

  • Common Mode Currents and
  • Harmonic Distortion

Much of the content of this article is likely new to most people, including many electricians and many marine electrical technicians. The reason boat owner/operators should be aware of these concepts is that when they occur, they can and do affect the performance of electrical equipment onboard boats, and often with intermittent and obscure symptoms.  “Honey, it never did THAT before!”  Symptoms of these issues DOES NOT mean that affected equipment, itself, is necessarily faulty or failing.

Figure 2 expands upon the conceptual depictions of Figure 1. Here we see the waveform detail that characterizes and defines “Power Quality” problems. These waveform distortion “anomalies” can occur as amplitude and wave shape distortions, and can be present one-at-a-time or in random combination of several-at-a-time.  All voltage waveform anomalies can cause customer premises equipment to fail to operate correctly or to fail to operate at all.

Figure 2: AC Voltage Waveforms Associated with Select Power Quality Issues

When, where and how can this appear to boaters?

  • anywhere, any time of day, any season of the year, randomly…
  • as random equipment shutdowns on boats in a marina…
  • as random equipment malfunctions on genset power but not on shore power…
  • as random HVAC equipment power errors…
  • as random refrigeration system shut down…
  • as random TV picture distortion anomalies…

What can the cause(s) be…

  • equipment not designed or intended for use in “mobile” (marine) applications…
  • ungrounded or improperly grounded equipment on boat…
  • faulty equipment on other boats at a dock
  • faulty premises equipment in a boatyard or commercial facility…
  • anomalies in the campus power delivered to a dock by the facility power distribution system, including inadequate dock wiring…
  • anomalies in the power provided to the facility by the local electric utility at it’s “Point of Common Connection” (PCC) to the grid…

In order to understand “Power Quality” issues, some introduction to technical concepts is helpful and necessary.  The following is written to my dad (banking and finance), my dock neighbor (911 dispatcher) and my best friend (printer) in discussing these topics. In other words, to those with little or no electrical background. The next three topics are building blocks for understanding  and becoming familiar with the larger issues that cause problems for boaters.


While much of what follows is new, most readers will (I assume) have heard of “Ohm’s Law.” In my era, Ohm’s Law was a high school science topic. Ohm’s Law is a fundamental law of electro-physics, that describes the inter-relationships of resistance (Ohms), voltage (Volts) and current (Amps).  Ohm’s Law recognizes that electrical circuit components all have the property of “resistance” (“impedance” in AC circuits). In a circuit, as one quantity changes, the others follow in direct linear or direct inverse proportion.  In our homes and on our boats, we expect the incoming electrical voltage (120V/240V) to stay mostly constant, so as the resistance of the circuit changes, current follows.  For example, in a 120V/240V residential system:

  • turn lights “on” throughout the house, total current used goes up;
  • temperature satisfied in water heater, water heater shuts “off,” current use goes down;
  • thermostat tells Air Conditioner to turn “on,” current use goes up;
  • toaster completes your breakfast muffins, current use goes down;
  • induction fry pan turned “on” to make hash-browns, current use goes up.

Figure 3: Linear AC Loads – Typical

Ohm’s Law describes the relationship between current and voltage as “Linear.”  As shown in Figure 3, the current waveform follows voltage waveform in a perfectly proportional and aligned manner.  In technical literature, electrical loads that elicit this behavior are characterized as “linear loads.”

There are many kinds of electrical equipment and appliances that are not linear in behavior. Electrical circuits where current does not follow voltage are characterized as a “non-linear;” simply stated, they do not adhere to the simple proportionality of Ohm’s Law.  Figure 4 shows an example of a non-linear load, where current behaves quite independently of voltage.

Figure 4: Non-Linear AC Load – Typical

In homes and on boats, we find many examples of both linear and non-linear loads. Water heaters, clothes dryers, cook tops, crock pots and “old fashioned” incandescent lamps are “purely resistive” linear devices.  HVAC and refrigeration compressors, florescent lighting ballasts, “new fangled” LED lighting fed by AC power bricks, microwaves, inverter/chargers, DC-to-DC converters, engine alternator Voltage Regulators and Switched-Mode Isolation Transformers are non-linear devices.


A “Switched Mode Power Supply” (“SMPS”) is now by far the most common type of power supply found in modern consumer electronics, especially digital electronics, both AC and DC. As shown below in Figure 5, an SMPS is a “non-linear device” that utilizes solid state switching devices (IGBT – Insulated Gate Bipolar Transistor) to continuously switch power “on” and “off” at very high frequencies (more on that in the section on “Pulse Width Modulation”).

Figure 5: Capacitor Voltage and Current in a Power Supply Application

A sidebar of “geek speak” follows, for those interested, to illustrate the cause of the non-linear current. Others can “skip it.”

In an SMPS, incoming power is fed to “energy storing components” (capacitors and inductors). The energy storing devices smooth DC voltage and supply power to the circuit during the non-conduction state of the switching transistors.

The basic SMPS design variations are categorized based on input and output voltage type. The four principle groups are:

  • AC to DC – DC power supplies as found in many end-user devices;
  • DC to DC – Converter to change or regulate DC voltage;
  • DC to AC – Inverter;
  • AC to AC – Cyclo-converter (“frequency changer;” i.e., 60Hz AC to 50 Hz AC or vice versa).

​SMPS Advantages:

  • More compact and use smaller transformers; smaller size and lighter weight is an advantage for electronic devices with limited space and in mobile applications;
  • Regulated and reliable voltage outputs regardless of variations in input supply voltage;
  • High efficiency: 70% to 90% vs 45% for traditional power supplies.

SMPS Disadvantages:

  • Generate Electro-Magnetic Interference (EMI/EMC) and electrical waveform noise/distortion;
  • Complex electrical designs;
  • More components resulting in greater expense vs traditional linear supplies.

The main internal components of an SMPS are:

  • Input rectifier and filter;
  • Inverter (consisting of a high frequency signal and switching devices);
  • Power transformer;
  • Output rectifier and filter;
  • Feedback system and circuit controller.

Figure 6 is a very highly conceptualized block diagram showing power flow through a SMPS. This example is typical of a DC power supply in a TV, VCR, computer/printer/copier power brick, all kinds of battery chargers and other electronic equipment. This example uses 120V AC wall input and produces clean, highly regulated DC Output.

Versions of this same technology can use DC input power to generate Pure Sine Wave AC, and can be used to change AC line frequencies (50hz to 60Hz, or vice versa). And, versions of this same technology are used extensively in DC-to-DC applications, like the Balmar external alternator voltage regulator, Victron solar DC-to-DC controllers or Sterling Power DC-to-DC Converters used in battery charging, voltage doubling or voltage halving applications. These power supply designs are also used in DC navigation equipment, VHF radio equipment, and other DC equipment since the internal “high speed switch” is, electrically, an inverter.

Figure 6 shows an “InAC” Input and an “InDC” input; likewise, an “OutAC” and “OutDC” output.  All of these input and output types do not usually appear in the same device, but different mix-’n-match combinations of AC input and output, and DC input and output designs are manufacturer’s-choice product alternatives. Because of the PWM signal generator, the “AC” output is an AC “Pure Sine Wave,” such as what is found in 12V/24V PSW inverters on boats. The “DC” output is a highly-regulated DC voltage, such as found in a DC-to-DC Converter, or in the power supply inside sensitive made-for-purpose navigation electronics.

Figure 6: Block Diagram of a “Typical” SMPS Stand-Alone DC Power Supply

Pulse Width Modulation in a SMPS:

Figure 6 shows a PWM Signal Generator (the signal in the red oval).  This is both “the heart of the magic” and the source of some of its problems. A “PWM Signal Generator” produces a DC Square Wave, where the individual pulses have varying widths. A DC Square Wave is technically also an AC waveform, so it can be fed into a transformer just like any other AC waveform.

PWM Signal Generators use very high internal DC square-wave signal frequencies (50kHz).  This enables the use of smaller, lighter transformers in power supply applications, and greatly simplifies filtering of the DC output voltage. A significant potential penalty of this technology is electrical noise and waveform distortion reflected backwards into the local dockside electrical system, as well as local RF interference, which is very common with LED lighting that isn’t filtered well enough.

Another sidebar of “geek speak” here, for those interested, to frame the operation of “Pulse Width Modulation.” Others can “skip it.”

In a SMPS circuit, a PWM signal is generated by feeding a reference signal and a carrier signal through a comparator. The output signal is based on the difference between the two inputs. In an inverter application, the reference is a sinusoidal wave at the frequency of the desired output signal. The carrier wave is a triangular, or “sawtooth,” waveform which operates at a frequency significantly greater than the reference. During times when the carrier signal voltage exceeds the reference signal voltage, the output square wave is in one state, and at times when the reference voltage exceeds the carrier signal voltage, the output square wave is in the opposite state. Figure 7 shows the signals, with the carrier signal in blue, the reference wave in red, and the PWM DC output square wave in green.

Figure 7: DC “Square Wave” Pulse Width Modulation Signal


Two electrical concepts with which I would expect most laymen to be unfamiliar are “Differential Mode” and “Common Mode” voltages and currents in a circuit. Common Mode currents are usually noise; that is, an AC disturbance between one or more signal or power conductors and an external conduction path, such as an earth or chassis ground or miscellaneous conductive material not intended to conduct the power or signals (including ground fault current). Even to power engineers, this is arcane stuff. Arcane, that is, outside the marine environment. On a boat, it can rear its ugly head as power quality issues in onboard electrical equipment.

Pictures will make this discussion much easier to follow, so the next several drawings are a progressive sequence of views of the same thing, each building on the previous one, to help understand Differential Mode currents and Common Mode currents.

Figure 8: Typical Components and Circuits in a SWPS

Figure 8 is the starting point; the same basic electrical circuit shown in Figure 6, but this time, with some of the internal circuit details shown. The voltage waveforms in the various parts of the circuit are as shown earlier, and have the same meanings here.

Figure 9: Diagram as above, Showing Electrically Isolated Metallic Case

In Figure 9, the very same circuit diagram is repeated, but here, the metallic equipment case of the device is portrayed as a grey box in the background of the diagram.

Notice (lower left) that the equipment case is grounded to the incoming AC power source, but the logic circuit itself is electrically isolated from the case. The electrical isolation from the case of the unit is to help minimize the presence of Common Mode signals and other types of electrical noise.

Figure 10: Sources of Common Mode Voltages/Currents in SWPS


in Figure 10, we begin to see the emergence of the “noise problem” (or “magic,” as some might see this).

Capacitors are electrical components that block DC but pass AC. It’s actually much more complex than that, but that’s enough for now.

A Switch Mode Power Supply (SMPS) develops high frequency signals that also vary in frequency, cyclically over fixed time intervals, in normal operation. These high frequency AC and AC-like DC signals couple through internal parasitic capacitances (stray capacitance between electrical circuit components) directly to the equipment ground, and also couple through the inverter circuit via magnetic field coupling.  This generates undesirable noise currents which find their way back to the external power supplying source.  Here, the little red capacitors show the parasitic capacitive connections between the darker grey component heat sinks and component metal part content and the metallic case of the equipment.

And following the electrical path of noise currents from the parasitic capacitances shown in Figure 11, we see the noise currents reaching the power source’s “safety ground” conductor.

Figure 11: Common Mode Noise Currents Find Their Way Back Into the Host Power Distribution Network


For simplicity, the Earth connection appears on this drawing, but remember from many previous discussions that the actual earth connection is back at the “derived source” in the facility’s infrastructure.

Finally now, we can see, and point to, the distinctions between Differential Mode signals (voltages and currents) and Common Mode signals in a Power Distribution System on a dock.

Figure 12 shows both kinds of signals in the distribution system and attached equipment. In this case, Differential Mode signals are the desirable, wanted voltages and currents that make attached equipment work. They are portrayed in blue. They originate at the facility power source, travel to loads on the line conductor, and return from loads on the neutral conductors.

I want to emphasize that Differential Mode currents and voltages are the same old AC currents that we know and love and have always talked about.  They are the currents flowing from the source to the load in the Line conductors (L1 and L2) and returning from the load to the source in the Neutral (N) conductor.  We have just never needed to talk about them as “Differential Mode Currents” (or “Differential Mode Voltages”) before. It’s not language that’s commonly found in the ordinary course of “electricity” discussions, because we have never needed to differentiate these normal currents from anything abnormal; until now.

Common Mode currents are undesirable and engineers work hard to minimize and eliminate them. They are portrayed in red in Figure 12.  The electrical “source” of Common Mode signals is in the SMPS of the premises equipment.  In most marine environments, there are many, many, many of these devices on any given dock. AC non-linear loads produce undesirable Common Mode signals that
require suppression by complex and costly circuits designed specifically for noise filtering and suppression.


Figure 12: Differential Mode Currents in Blue, Common Mode Currents in Red


Again in Figure 12, the high frequency Common Mode Noise Currents ORIGINATE in the Rectifier and Inverter sections of the SMPS, capacitively couple to the equipment ground, and flow along the ground conductor into the external system’s power source. From there, they can flow as electrical noise in many directions. Above, they are shown flowing in phase with each other (which is the technical definition of Common Mode Currents) back to the SMPS in which they originated (Rule 1).  They are flowing in the same direction on BOTH the Line and Neutral conductors of the device (Rule 2).  But, since the line, neutral and ground conductors are shared in parallel across many, many end-user circuits, that noise will ALSO flow on those parallel paths (Rule 2).  And since the device ground is connected to earth ground, Common Mode currents can also flow through the earth/water to impact other parts of the common, shared system.  And so, a noise-producing fault on one boat can and will propagate to other nearby boats.

And folks, that’s the reason to care about any of this stuff in the first place!

Figure 13: Typical Oscilloscope Traces of “Electrical Noise”

What does “electrical noise” look like? Figure 13 shows screenshots of electrical noise from articles I’ve found online.

Instead of a nice, clean waveform, it’s a distorted jumble of spikes, ripples and gaps.


In systems with Common Mode Currents causing electrical noise, both the line conductors and the neutral conductor will be oscillating up and down at the noise frequency, so Differential Mode Currents can appear almost normal.  But excessive Common Mode noise can cause equipment circuits to malfunction. Lots of money is spent to design power supplies to recognize and suppress these undesirable noise components. That works; to a point. But, sometimes under some conditions in some places, the noise components become too large to be fully suppressed by “standard” means, and then end-user equipment may fail. Noise filtering adds cost to components, and so may not be found in all equipment. Buyer beware.

Equipment manufacturers do know about this kind of noise and its causes, and try to design filtering circuits to minimize it. Let’s look quickly at another source of Common Mode noise that is found on many, many cruising boats.

Refrigerators with the ever-so-common BD35, BD50 Danfoss/Secop compressors are advertised as 12V and/or 24V DC appliances. And as far as the power supplied to the refrigerator is concerned, that’s true. However, the compressor motor itself IS NOT a 12VDC or 24VDC motor. That little compressor motor is a 3-phase, variable frequency motor that runs at nominal 277VAC.


Indeed, I did say that. Cowabunga, dude!

The Danfoss/Secop power module (101N0510) in my Vitrifrigo fridge accepts either 12VDC/24VDC or 120VAC and converts that input voltage into 3Ø, 277VAC to run the little compressor. The conversion is via a 3Ø AC SMPS. Yes, that makes the fridge a non-linear device to incoming 120V AC. Figure 14 is a highly conceptualized diagram of the 101N0510 SMPS:

Figure 14: Three-Phase SMPS with DC Input


The 3Ø SMPS is a Variable Frequency Drive application which spins the compressor at slower speeds when cooling demands are low and higher speeds when cooling demands are high, all while regulating the AC Voltage required by the motor.

For those familiar with three-phase systems, the Danfoss/Secop compressor motor is a Wye-connected 3Ø circuit with a totally isolated, floating neutral.  The 3Ø electricity is created by 6 IGBT switches, so as the phases rotate, the motor’s neutral star-point DOES NOT stay at 0 v with respect to frame ground.  Great pains are taken to avoid capacitive coupling from the motor’s star point neutral to the frame of the motor, because that becomes the source of a Common Mode Current. But yes, there is capacitive coupling from that neutral to the frame of the fridge… And. Insulation does break down as it ages and goes through thousands of heating/cooling cycles…

Here’s a link to an 11-minute Danfoss video, explaining the above, that readers may find interesting:


“Harmonic Distortion” is another true AC sine wave malformation. The math describing Harmonic Distortion is called “Fourier Analysis,” or a “Fourier Transform.” No, we’re not going to look at Fourier math in this article! The concept is, if a waveform is a true sine wave, it is made up of one, and only one, fundamental frequency. So, ANY sine wave that is not “prefect,” is actually not a “sine wave.” That waveform contains, by definition, “harmonic frequency components.” Harmonic frequencies are even and odd multiples of the fundamental sine wave frequency. For a 60-Hz sine wave, the 1st harmonic is 120 Hz, the 2nd harmonic is 180 Hz, the 3rd harmonic is 240 Hz, the 4th harmonic is 300 Hz, the 5th harmonic 360 Hz, etc, etc, etc. Fourier analysis will tell engineers exactly what harmonics are present, as well as their real and relative amplitudes. There is expensive handheld test equipment, such as the Fluke 40, 41 and 345, AEMC 8336, and others, that can identify harmonics for electrical technicians working on Power Quality problems at customer premises.

Harmonic waveform voltages combine with (add to and subtract from) the fundamental waveform voltage to produce the observed “apparent waveform.” OK. Time for a picture:

Figure 15: Waveform Distortion Caused by the Presence of Harmonic Frequency Components

Figure 15 shows a 60Hz fundamental frequency waveform (solid black line) and for simplicity, only 3rd and 5th harmonics (dotted lines) of the fundamental frequency. Instantaneous voltages of the fundamental and harmonic waveform voltages “add up” to produce the resulting voltage waveform that is actually experienced by equipment in the system.

Depending on the specific mix of harmonics and their amplitudes, many variations of malformed voltage wave shapes are possible. Two additional, “typical” malformed AC mains power waves are shown in green and blue in Figure 16. The red wave shape above is called “Flat Topping,” for obvious reasons. Flat topping is characteristic of harmonics injected into the mains power by SMPS power supplies. Other malformed waveforms are characteristic of inadequately filtered Variable Frequency Drives (VFD). There are many more harmonics than are shown in this highly simplified chart.

Figure 16: Infinite Malformed Wave Shapes Are Possible, Depending on Details of Specific Harmonic Content

All electrical equipment is rated to several industry quality standards by its manufacturer for its tolerance of Harmonic Distortion and its contribution to back-feeding distortion into the AC service mains and onto the grid in the neighborhood. The applicable utility company Power Quality standards (IEC 1000-3-2 or EN61000-3-2 and IEEE-519) are to enhance the quality, reliability and stability of the electrical power grid and it’s infrastructure.

The issue of Harmonic Distortion can become symptomatic on boats when digitally controlled equipment is running on generators vs when running on shore power. The reason traces back to Ohm’s Law. ALL ELECTRICAL CIRCUITS have the property of electrical resistance. In AC circuits, its called “Impedance,” but it works the same way as resistance in Ohm’s Law math.

Another sidebar of “geek speak” follows, but please don’t skip this one, because the punch line may be worth the price of reading through it.

Figure 17: Effects of Linear and Non-Linear Loads Fed by a Weak Grid

Figure 17, left side, represents “source impedance” as a combination of the source’s electrical resistance, Rg, and inductance, Lg. Ohm’s Law predicts there will be a voltage drop, Vdrop, across that source’s impedance as a result of current, Ig, flowing from the source into attached loads. The effects of that current flow is reflected in the waveform corruption seen on the drawing.  In the case of commercial utility distribution systems, the subscript “g” means “grid.” In the case of a boat’s onboard genset, the subscript “g” means “genset.” For utility systems, the heavy vertical line in the center of the drawing is the “Point of Common Connection” between the utility grid and a collection of shared loads, such as residential neighborhoods or commercial facilities. On boats, the PCC is where the genset connects to the onboard electrical panel; it’s easy to think of that point as being the Generator Transfer Switch. The effective impedance of the AC shore power utility grid system is proportionally much smaller than the effective impedance of an onboard genset. Per Ohm’s Law, the effect of harmonic currents is therefore much greater and more significant within the proportionally larger impedance of the proportionally smaller capacity generator power source. Especially when that small source is running above 60%-70% capacity, where magnetic saturation issues begin to come into play. The result of all of this is that onboard AC equipment may run just fine when on shore power, but experiences intermittent failures when running on the genset.


I’m sure some readers ask themselves that question from time to time.  Well, how’s about I show you some oscilloscope waveforms and you decide for yourself?

Aboard Sanctuary, we have a Dometic model DTU16-410A, 16kBTU, 120V, 1∅, mfg. no. 205160160,  heat pump installed.  User input is provided by a Dometic model SMX II remote control unit.  The SMX II stages loads “on” in order to more evenly distribute inrush currents and avoid tripping the HVAC branch circuit breaker.  When calling for heating or cooling, first the fan come “on,” then after short delay, the raw water circulator stages “on,” and again after a short delay, the compressor stages “on.”  These units are staged “off” in reverse order once the thermostat becomes satisfied.  Aboard Sanctuary, this heat pump provides for our main heating and cooling needs.  Many readers will have this unit, or very similar and equivalent units, aboard their boats.

The blower in this heat pump is a variable speed drive motor that speeds up when heating or cooling demand is high and then slows when the thermostat is satisfied.  Figure 18 show the 120V voltage waveform (yellow) and the current waveform (blue) with the blower-only running, on its “slow speed:”

Figure 18: AC Current Waveform – Blower only

Clearly here, the current drawn by this blower IS NOT a sine wave, with somewhat more than 50% of the leading portion of the waveform missing.  This is reminiscent of what we saw above in Figure 4.  This motor is a typical example of a non-linear load.  Because it’s a non-linear load, it contains many harmonic frequencies.

Figure 19 shows the current waveform (blue) when the raw water circulator stages “on,”  in this case, in a “heating” cycle example:

Figure 19: Current Waveform for Fan and Raw Water Circulator.

In this waveform, we can see the current drawn by the non-linear blower superimposed on top of the current drawn by the linear raw water circulator pump.  The pump motor’s waveform is a sine wave, and this composite waveform shows the “bump” of the fan’s non-linear load atop the motor’s linear sine wave.  This resultant waveform is non-linear, itself with harmonics, but different harmonics than were found in the fan waveform by itself.

Finally, Figure 20 shows the current waveform when the fan, raw water circulator pump and compressor are all “on,” running, at the same time.

Figure 20: Current Waveform: Blower plus Raw Water Circulator plus Compressor

This waveform is the composite result of the current drawn by the Blower, the current drawn by the raw water pump and the current drawn by the compressor motor.  This waveform is still not a sine wave, distorted as it is by the blower’s non-linear components.  And, as can easily be seen here, the top of the wave is “flattened” by the fan component.  And again, there are lots of harmonics here.

My oscilloscope has a built-in Fast Fourier Transform (FFT) math function to analyze the harmonics present in waveforms under analysis.  Figure 21 shows the FFT Analysis for the composite waveform of the Fan, the circulator pump and the compressor:

Figure 21: Fast Fourier Transform (FFT) of the Overall Load Drawn by the Heat Pump in its “Heat” Mode.

This FFT waveform shows the 60 Hz “fundamental frequency” as the highest peak on the left side of the screen.  Then, the peaks show several succeeding harmonics, each with their characteristic descending magnitude.

ALL OF THESE HARMONICS contribute to the shape of final waveform shown in Figure 20 and experienced by the equipment on this particular power inlet to our boat.  This (and one other, smaller, heat pump) is the only equipment on this 120V line into our boat, so what happens here is exactly what is drawn from the dock feeder by our boat.

The day I took these screen shots was “chilly” and rainy (downright “cold” for La Florida), and many live-aboard boats on the dock feeder probably had heating systems in various stages of “working.”  Notice that there is minimal distortion of the voltage (yellow) sinusoid.  That’s because the dock infrastructure is capable of providing power to the loads attached to the dock feeder; in electrical speak, the feeder has a “low internal impedance.”

One final waveform is shown in Figure 22.  I have not described this at all in the preceding text because it is generally not an issue for end users of electric power, but it does contribute to waveform distortion if/when aggregated into the electric utility.

Figure 22: Power Factor, Showing Current Waveform Lagging the Voltage Waveform for this Device

In this screenshot, I adjusted the amplitude of the current waveform (blue) to show it visually to the same scale as the voltage waveform (yellow).  Notice the obvious non-sinusoidal shape (distortion) of the current wave form; proof positive of the presence of Harmonic Distortion.  But also notice that the current waveform lags the voltage waveform in time; that is, these two waveforms are slightly out-of-phase with one another.  The voltage waveform makes its “zero crossing” before the current waveform makes its “zero crossing.”  This phenomena is an electrical “characteristic result” of “inductive loads” (motors, transformers) in AC systems, and it’s called “power factor.”  It doesn’t mean much to boaters, and there is nothing end users of electric power can do about it, which is why I haven’t discussed it in more detail, but it can and does mean a lot to the utility supplying the docking facility with electric power.  It means the electric feeder cables need to be bigger – sometimes significantly bigger – than they otherwise would in order to carry the total load of the facility.


What was the impetus for this article? Consider the question, “what is an acceptable limit for neutral-to-ground voltage?” The National Electric Code says 2.0 VAC is the max, and that’s based on resistive voltage drops that can occur in the neutral conductor in long cable runs as found on boat docks. But, since some of these Power Quality signals can look like Differential Mode currents between neutral and ground, it is possible in unusual circumstances to experience very high neutral to ground voltages. Anything higher than 2.0VAC should be treated as suspicious, and should be investigated. These waveform distortion faults are not simply aggravating and frustrating, they can be economically significant.”  They degrade wire insulation and the mechanical parts of equipment (especially, motor bearings), so they can have real economic impacts to equipment service life.


So, to wrap it up, there are several power quality issues that can occur on boats, and particularly when running on relatively “small” power sources (compared to facility shore power), like inverters and generators. Power Quality issues can cause attached equipment to experience intermittent or continuous failures. These problems can appear alone or in combination with other types of electrical
disturbances. Some of these disturbances are intermittent and very complex and arcane. When they happen, they can be enormously frustrating, time consuming and expensive.

In our “modern” “throw it away” vs “fix it” culture, it’s easy to fall into a “replace it” trap without identifying or realizing the actual root cause of the problem. Since the equipment itself may not be the cause of the fault symptom(s), replacing equipment with new, like-for-like equipment is just as likely to result in continued exposure to the same old performance issues with the new, replacement equipment. So I bring this to your attention for awareness. I haven’t talked much about how to identify these issues, because that is definitely an advanced skill requiring expensive, time-interval and data recording test equipment. But simply being aware of these issues can enable boaters to ask the right questions and get to qualified service technicians to reach a good, and probably permanent, problem correction.


Electrical Lingo: What Is He Talking About

10/18/2021 – Initial post
10/19/2021 – Added “AIC”
11/28/2021 – Added “Charge Acceptance Rate,” “Self-Discharge”
12/12/2021 – Added “Power Quality,” “Switch Mode Power Supply”
  4/25/2023 – Added “Polarity/Reverse Polarity,” “Sine Wave”
5/13-14/2023 – Added “UL;” fixed bad links; editorial updates to add NEC and UL 489 definitions
5/15-24/2023 – Major expansion; added many additional entries; editorial clean-up.

This article is an effort to try to help electrical “laymen” and others who may be new to electrical terminology.  Following are my attempts to explain and describe what is meant by some of the “lingo” that appears frequently in Internet and dock conversations about electrical topics. This is not a classic “glossary,” because I also try to explain what the terms mean in the context of boat electrical systems.  Hopefully, this list will make the inevitable future conversations about boat electrical topics less intimidating.

It is not possible to discuss electrical topics without encountering some “jargon.”  Even in fairly basic discussions, some technical background knowledge is always assumed. The very basic stuff, like Ohm’s Law, is High School science, but that is often not quite “enough.” Online forums are places for people to exchange information and learn. Some minimal “knowledge” is required to read and understand electrical technical exchanges. Many words have different meanings depending on their context of use.  Individuals directly involved in these discussions may understand the context, but those who are generally not familiar with electrical topics may not know what the lingo being used really means. Those who would otherwise benefit from the information exchange may decide to skip the topic, and miss the benefit.

Some of the descriptive sections below are more detailed than just simple terminology descriptions.  That’s because I have not written about them before in other places.

I don’t know that I’ve included all of the “lingo” that those unfamiliar with “things electrical” (“laymen,” “novices,” “newbies”) may find obscure or confusing.  If ANY READER is interested in terminology that I have not captured here, please contact me to let me know that. My plan is to maintain this document as a “living document,” not a one-time monograph. Please help make it more complete and understandable.



Standards organization; American Boat and Yacht Council; organization that creates minimum manufacturing performance standards for various aspects of pleasure craft; primary jurisdiction is manufacturers in the United States, and manufacturers who sell boats into the US market, but ABYC standards are used in Canada, and have a very influential role in Europe and SE Asia as well. Intended for use by:

  • boat builders
  • boat equipment manufacturers
  • Surveyors
  • certified marine technicians and
  • lawyers involved in litigation.

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Electrical Standards; National Electric Code; created and owned by the National Fire Protection Association. Adopted in the United States for use by the hierarchy of various political subdivision (state, county and municipal) and the construction industry. Ranges from installation codes for single-family houses, condos and apartment buildings, light commercial and industrial buildings, marinas and floating structures, elevators, hospitals, airports and heavy industry.  Adopted as the minimum legal requirements for electrical installations performed by installers of electrical systems.  Enforced by municipal Code Compliance Officers.

Canadian Electric Code; adopted in Canada for use as described above.

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Standards organization; National Electrical Manufacturers Association; provides mechanical and electrical standards for electrical equipment and components. NEMA publishes more than 700 electrical and medical imaging Standards and technical whitepapers that cover millions of Member products.

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Standards organization; National Marine Electronics Association; NMEA created uniform interface standards for digital data exchange between different marine electronic products; most familiar are:
1. NMEA0183 and
2. NMEA2000.

NMEA2000 is an extension of CANBUS that suits marine-specific application and equipment needs.

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Testing Standards Organization; Underwriter’s Laboratory; develops standards against which products are tested. The UL Certification Marks serve as a recognized symbol of trust in products and reflect the UL goal of advancing product safety.

The ABYC safety standards for boats frequently refer to UL testing standards compliance for components, materials and equipment installed and found on boats.

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Standards Organization, the Society of Automotive Engineers, international in scope,  develops standards which apply specifically to vehicles (cars, trucks, busses, semi-trucks, ambulances, emergency vehicles, etc). These standards include a significant body-of-work around electrical requirements.

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An industry standard; the American Wire Gauge; widely deployed and used across North America; describes electrical conductor wire types, construction, insulation and diameter.

There is a similar SAE standard specific to conductors used in vehicles, but the AWG and SAE conductor standards are not equivalent. SAE wire sizes of the same numerical designation are generally about 9% – 12% smaller in diameter.

Both AWG-rated products and SAE-rated products are allowed on boats by the ABYC electrical standard for boats, E-11. It is the installers responsibility to understand where and how the standards are different and make product selection decisions that are compatible with the intended use.

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Ohm’s Law

Formula that describes the mathematical relationship between voltage, current, resistance and Power in electrical circuits.

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Kirchhoff’s Law

Also called Kirchhoff’s “first law,” or Kirchhoff’s “junction rule,” first described in 1845 by German physicist Gustav Kirchhoff, states that for any node (connection) (junction) in an electrical circuit, the sum of the currents flowing into that node is equal to the sum of currents flowing out of that node.  That is, the algebraic sum of currents in a network of conductors meeting at a node point is zero.

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Peukert’s Law

Peukert’s law, first described in 1897 by German scientist Wilhelm Peukert, expresses the approximate variability in energy storage capacity of a given rechargeable lead-acid battery when discharged at different rates.  As the rate of discharge increases, the energy returned by the battery (battery capacity) decreases. Peukert’s law defines the approximate abgebraic relationship of rated battery capacity, rate of discharge and time to dead battery.

In the chemistry of a lead-acid battery, the chemical process of diffusion is responsible for transporting electrolyte around the battery.  Diffusion of electrolyte into lead progresses at a finite rate, so discharging the battery quickly causes the voltage to fall off prematurely, before all the active material in the battery is used up. Given sufficient recovery time, a battery which has been discharged at a very high rate will recover some usable capacity if it is left “at rest” for several hours or a full day.  This engineering factoid is of interest only to engineers in laboratory settings.  On a boat, it is not practical to suspend battery use for several hours in order for it to recover from overly rapid discharging.  Additionally, overly rapid discharging shortens battery service life and is considered by manufacturers to be abuse.

Discharged at a rate of 10 Amps, a battery with a C20 manufacturer’s rating of 200 aHr will last 20 hours, but the same battery, if discharged at a rate of 20A, may only last for 5 hours.  Therefore, when discharged at a higher than rated load, this 200 aHr-rated battery only delivered 100 Ah of usable energy, and only for a very few hours.

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The name of the quantity associated with “Electromotive Force” (EMF), “propulsive energy” that acts on a circuit and causes electrons to flow into a load.

Voltage (“Electromotive Force”) is measured across two points in an electric circuit.  The unit of measure is the “Volt” (or “millivolt,” or “kilovolt”). Common symbols for “Volt” are “mV” for “millivolt” (thousandths of a Volt) or the letter “V,” for whole “Volts” in the range 1-999, or “kV,” for thousands of “Volts.”

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The name of the numerical quantity of electrons flowing through a metallic conductor, or of ions flowing through a liquid medium like salt water, at any particular point in time.  The unit of measure is the “amp” (or “milliamp”). Common symbols for “Amp” are “mA” for “milliamp” (thousandths of an Amp) or the letter “A,” for whole “Amps” in the range 1-999.

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The physical property of any/all electrically conductive material(s) that tends to retard or impede the flow of electrons through it.  The unit of measure is the “ohm.”  The symbol for resistance is the Greek letter Omega (Ω).  Measurement quantities are “ohm” (Ω), “kilohm” (kΩ), or “megohm” (mΩ).

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Power Factor

Electrical phenomena common to the behavior of AC electrical equipment. They are particularly associated with large electrical equipment and all solid-state electronic equipment. These become increasingly important as voltages, frequencies and power consumption rise.

“Inductance” is measured in “ohms.”  “Capacitance” is measured in microfarads, millifarad and farads.  “Power factor” is measured in a decimal percent.  Power Factor represents the angular displacement in time of current and voltage waves in the same conductor, or the angular displacement in time of either the voltage waves or the current waves in two different conductors.

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A circuit component(s) that transports electric current around a circuit.

In DC circuits, the battery is considered as the source, and the polarity of the terminals never changes.  The conductor attached to the positive battery terminal is known as the “B+” (B-plus) conductor.  The conductor attached to the negative battery terminal is known as the “B-” (B-minus) conductor.

In AC circuits, the polarity of the voltage changes in a regularly-repeating cycle.  AC systems on boats are required by ABYC to have “grounded neutral AC systems,” and the “grounded neutral requirement” establishes the agreed convention for the “working polarity” of the conductors of the system.  The “positive” conductors are known as “ungrounded current-carrying conductors,” and the grounded conductor is known as the “grounded current-carrying conductor,” or more ordinarily in electrical conversation, the “neutral conductor.”  In AC systems, it is possible to have an ungrounded neutral and the circuit will still work normally.  The neutral grounding requirement is to handle transient, dangerous fault conditions.

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A measure of the amount of energy a device needs to operate, important on boats because energy for appliances comes from batteries, and owners need to know how long the battery power “will last.” In purely resistive applications, “work” is the creation of light or heat.  In turning a motor, “work” is the creation of torque.  Power is cumulative.  Each separate appliance needs some power, but batteries have to supply all of it.

In electrical engineering, the unit of measure is the “Watt” or “Joule.”  In mechanical engineering, the unit of measure is the “inch-pound” or the “foot-pound.”

Common symbols for “Watt” are “mW” for “milliwatt” (thousandths of an Watt), “W,” for whole “Watts” in the range of 1 – 999, or “kW” for “kilowatts” (thousands of watts).

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The number of times a repeating alternating wave goes through a complete cycle in a standard measurement time interval, usually one second.  The unit of measure is the “Hertz” (or “Kilohertz,” “megahertz,” or “gigahertz,” etc).

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The point in an electric circuit from which electric energy (voltage and current) originates to energize a circuit.

In DC electricity, this can be a battery, a fuel cell or a solar panel.

In AC electricity, it can be a generator, an inverter, or a point in the commercial distribution grid network defined by the NEC as a “newly derived source,” which in turn is fed by a higher level utility distribution circuit.

The term must be understood in the context of the circuit being described.

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The electrical equipment components of an electric circuit where energy is consumed to do useful work: “useful work” includes production of heat or light, or torque in the case of a spinning a motor.

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A network of electrical components consisting or a source, one or more loads, and the interconnecting conductors, protective devices and control switches. A “circuit” only exists if there is an unbroken electrical path from the source, to the load, and back to the source. If that path is interrupted anywhere, as with an “on”/”off” switch, there is no “circuit;” only wires and devices, but no “electric circuit” for electric current to flow through.

Following is an “electrical diagram” of a simple DC “circuit::


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In AC circuits, “neutral” is the “common conversational term” for the current-carrying conductor that returns electric current from the circuit’s load to the circuit’s source. Sometimes conversationally – but incorrectly – called the “ground.” The neutral conductor is a current-carrying conductor.  with shore power, the neutral is bonded (electrically connected) to the system’s “earth” connection at the source.  With onboard AC system sources (transformer, generator, inverter), the system neutral is bonded to the system ground conductor at the source device of electric power (at the shore power transformer secondary, the genset or within the inverter).

In DC systems on boats, the DC source is considered to be the vessel battery, and there is a return conductor from the DC Load to the negative battery terminal known as the “B-” conductor.  In recent years, that conductor was specified to have yellow insulation, but previously, that conductor had black insulation.

In AC systems on boats, all utility and equipment circuits are made up of three conductors.  The energized current-carrying conductor(s) are either black or red in color. The returning conductor – the “neutral” – is a current-carrying conductor that is white in color. The safety ground is always green in color. This separate and independent green “ground” conductor should never carry current except during the time an actual fault condition is present.

Although the term “neutral” is not used with DC circuits, the purpose of the B- conductor is the same as the purpose of the “neutral” conductor in AC circuits.  The purpose of the DC “B-” and AC “neutral” conductors is analogous.  That is, they carry current back to the source from the load.

“Common” is a term that refers to a portion of a circuit where many branches of other circuits are also connected and used as part of the return circuit.  “Common” is a word that is was used in electricity, and more so in electronics, to refer to a shared return path for current.  It was commonly used in electronics in connection with devices built on metal chassis, where multiple circuits all shared the power supply return path of the metal chassis itself, instead of using individual return conductors.  Metal chassis are gone with the advent of printed circuit electronics, so this term has faded from use, but the concept remains and the term is occasionally heard, used mainly by us old-timers.

Today on boats, the metal block of the engine is an example of a shared, “common” conductor.  The engine block has a number of gauge sensors (oil pressure, coolant temperature, tachometer), controls (fuel solenoid) and alarm sensors (loss of oil pressure, high temperature) which use the engine block as their DC return circuits.  On many boats with engine-mounted alternators, the negative alternator conductor is the mechanical connection to the engine established by the mounting bolts.

“Buss Bars” are also an example of shared path, “common” conductors. Language analogies: “ground,” “B+,” “B-,” “buss.”

Language opposite: “home run.”

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Short Circuit

A specific type of electrical fault resulting from an unintentional direct connection of an energized conductor to a return circuit or to an earth ground. This very low resistance, unintentional connection, results in the flow of extremely large fault currents and causes overload protection devices (fuses, circuit breakers) to “open,” or “trip,” in order to disconnect the energized power source and protect system wiring from overheating damage.

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A rating of the ability of a conductor of a given material, cross-sectional diameter and insulation properties to conduct an electric current within the temperature limits established by the properties of the conductor’s insulation composition.

Current-carrying capacity is specified in “amps;” Temperature is specified in “degrees centigrade.”

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A “Surveyor” is an independent tradesman in the marine industry. “Surveying” is the process of evaluating the current condition of the boat against objective criteria. Surveying necessarily involves a number of objective standards, subject-matter knowledge, work experience and subjective personal judgement on the part of the surveyor.

In evaluating boats, surveyors make extensive reference to industry-recognized and accepted standards. The principle collection of objective standards in North America are those of the United States Coast Guard (USCG) and American Boat and Yacht Council (ABYC). ABYC standards are voluntary, and boats are “grandfathered” to compliance standards that applied at the time the boat was built (not necessarily to the standards in place as of the time of survey). If the boat has undergone “significant retrofit and upgrade,” current ABYC standards apply.

The surveyor’s written work-product is a “Survey Report,” or “Survey.” Survey content, and the way that content is phrased, can result in insurance companies mandating certain repairs, determine if the client can get insurance at all, and can affect the pricing of premiums.

There are several kinds of “surveys,” including:

  • Condition and Value
  • Insurance
  • Electrical
  • Engine
  • Corrosion

Statistically, electrical problems account for a large percentage of maintenance and performance issues that boaters typically encounter during their period of boat ownership. Previously-owned boats may have had OEM equipment installed that cannot meet today’s requirements or may have had prior electrical work done by unqualified or under-qualified tradesmen or DIY owners. Buyers, particularly of larger, previously-owned boats, should consider commissioning a pre-purchase “Electrical” Survey. Buyers of wooden-hulled boats or of metal-hulled boats should also commission a pre-purchase “Corrosion” Survey. Electrical and Corrosion surveys are done by technicians who specialize in those specific marine specialty areas.

🎓Click link for additional information on Boat Surveys

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Interoperability” is the term for the technical capability that allows consumers to buy products from different manufacturers with confidence that the products will operate properly in a system containing equipment from multiple different manufacturers.

The word “interoperability” is a conceptual expression used across organizations and manufacturers involved in electronics and communications technologies.  It means that various different pre-programmed electronic devices made by different manufacturers can be networked together and will then be able to “communicate with,” “understand” and “respond appropriately” to one another. This is done by defining a common “language” (called a “data protocol”) to which individual manufacturers agree to design their products to be compatible.  For example, “interoperability” means that products made by Garmin can interact correctly with products made by Sitex, Sitex products can interact correctly with products made by Raymarine, Raymarine products can interact correctly with products made by Furuno, and Furuno products can interact with products made by Garmin, all can interact with various engine manufacturers, and all can be reported by Mareton network monitors

The marine standard communications language “data protocol” is either NMEA0183 or NMEA2000, which are different vintages of communications protocol.  They are mutually incompatible.  The standard communications protocol in the computer world is Ethernet (IEEE 802.3) or Wireless Ethernet (IEE802.11 a/b/g/n/an).  The cellular communications standards include GSM and CDMA. Older equipment protocols, like RS232, are still found in use in legacy equipment, like Raymarine Seatalk, but are no longer found in new equipment, and are disappearing in favor of newer, faster, more functionally rich, network technologies.

🎓Click link for additional information on Marine Data Networks

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DC Electricity;
AC Electricity

Direct Current (DC) is electricity that flows in one, and only one, direction from the source, through a circuit, and back to its origin. By convention, engineers treat DC current as if it flows from a “positive” terminal to a “negative” terminal.  Common sources of DC electricity are batteries, solar cells and fuel cells.

🎓Click link for additional information on DC Electricity On Boats

Alternating Current (AC) is electricity that flows in two directions, with the “positive” and “negative” terminals of the source periodically reversing their relative roles of outbound and return conductors. The time period of the reversal of direction is called “frequency.”  In North and Central America, the standard frequency for commercial AC power is 60 Hertz, meaning it reverses polarity “60 times per second.” In other parts of the world (Eastern Europe, Asia and Oceana), the standard frequency is 50 Hertz. The most common sources of AC electricity are rotating machines (generators) driven by water or steam turbines or fossil fuel engines, and inverters that produce AC electronically from DC sources.

🎓Click link for additional information in AC Electricity Fundamentals, Part 1
🎓Click link for additional information in AC Electricity Fundamentals, Part 2
🎓Click link for additional information in Electrical Behavior of a 208v/240v Boat

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Sine Wave

In Trigonometry, a “Sine” function is a smoothly recurring curve produced by a constantly repeating vertical quantity (in electricity, usually voltage or current) plotted against a horizontal axis delimited in units of time. In AC power electricity, the time axis is 60 Hertz (60 “positive and negative half-cycles per second”) in North and Central America. The time period is 50 Hertz (50 “positive and negative half-cycles per second”) in Europe, Asia and Oceania.

Following is a diagram showing typical AC voltage and current sine wave curves:

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Single phase;
Three phase

“Single Phase” and “Three Phase” are terms that specifically refer to types of alternating current electricity. These terms refer to how the AC electricity originates in generating equipment and is then distributed to end users.

Single Phase” (or 1∅) refers to alternating current systems in which any/all of the voltages in the system rise and fall together, at the same time.

Three Phase” (or 3∅) refers to alternating current systems in which there are three separate voltages in the system which rise and fall at intervals displaced 120° in time apart from one another.  In English, that means they do not all rise and fall together, but rather in a time-displaced, repeating pattern.

This distinction is extremely important to electrical system and equipment designers, but it is of no significant interest to end users of electric power in buildings or boats except as noted in the following section on 208V vs 240V services found in marinas and other facilities that provide shore power to boats.

🎓Additional description and drawings in AC Electricity Fundamentals, Part 1

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120V/208V AC Power;
120V/240V AC Power

Throughout North America, the AC power present in commercial buildings (apartment houses, condos, light commercial businesses), can be either 120V/240V or 120V/208V.  The AC power delivered to single family residential homes is 120V/240V. 120V/240V originates with Single Phase sources, so that the end user gets 120V/240V from the source; 120V/208V originates with Three Phase sources, so the end user gets 120V/208V.  For 120V-only appliance and attachment needs, both are identical and building occupants would be unaware of any difference at all.  There is only a significant difference for devices that might need 240V to operate correctly.

In larger buildings and multi-building complexes, Three Phase distribution is significantly less expensive to install and maintain than Single Phase distribution, so whichever type is installed in a facility is the facility owner’s capital cost choice.  Because of the voltage differences between the systems, the facility manager is generally the designated party responsible for replacing dwelling unit equipment (ranges, dryers, water heaters) that requires both 120V and either 208V or 240V voltages. Making equipment replacement the facility manager’s responsibility means the occupant of the dwelling unit doesn’t need to know or care about their electric service.

In boating, many marinas are served with Three Phase systems, which means it is inevitable that boaters will encounter 120V/208V power at some marinas, and 120V/240V power at other marinas.  Boat equipment that requires both 120V and 208V/240V voltages (especially refrigeration and HVAC equipment and water pumps) MUST BE DESIGNED to tolerate both 208V and 240V sources. The boat owner is responsible to ensure that equipment is properly rated, and to know what the supply voltage is to the vessel when hooking up to shore power.  This is an area that can impact boaters who install 120V/240V household appliances aboard boats.

🎓More information in “Why Do I Get 208VAC At Some Marinas?

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Electrically, “Ground” is just a physical connection to the earth.

  • The NEC defines “Ground” as: “The Earth.”
  • ABYC defines “Ground” as: “11.4.16 Ground – the potential of the earth’s surface. The boat’s ground is established by a conducting connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.”

“Grounding” is the act of making an electrical connection to the earth.  Because the earth is electrically conductive, the act of electrically connecting one of the current carrying conductors of an electric circuit to the earth “references” the voltage of that conductor to the voltage of the earth itself.  This also establishes the “polarity” (discussed further down) of the conductors in the distribution system.

The “conductivity” of the earth from place-to-place varies greatly, and the earth is not a particularly good conductor of electricity. It is not “good enough” to provide any kind of guarantee of electrical shock safety.  Yes, the act of referencing a building’s electrical system to earth is a safety mechanism of sorts, but it is not primarily about electric shock mitigation to protect living beings.  It is primarily about mitigating facility and equipment damage caused by spike voltages from static electricity, lightening and abnormal events that can occur in the commercial electric power grid.


So if the system’s ground connection isn’t about shock mitigation, then how is shock mitigation accomplished?

Throughout North America, AC systems in buildings and on boats have a conductor (green insulation on boats, bare copper in buildings) that is commonly called the “Safety Ground.”  It’s called the “Safety Ground,” or “Ground” by almost everyone EXCEPT the national standards-writing authorities, who call it an “Equipment Grounding Conductor” (EGC).  All metal objects (plumbing, furnace, air ducts, heat pumps, appliance cabinets, utility outlets, etc) installed in the building/boat are connected together by a network of EGCs, and that entire network is attached to the earth ground back at the power’s source.  If there is an electrical fault anywhere in the system which causes power to be wrongly applied to metal contact surfaces anywhere in that network of connected metal objects (dangerous shock “touch potential” to living beings), that EGC network provides a low resistance electrical path that will cause a circuit breaker to “trip,” thereby clearing the fault by removing electrical power from the faulting circuit.  “Bonding” or “Bonding conductors” are terms commonly applied to EGC conductors.

So the way we boaters might see “Grounding” discussions is context sensitive.  It could be in reference to either one of two different applications:

  • Connecting the system to the electrical potential of the earth, or
  • Interconnecting metal objects in a network to provide a low resistance safety path that will trip circuit breakers.

By far the most common way the term “ground” will be encountered by homeowners and boaters is the latter case; i.e., a reliable low resistance path that will clear a fault by tripping a circuit breaker.

Note: the terms “ground” and “grounding” as used in conversations of VHF and UHF radio transmitting equipment and antennae is a related but very different technical discussion.

🎓More information in Earthing and Grounding
🎓More information in Bonding System Design and Evaluation

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Circuit Breakers –

  • single pole
  • double pole
  • multi-pole

The purpose of any “Circuit Breaker” (fuse or mechanical switch) is to protect its attached wiring from electrical overload. 

Testing Standard UL 489 defines “circuit breaker” as “a device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overcurrent, without damage to itself when properly applied within its rating.

Wiring that is overloaded will become hot, and with a large or prolonged overload, can get hot enough to cause ignition of nearby flammable materials or its own insulation wrap.  It is urgently important to ensure this DOES NOT HAPPEN with wiring that is hidden inside the walls of a home, or on or behind liner panels of a boat, where the wiring is likely to be surrounded by, or in contact with, flammable materials.  In such situations, overheating can cause a fire danger long before a human occupant becomes aware of impending danger.

Circuit breakers (fuses) used for “overcurrent protection” (OCP) in customer premises electrical systems come in a wide range of standard trip ratings, 15A, 20A, 30A, 50A, 100A, 200A, etc. When the amount of current in the circuit exceeds the overcurrent rating of the circuit breaker, the circuit breaker “opens” to protect the circuit’s wiring.  Wire material, wire diameter, conductor bundling, insulation rating and circuit length are the determinate factors in specifying the amount of current a given wire conductor can carry safely in any given application.  That current carrying rating (“Ampacity”) and circuit breaker over-current protection (OCP) ratings MUST BE MATCHED to each other.  (Fuses also serve to protect wiring.)

Circuit breakers are made in “single pole,” “double pole” and “multi-pole” configurations. “Single pole” breakers in a 120V single phase system open only one conductor; normally the “hot,” or “energized” “current-carrying conductor” that feeds power to a circuit.  “Double pole” breakers in 120V single phase system open BOTH the “hot” conductor AND the conductor which returns power from the circuit.  “Double pole” breakers in a 120V/240V single phase system open the two “hot” conductors, but not the return conductor.

Multi-pole breakers are available for use in 3∅ systems and for certain special uses in single phase systems.  One such “special case” in a 120V/240V single phase system on a boat is the “Generator Transfer Switch,” which in a 120V/240V case, must transfer BOTH “hot” conductor(s) (L1 and L2) AND the return conductor.

It is always OK to use a circuit breaker rated at a smaller capacity than required by the rating of the conductors it protects.  It is NEVER OK to use a circuit breaker of larger capacity than required by the rating of the conductors it protects.

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Ground Fault;
Ground Fault Sensor

A “fault” is an undesirable, unwanted condition where one of two things occur.  Either:

  • electric current DOES NOT flow when it should, or
  • electric current flows someplace where it SHOULD NOT go.

Current that flows where is should not flow creates shock hazards that can be dangerous to people, pets and wildlife. All currents, especially including fault currents, flow ON ALL AVAILABLE PATHS.

Depending on the “conductivity” of the medium through which the fault current flows, “faults” can occur in a range of very small electrical currents to very large electrical currents. Fuses and circuit breakers protect against large electrical faults.  A large fault will cause a fuse or circuit breaker to “open” (“trip,” “blow”). However, small faults that are too small to cause an OCP device to “open” can exist in any electrical system.

Ground Faults” are a condition where electric current “escapes” from the normal electrical wiring of an electric circuit and finds its way back to its source via some unintended path (ex: see figure, above).  That path can be through soil, or it can be through water, or it can be through any other medium that conducts electricity, such as a wet, wooden or concrete decking or docks.

The NEC defines a “Ground Fault” as: “An unintended, electrically conductive connection between the ungrounded conductor of an electrical circuit and the normally non-current carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.”  (Note the word “earth,” meaning “soil” and/or “water.”)  The scope of the NEC definition includes all of the things that can and do happen on a boat, including current running through the “earth”.

In the case of a “Ground Fault,” which can be quite small but quite lethal, a different kind of circuit breaker from a standard over-current protection (OCP) breaker is needed to protect people, pets, wildlife and faulting equipment. In residential settings, “Ground Fault Circuit Interrupters” (GFCI) are installed throughout residential dwelling units.

The NEC defines “Ground-Fault Circuit Interrupter” as: “A device intended for the protection of personnel that functions to deenergize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device.

Testing standard UL489 defines “Circuit Breaker and Ground-Fault Circuit-Interrupter” as “a device that performs all normal circuit breaker functions and provides personnel protection by functioning to de-energize a circuit within an established period of time when a current to ground exceeds the values established for a Class A device as required ny the National Installation Codes in Annex B, Ref. No. 1.”

GFCI devices are intended to protect “personnel” (living beings) from shock hazards.  In commercial and industrial settings, “Equipment Protective Devices” (EPD) with a higher trip setting are intended to protect sensitive equipment from leakage hazards.  EPDs protect equipment rather than personnel, but in the context of a boat floating in electrically conductive water, they do ALSO protect personnel.  EPDs and GFCIs are a class of device called “Ground Fault Interrupters.”

The NEC defines “Ground-Fault Protection of Equipment” as “a system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.”  To electrical manufacturers, devices that meet this statement are called “Equipment Protective Devices,” but have many other names, including “Residual Current Device” (RCD), “Ground Fault Protector” (GFP), “Ground Fault Interrupter” (GFI), “Equipment Leakage Circuit Interrupter” (ELCI), and others.

Regardless of what they’re called, these devices all do the same thing; they monitor the amount of electric current going into the circuits they protect and compare that input quantity (current, in Amps) to the quantity of electric current coming back. The outbound and returning quantities must match within a very small tolerance (5 mA for GFCI personnel protection, 30 mA for EPD equipment protection). If the quantities do not match within the allowed tolerance, by definition, electricity is “leaking out” of the circuit somewhere it should not be going, and the “Ground Fault Sensor,” whatever its name, will “open” the electrical feed to protect the personnel or attached equipment from the risks presented by that leak condition.

In terms if physical construction, there are two types of both UL489 “circuit breakers” and UL 489 “Circuit Breakers and Ground-Fault Circuit-Interrupters.”

  • Fully self-contained mechanism in a single physical assembly, and
  • Shunt-Type units, made up of discrete, often remotely-separated, physical components.

We are all familiar with the self-contained breaker units, which are virtually the only style encountered in residential dwelling units and on boats.  Self-contained non-GFCI breakers have a toggle handle that manually sets the breaker “on” and “off.” These trip when over-current rating is exceeded. Self-contained breakers with GFCI or EPD also have a toggle handle that sets the circuit “on” and “off,” but also have a “Test Button” that activates the ground fault trip function and a “Reset” button that restores the device to normal operation.

Most boat owners are less familiar with “shunt-type” devices, but on boats, they can be used very effectively as “ELCI” devices, especially when space limitations are highly constrained.

Testing standard UL 489 defines “Shunt Trip Protector” as “a protector with a trip mechanism energized by a separate source of voltage or current that is derived from the main contact circuit or from an independent source.  The trip mechanism is either of the overcurrent type or of the voltage actuated type.”

For EPDs on boats, several manufacturers (Carling, Eaton, Siemens, others) make the self-contained EPD breakers.  But an alternative is a shunt-type arrangement made by Sensata Technologies and North Shore Safety Systems.  This shunt-type arrangement allows the sensor assembly to be located remotely from the breaker assemble.  This “solution” consists of the North Shore Safety System “PGFM” Series ELCI Remote Sensor and the Sensata Technologies AirPAX LEL Series circuit breaker.  The sensor mounts is a standard Marino/Hubbell 120V, 30A shore power inlet housing, and contains the “test” and “reset” buttons for the breaker.  The breaker can be mounted in any space available – if out-of-the-way – location.

🎓More information in AC Safety Tests For Boats
🎓More information in Ground Faults and Ground Fault Sensors
🎓More information in ELCI Primer
🎓More information in Difficult to Hire Troubleshooter

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Reverse Polarity

There are two ways in the overall world of electricity and electrical practice to think about “polarity.”

  1. As the relative positive or negative relationship of voltage at any given place in a circuit, and
  2. The manner of the physical layout of the wiring of a system, denoted for human convenience by the color of the conductor’s insulation.

In DC systems, there is a permanent positive and negative relationship of the voltages in a circuit.  The electro-physics of batteries, solar panels and fuel cells establishes permanent positive-voltage and negative-voltage terminals that never change with respect to one another.  This is case 1, mentioned above.

In AC systems, the voltage polarity changes in a rhythmically repeating sequence, reversing once in every AC voltage cycle (“frequency,” measured in “Hertz,” with voltage reversing direction 60 times per second in North American systems).  Since the “polarity” for the voltage relationship changes each cycle in AC systems, the term “polarity” refers to the way the wires in the AC system are laid out; this is case 2, mentioned above.

All electric circuits, whether DC or AC, require two wires to operate.  120V AC circuits are normally wired with three wires, but only “require” two wires to operate.  To operate, what’s “necessary” is one individual wire to transport power from the AC source to the power consuming device and a second individual wire that returns power from the power-consuming device back to the AC source.  In North America, AC systems are attached to ground, as discussed earlier.  At the “designated source” of AC electricity on any customer premises, one of the source wires is intentionally, deliberately connected to the earth; “grounded.”  That specific connection to the earth is what establishes the electrical “polarity” of the AC system.  In the lingo of the electrician, that wire is from then on known as the “grounded current-carrying conductor” of the system, nicknamed the “neutral” conductor.

The neutral conductor will always and only have “white” insulation (in the North American color scheme; the European color scheme found on some boats is different).  In that way, anyone, anywhere in North America working on a 120V/240V AC single phase system, will always be able to assume that the “white” wire is normally “grounded” at its source.  That also means that the working voltages of the not-grounded (ungrounded) conductors in the system (the “hot” conductors) are measured with respect to that “grounded neutral” conductor.  The colors used for the insulation of the various “hot” conductors communicates their roles in the circuit to humans working on the systems; that is, communicates their “polarity” as referenced to ground.  In North America, the colors “black” and “red” denote the “ungrounded current carrying conductors” (the “hot” conductors) that transfer current from the source to the circuit, and the color “white” denotes the “grounded neutral” conductor that returns current from the circuit to the source. “Green” denotes the “Equipment Grounding Conductor” network, or “Safety Ground.”

From the above, we can appreciate the arcane lingo used in the electrical standards.  The ABYC definition of a “Polarized AC System” as: “a system in which the grounded and ungrounded conductors are connected in the same relation to terminals or leads on devices in the circuit.”  The above is also the language of the National Electric Code (NEC) in North American single phase systems.  This standards requirement is “enforced” by the insulation colors of the AC conductor(s).

The NEC and ABYC standards require that “hot” wires and “neutral” wires ALWAYS appear in the same relative positions on plugs and receptacles. Ubiquitous 120V utility outlets and plugs have blades of different width to denote “hot” and “neutral.” Marine 30A twist lock receptacles and plugs have blades that are also of different widths and shapes which make it possible to connect them together IN ONLY ONE ORIENTATION.  Again, this is because the electric codes REQUIRE that “hot” and “neutral” are ALWAYS maintained IN THE SAME RELATIVE ELECTRICAL (and physical) RELATIONSHIP.

In parts of the wiring system where wires are connected together permanently, with splices or via ring terminals at busbars rather than disconnectable plugs – as internally within household appliances – the physical wiring relationship must also be maintained throughout. That is, “hot” wires always connected only to other “hot” wires and “neutral” wires always connected only to other “neutral” wires.  It is WRONG and DANGEROUS to connect a circuit’s “black” wire to an appliance’s “white” wire, and a circuit’s “white” wire to an appliance’s “black” wire.   Although the device will work correctly that way, it is a “reversed polarity” condition, and particularly dangerous to living beings.

Reversed polarity will not prevent the connected appliance from working, but reverse polarity elevates the metal cabinet of the appliance to a dangerous “touch potential” voltage compared to the case of the appliance that’s next to it, or nearby to it.  That is a potentially lethal combination, especially in galleys, laundry areas and equipment bays, where multiple separate appliances are in close proximity to one another. Consider, for example, a washing machine wired with a reversed connection that is placed adjacent to a dryer wired correctly.  A fault to the metallic case of the washer would put 120V on the washing machine’s metal cabinet. The family member tasked with doing the laundry, while transferring wet clothes from the washer to the dryer, could sustain an electric shock; a potentially lethal shock.

So we see that the fixed relationship of wiring in an AC system MUST be maintained throughout the system. To accomplish that goal across society, NEMA standards define the blade locations on all plug and receptacle devices made/sold/used in the United States.  Similar standards associations do the same in Canada. The blade locations and orientations on household 15A/20A plugs and receptacles are standardized; 30A dryer and 50A range plugs and receptacles are standardized; the blade locations on all 30A marine twistlock receptacles are standardized; the blade locations on all 50A marine twistlock receptacles are standardized.

Why do we do all this?  Because the intent is to maintain the relative position and location of “hot” and “return” conductors to ensure personnel safety from lethal electric shock.

Understanding the above, it’s clear that “Reverse Polarity” wiring ONLY occurs from a man-made installation mistake in wiring a customer premises system; that is, incorrectly wiring plugs and receptacles or appliance internal wiring connections. By definition, “Reverse Polarity” means the “hot” and “neutral” conductors are “reversed” in the plug/receptacle or in the appliance.  When an appliance is plugged into a mis-wired outlet, or an appliance is wired backwards, that becomes a shock hazard that can kill.

Fortunately for all of us, “Reverse Polarity” is a relatively rare situation, but it does happen, and it has been responsible for deaths.  It can happen in a home, on a dock or on a boat.  It happens on docks if/when maintenance personnel make a wiring mistake. When it happens on a 120V, 30A marina dock receptacle, the properly wired boat that plugs into that receptacle will have a potentially lethal “shock fault” between dock and boat. So when your crew member (self, spouse, guest, grandchild) steps off the boat with a properly grounded handrail and grabs the properly grounded metal dock railing, a very possibly lethal shock occurs.

On boats with 120V shore power cords, the ABYC requires that Reverse Polarity “Detector” circuits be installed.  These “detector circuits” sound an alarm or light a prominently displayed warning light, or both, to warn of a “Reverse Polarity” situation in dock wiring that reverses the polarity of circuits on the boat.   If this condition ever presents itself, boaters should immediately disconnect from that shore power source, and as a matter of personnel safety, NOT use it.  The presence of the fault should be immediately reported to facility management for URGENT investigation and correction.

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Adapters are physical devices that change the manufacturer-installed blade configurations of electrical cords while maintaining polarity consistency.  Adapters allow plugs of one NMEA blade configuration to be changed to use outlets of a different NMEA blade configuration.

For cruising boaters, there are many cases and conditions where adapters are necessary and can be extremely useful.  Many facilities have marine 20A outlets, which are very similar, but not identical to, Marine 30A outlets.  An “adapter” can be used to access the 20A pedestal power source from a 30A or 50A shore power cord.  Often, only a conventional 15A/20A utility outlet is available ON A DOCK, but with a proper “adapter,” can be accessed to charge batteries and keep a refrigerator running. “Adapters” make it possible for 30A shore power twistlock cords to access 50A pedestal outlets.  These cases often mean not being able to run everything aboard the boat at the same time, but can provide for essential, if minimum, electrical needs.

“Splitters” (also called “Wye Adapters”) are a form of electrical adapter.  There are two common types:

  1. One 240V/50A plug (fits into 50A pedestal outlet) to two 120V/30A outlets (fits 30A shore power cords to boat)
  2. Two 120V/30A plugs (fits into two 30A pedestal outlets) to a single 50A outlet (fits 50A shore power cord to boat)

Case one above is called a “splitter” (or “wye adapter”); case two is called a “reverse splitter” (or “Reverse Wye Adapter”)

The use of adapters in electrical systems ALWAYS REQUIRES VIGILANCE on the part of the user, because the adapter often creates a connection that is under-protected against electrical overload.  When an adapter is used to obtain power for an electric tool built for use on a 15A circuit from a 30A source circuit, conventional extension cords and the tool cord itself are not properly protected for overload. When an adapter or splitter is used to obtain power for a 30A circuit from a pedestal source that is protect at 50A, the 30A cord is not properly protected for overload.  So the use of adapters can create some net additional risk. It is ALWAYS better to use commercially-made adapters from reliable electrical equipment manufacturers than homemade adapters of lesser-known quality materials and assembly safety.

🎓More information in 50A Power From 30A Sources

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Isolation transformer
Polarization transformer

The ABYC defines an “Isolation transformer” as: “a transformer installed in the shore power supply circuit on a boat to electrically isolate the normally current carrying AC system conductors from the normally current carrying conductors of the shore power supply. NOTE: The shore power grounding conductor connection to the onboard grounding conductor is also isolated.

The ABYC defines a “Polarization transformer” as: “a transformer installed in the shore power supply circuit on a boat to electrically isolate the normally current carrying AC system conductors from the normally current carrying conductors of the shore power supply. NOTE: The shore power grounding conductor connection to the onboard grounding conductor is maintained.

The only difference between these definitions is the grounding connection of the boat. In one case, it’s isolated from the shore power grounding system, in the other, the connection to the shore power grounding system is fully maintained.

Shore power transformers can be very useful, but SHOULD NOT BE USED to mask “ground fault” issues on the boat. Transformers DO NOT “fix” wiring problems in the boat’s system, and if there are fire safety or shock safety issues on the boat, they will still be there after a shore power transformer is installed unless they are first found and fixed.

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Battery “Technologies;”
Battery “Types”

Beginning in 2015, and certainly thereafter, there have been two types of battery chemistry options available to boaters.  Each chemistry has multiple sub-types:

  1. Lead-Acid
    •          $ – Flooded Wet Cells
    •     $$$ – Absorbed Glass Mat (AGM)
    •     $$$ – Gel
    •   $$$$ – Carbon Foam
  2. Lithium Ion
    • $$$$$ – Lithium Iron Phosphate, LiFePO4, Lithium Ferrophosphate or LFP (all these terms are used in articles and advertising, and all mean the same thing in reference to the chemistry of the battery.)

Lead-Acid Batteries: Flooded Wet Cells, AGM, Gel and Carbon Foam are all Lead-Acid chemistry battery manufacturing techniques that all have identical electro-physics.  The individual names (wet cell, AGM, Gel, etc) reference the composition, suspension and retention techniques used in managing the cell’s electrolyte. Flooded wet cells have liquid, weak-concentration sulphuric acid electrolyte.  AGMs retain the sulphuric acid electrolyte in fiberglass “mats” sandwiched between lead plates.  Gel batteries have a gelatinized sulphuric acid electrolyte int the space between adjacent lead plates.  The details of their manufacture give them slightly different operational capabilities and characteristics, but all are fundamentally lead-acid chemistry batteries.

🎓More information in Battery Replacement
🎓More information in Batteries: Questions and Answers
🎓More information in Batteries: Charging and Care
🎓More information in Battery Bank: Separate vs Combined

Lithium Chemistry Batteries: Lithium Iron Phosphate (LFP) (LiFePO4) batteries are relatively new arrivals to the boating market.  Other lithium chemistry battery types have been around for years in miniature electronic equipment and commercial applications in different forms.  Lithium chemistry batteries are fundamentally different than lead-acid batteries, and require fundamentally different control and safety systems.

🎓More information in Lithium Chemistry Batteries On Boats
🎓More information in Lithium Batteries On Boats, Part 2/
🎓More information in Lithium Ion Batteries On Boats, 1Q23 Update/

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Battery “Service Types” 

Lead-Acid batteries are manufactured in two service application categories:

  1.          $ – Start Service
  2.     $$$ – Deep Cycle Service

“Start Service” batteries have thin lead plates, and are built in packages that contain many pairs of thin plates. These batteries do not store much total energy, but they can give up the energy they do store very quickly. They are very good for starter motor applications that require many, many hundreds of amperes to turn the motor and crank the engine, but they do not have the total storage capacity that allows that energy to be delivered for very long. The Battery Industry rates start service batteries in “Cranking Amps” (CA/CCA/MCA) and “Reserve Capacity.”

Compared to “Start Service” batteries, “Deep Cycle” batteries have pairs of lead plates that are relatively thicker, but fewer of them in their finished package. “Deep Cycle” batteries can store relatively much larger amounts of energy, but because the plates are thicker, they cannot give up their energy at as fast a rate as their “Start Service” cousins. “Deep Cycle” batteries are rated in “Ampere Hours” (aHr).

“Start Service” batteries will not be suitable for powering “house” and “inverter” loads, but a “Deep Cycle” battery of sufficient capacity will easily start a diesel engine. Batteries of the same type can be placed in parallel for long periods without degrading their service life, and many boaters leave separate battery banks switch-connected together “for the long haul” (although the defeats the supposed redundancy of having multiple banks). “Start Service” batteries are much less expensive than “Deep Cycle” batteries, and have long service lives. It’s a matter of personal preference and personal economics how individual boaters select and mix-‘n-match lead-acid battery use.

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Battery Monitor

A device that provides “overwatch” on the operational conditions and charge state (SOC) of a battery or battery bank. It is not, itself, a battery charger, but it does “watch over” the charging process in real time and does report the battery terminal voltage (V), the amount of current (A) entering (or leaving) the battery, and the remaining charge level (aHr) of the battery at the present time. Think of this as equivalent to the fuel gauge of a car. It allows the boater to know when to recharge and to avoid over-discharge.

EVERY BOAT – CERTAINLY EVERY CRUISING BOAT – SHOULD BE EQUIPPED WITH A BATTERY MONITOR. It is not sufficient to simply monitor battery terminal voltage. Battery terminal voltage is a late indication of charge and discharge condition, and it’s way too easy to over-discharge batteries, which impairs their potential service life.

There are two different types of battery monitors. One is a “Coulomb Counter” and the other is a “Conductance” technology. I personally prefer the former type. Installing a battery monitor is a one-time investment, and costs are similar. There are feature alternatives and marketing choices that determine the number of battery banks that can be monitored and the types of batteries that can be monitored. Monitoring lead-acid (today) and lithium batteries (in the future) is a feature consideration for a new buyer.

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Battery “Watering”

“Watering” is a term that applies ONLY to lead-acid Flooded Wet Cells.  All lead-acid batteries contain a liquid electrolyte, but only Flooded Wet Cells are built and function in a way that the electrolyte is accessible to owners and must be periodically refreshed. Lead-acid battery electrolyte is a dilute mixture of water and sulfuric acid.  As the battery discharges and recharges, the water/acid electrolyte mixture interacts chemically with the immersed lead plates to release (discharging) or absorb (recharging) electrons.  In the process, hydrogen gas can be given off.  Lost hydrogen amounts to lost water, and distilled (pure) water must be used to periodically replace what may have been lost in operation.

The process of replacing lost electrolyte volume is known as “watering the battery”.

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“Best practice”

As in all things in life, there are many ways to accomplish most tasks, and most electrical tasks. Many self-taught DIY electricians can “get things to work.” But, “things” on a boat are different than “things” in a residential dwelling unit, and even paid, professional electricians often do not understand marine design concepts and installation requirements.  “Doing things” on a boat as they are normally done in a single family residence creates MISTAKES and DANGEROUS CONDITIONS on boats, and often results in unexpected inconvenience for the unsuspecting boat owner.

There are “best practices” around material and equipment selection and “best practices” around user/owner operational procedures.  For materials “best practices,” ADHERING TO SAFETY CODES IS THE ABSOLUTE MINIMUM REQUIREMENT FOR WORK ON ANY BOAT.  The NEC and the ABYC Electrical standard, E-11,  are MINIMUM INSTALLATION AND PERFORMANCE STANDARDS.

Work done exactly to ABYC standards in an installation will be safe, but not necessarily “best practice.”  For example, single-pole breakers are allowed in 120V branch circuits, but double-pole breakers protect from “Reverse Polarity” and are more safe, so would be “best-practice.”   Use of Class T fuses in high current circuits vs other fuse types of lesser but allowable ratings would be “best practice.”  Inverters rated to UL458 over units not rated to UL458 would not only be a minimum requirement, but would be “best practice” in an electrical proposal.  Marine rated appliances made with double insulation, DC compressors and corrosion resistant internal components would be “best practice” compared to household appliances. Type THHN untinned copper wire with 90℃ insulation is allowed, but UL 1426, Type BC5W2 tinned Boat Cable with 105℃ insulation would be “best practice.” “Best-practice” is often virtually always “more expensive.”

“Best Practice” in operations is also important to service life of electrical equipment.  It’s always best to apply power to boat circuits “from source to load:” i.e., “outwards.”  In that model, we assume that EVERYTHING starts out “off.”  The sequence is, first, attach the shore power cords to the boat at both ends.  Then, turn the pedestal breaker “on,” next, turn the Main AC Panel breaker on the panel “on,” while observing volt onboard volt meter(s), and then, last, branch circuit breakers can be turned “on.”  When preparing for departure from a dock, do it all in reverse; “from the away end back into the source.”  Turn all branch circuits “off,” then turn the Main AC Panel breaker “off,” and finally turn the Pedestal breaker “off.”  DO NOT disconnect the shore power cord until power is “off” at the pedestal.  Is all this “necessary?”  “Necessary” no; but it extends the service life of all of the switching devices in the entire circuit, and minimizes the chances of sparks and arcs, so it is “best practice.”.

People take shortcuts.  Homemade electrical adapters are never “best practice.”  Homemade stuff is virtually always made from components that come from big box or hardware stores.  Those components ARE NOT RATED for marine use, and are not rated to be installed in high-UV, high dust and dirt or wet locations. Most of them do not meet the requirements for marine use at all, and are certainly not “best-practice.”

Always employ professionals who are fully trained for boat and marine installations. Whenever possible, make “best-practice” choices.

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“Alternator” being a word derived from “Alternating Current,” an “alternator” is a device that creates AC electricity.    The term “Alternator” has two significant and very different meanings to boaters, so context is important.

The most familiar use of the word alternators refers to a device mounted on gasoline and diesel engines as a DC device that charge batteries.  Nothing there to suggest AC, right?  Well, the alternator on engines, as a machine, is actually an AC device (a 3∅ AC device at that), but inside its case, it uses a “diode” circuit to convert the generated AC into a DC output suitable to charging batteries.  So the first and most common use of the term is to refer to the belt-driven machine on the engine that charges batteries.

Now since the word “alternator” derives from the term “alternating current,” consider the boat’s AC generator.  The generator has two major internal components: one, it’s drive motor and two, the alternator, which is the mechanical part that creates AC electricity.  The drive motor is usually a gasoline or diesel engine, but the electrical end is called the “alternator.”  That alternator, like its on-engine cousin, is a repairable, replaceable electrical device component.

So context is important.  The device to which the word “alternator” refers depends on the topic under discussion.  The word “alternator” is used correctly in either context.

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Voltage Regulator

An electronic device, typically associated with an on-engine alternator of 12V or 24V. A voltage regulator can be “internal” to the alternator it controls or can be a discrete, “externally-mounted” device. The Voltage Regulator adjusts the output voltage of an engine-mounted alternator in a manner that is pre-programmed to properly charge batteries of the various different types discussed above. It works by adjusting the density (strength) of the magnetic field in an AC Generator’s alternator or a DC engine-driven alternator.

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Field Coil

The rotating (spinning) part of one design of an AC generator’s alternator or automotive DC “alternator” machine. This coil can be replaced by a DC permanent magnet in small alternators, but is typically an electromagnet. On one machine design, the field coil spins; in another machine design, the field coil is stationary.

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Stator Coil

The fixed (non-rotating) coils in one machine design of AC generator alternator, from which sine waves of AC electrical energy emerge. See “Armature,” immediately following.

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Analogous to “Stator,” above. The power producing component of an AC generator; the rotating part of a DC generator.

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Zener Diode
Surge Suppressing Device

A “Diode” is an electronic component device that conducts electricity in only one direction.  Diodes are widely used in circuits that convert AC into DC electricity and in anti-corrosion circuits.  This latter use is because when a diode does conduct, it creates a known, fixed voltage drop across it’s input and output terminals.  That voltage is very small, but so are the natural corrosion potential voltages of metals.  So the internal diode junction will block small currents that cause undesirable corrosion in underwater metals, but it will pass larger currents that are desirable or necessary to the operation of the circuit of which they are a part.

A “Zener Diode” is a special kind of diode.  Zener diodes will conduct, but not until a specific voltage threshold is reached.   So think about an outlet strip used to power your computer, printer, monitor, router, TV, and stereo equipment.  If there is a voltage surge on the line powering that equipment, all of that expensive electronics can be damaged.  But if that line is “clamped” with a surge suppressor, then the spike energy will be diverted to ground through the Zener diode and the attached equipment saved from damage.  That is the purpose of Zener Diodes and other “avalanche-type” diodes.

As a “best practice,” Surge Suppressor diodes should be connected to the DC output of engine alternators.  If the output line of an engine alternator is accidentally disconnected when the device is working, the magnetic field in the device at that time will collapse, and create a very large electrical spike.  The spike can damage the alternator diodes and also the electronics of devices attached to the DC buss of the boat.  A Surge Suppressing Device can mitigate, maybe prevent, serious damage.  It just sits there, maybe for years, maybe forever, until it is needed. just waiting to protect against a serious and expensive event that may never happen.

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Voltage Drop

“Voltage Drop” is the amount of voltage that will be lost across a length of conductor when the conductor is passing electrical current. All electrical wiring has the physical property of “electrical resistance.”  Small-diameter wire has more resistance-per-foot of length than a large-diameter wire of the same material. Aluminum has more resistance-per-foot than copper.

When installing electrical equipment, it is extremely important to match cable material and diameter to the requirements of the attached device. Included in that calculation is the length of the round-trip wiring from power source to appliance AND back. It is always OK to use wire that is of larger diameter than needed.  It is NEVER OK to use wire than is of smaller diameter than needed. For both DC and AC circuits on boats, ONLY copper wire should be used; preferably, wire/cable rated as Type BC5W2 Boat Cable.

“Voltage Drop” is not usually a “primary complaint.” “Voltage Drop” is usually a secondary effect or finding arising from analysis of some other equipment performance or reliability symptom. Often, this will be an issue that has emerged over time in the operation of a particular appliance or device, and has reached the point of being an annoying operational problem; ex:

  • a slow running or stalling water pump, circulator pump, thruster motor, windlass motor or davit crane,
  • autopilot dropping out
  • gauges reporting a lower voltage at one helm station than is reported at another helm station,
  • random alarms from electronic equipment, for no apparent reason,
  • lights dimming or flickering when other equipment cycles “on.”

“Voltage drop” affects the electrical performance of equipment.  “Voltage drop” can be an issue in all electrical circuits, both AC and DC. “Voltage drop” is ALWAYS associated with either:

  1. the quality (electrical integrity) of electrical connections between power source and attached appliance, or
  2. the gauge (diameter) of the wiring between the power source and the attached appliance.

“Voltage Drop” is often caused by poorly made electrical splices, screws or nuts/bolts that are not tight, or corrosion of a splice connection. It can also be associated with circuit breakers having weak internal springs or cycle-life wear-and-tear to the internal electrical contacts. Often, there will be signs of overheating at the site of connections that are the cause of voltage drop.

The following chart shows significant voltage drop in a 30A shore power cord as appliances cycled “on” and “off,” and illustrates that voltage drop increases (voltage loss increases, actual voltage present decreases) as power demand increases.  The red line is shore power voltage starting at the pedestal at ~118VAC at “no load,” and dropping to ~109V under load of several appliances.  The blue line is power, measured in kW.  This measurement shows total power consumption across two, nominally 120V, shore power circuits (so includes heat pump power consumption), but the voltage is measured at the power panel for the boat’s house AC loads.  Since the heat pump was on the other 120V shore power cord, the voltage on the house circuit was essentially unaffected.  The timeframe of this measurement is ~1/2 hour.  Voltage drop in this example is significant and excessive.

Special attention and consideration should be given to the requirements of bilge pumps. Bilge pumps are frequently small motors that do not draw greatly excess current when mechanically stalled.  Bilge pumps are particularly susceptible to debris floating in the bilge. Debris can block the impeller of a bilge pump, effectively creating a stalled rotor condition. ABYC requires that motors be fused at a level that will prevent overheating of the pump in a stalled rotor condition in cases where the stall condition lasts for seven hours or longer. Per ABYC: Motors and motor operated equipment, except for engine cranking motors, shall be protected internally at the equipment, or by overcurrent protection devices suitable for motor current. The protection provided shall preclude a fire hazard if the circuit, as installed, is energized for seven hours under any conditions of overload, including locked rotor.  So if the manufacturer of a bilge pump calls for a 10A fuse, installers MUST NOT simply use larger diameter supply wire to the pump and protect that circuit at too high a current.  True, there won’t be a voltage drop issue, but a stalled motor fused too high CREATES a fire hazard.  I wonder, do you suppose a surveyor would catch such an issue on a boat?

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Amps vs Amp Hours

Amp Hours and Amps ARE NOT the same thing.  These two terms are very frequently confused and used incorrectly.

“Amp Hours” (abbr: “aHr”) is a measure of quantities of energy storage capacity, or a statement of energy requirements.  That can be either the storage capacity of a battery or battery bank (ex: a fuel tank able to hold some fixed number of quarts or liters or gallons of fuel; its “capacity”) or the amount of energy needed to support the operation of an appliance over a period of operation (ex: it takes 500 gal of diesel to move Sanctuary from Baltimore to Punta Gorda; it takes 250 Amp Hours of 12V DC to power Sanctuary overnight at anchor).  This electrical term relates to quantities of energy over time.

When sizing batteries or battery banks, the planning assumption is that lead-acid batteries should not be discharged more than 50% for maximum service life. This is called “the 50% rule.” The 50% rule means that lead-acid battery banks need to be sized large enough to have TWICE the energy rating as that required per period of time in use on the boat. Sanctuary needed 300 aHr for a winter overnight at anchor for refrigeration, space lighting, TV watching, computer and network use overnight, anchor light overnight, and making coffee in the morning.  We had a battery bank of 690 aHr capacity, which ensured we did not exceed 50% state-of-discharge in routine operation. Sanctuary had only one battery bank, and it was also used to start our propulsion engine.  Engine starter motors need lots of amps, but take very little amp hour capacity for starting, and we NEVER had a problem starting our engine in the morning with out single battery bank.

“Amps” (abbr. “A”) is the measurement quantity of electrons moving in an electric circuit at a particular moment in time; i.e., the rate at which energy is being consumed in the moment (ex: currently consuming 4 gallons per hour, against an operating range of 2 gallons per hour at idle, 6.0 gallons-per-hour at “hull speed” and 32 gallons-per-hour at “planing speed”; or, drawing 15A because the water heater is “on”.

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Ampere Interrupt Capacity

“Ampere Interrupt Capacity” is the amount of current that a switching type of device (circuit breaker, fuse, switch, relay, solenoid) can safely stop during overload conditions.

All electrical switching devices of all sizes have a rating for the amount of current that they can safely switch “on” and “off.” Actually, it’s the amount of current they can switch “off” when they are “opened;” i.e., “turned off.”

Whenever an electric current is interrupted by a switching device, and particularly current flowing to any kind of electric motor or transformer containing coiled windings, there is an electric arc at the contacts as the contacts part (open up). These arcs produce extremely high spot temperatures and erode the conductive metal contact surfaces of switches and relays. To be effective in a circuit over time, switching devices must be able to withstand the spot temperatures that occur while the contacts are parting and until they are fully open. Equally important, the gap of the open contacts must be able to actually stop the flow of current. Arcs are composed of a “plasma” of flowing electrons. If the switch contacts don’t open far enough, the arc will not be quelled, and the electric current will continue to flow. In fact, we are all familiar with this phenomena; it called “welding.” In any kind of application OTHER THAN welding, that arc will melt the switch and start a fire in nearby combustible materials.

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Watt Hours

Like “Amp Hours,” “Watt Hours” is a measure of energy; specifically, the energy consumed by equipment.  “Watt Hours” refers to the amount of energy consumed over a period of one hour. AC electrical equipment is rated in Watts.

Boaters may find it helpful to convert “Watt Hours” into “Amp Hours” when estimating the amount of stored battery energy an item of 120V AC equipment may demand from the house battery bank when running via an inverter.  The conversion calculation is: (aHrDC) = (WhAC) / (VAC/VDC).

Not including inverter efficiency, a 100W/120V lightbulb would take 100W / (120VAC / 12VDC) = 100W / 10 = 10 aHr from the batteries per hour of operation.  Running for 4 hours, that’s 40 aHr just for that one lightbulb.  (Makes the case for LEDs, eh?!)  A 50W electric blanket running for 8 hours would need (50W / (120VAC / 12VDC)) * 8hr = (50 / 10) * 8 = 40 aHr for overnight sleeping comfort on a 45℉ overnight.

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Magnetic Field Collapse

Managing the collapse of a magnetic field is an important design consideration or any magnetically operated electrical device (motor, generator, transformer, relay, solenoid, etc), and many solid state devices.  A collapsing magnetic field can create a very large voltage spike that can result in an electrical arc. A spike can easily damage electrical equipment, and is a significant safety consideration for electrical maintenance and service technicians.

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Solar controller

An electronic device that adapts the “raw voltage” produced by solar panels to voltages that are compatible with 12V or 24V boat battery systems.  The “raw voltage” that comes from a bank of solar panels can vary to voltages upwards of 60V DC as the sun moves through its daily arc through the summer sky.  The solar controller adapts that “raw voltage” to 12V or 24V for boat DC system use.

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The term “Relay” is a generic term that applies to a class of electrical switching devices having a wide range of physical sizes and electrical capacity ratings.  They range from tiny control circuits mounted on printed circuit boards and installed inside electronic equipment all the way up to multi-megawatt grid-switching applications in power-grid transformer substations.

Specifically to typical cruising boats, “relays” and “solenoids” are used to control both high current draw, high power appliances and low current draw devices like those found in lighting circuits.

“Solenoids” are widely used as a technique to enable “user friendly,” convenient, low space-occupying switches to control very high powered appliance circuits that require large gauge (large diameter) cable between battery and appliance.  Windlass and thruster motors are usually controlled by solenoids.   Solenoids can be implemented with multiple activation points, so they can control high power devices from several locations that are all relatively distant from the heavy duty device itself. Solenoid activation (control) circuits do not require large gauge (diameter) cabling, so are much easier and more cost effective to install.  The most common example of these applications is the “engine starter motor.”  Starter motors draw many hundreds of amps from battery banks, and require very large feeder cables. In circuits of that kind, “voltage drop” along the feeder cable length is always an engineering constraint, so reducing the length of power cables is a very high priority to reliable device performance. “Solenoid devices” enable that capability.

Relays and Solenoids have a magnetic coil that operates the contacts for the high-current (controlled) circuit. Starter solenoids are usually mounted directly on the starter motor. Only small gauge wiring is needed to operate the starter solenoid. So its typical for the ignition key to pick a relay, and for that relay to, in turn, to pick the starter solenoid to start the engine. That minimizes cable length losses, minimizes materials costs and minimizes the size of switches that occupy space on control panels and at operator console stations.

Because of relays, it’s also easy for switches at multiple locations to activate a single heavy duty Solenoid. This is a technique that allows operation of larger motor-operated appliances from multiple operator locations.  Examples of solenoids used for high power equipment include thrusters, anchor windlasses, davit hoist motors and power capstans. Solenoids are also widely used to parallel battery banks, and to provide safety disconnect functions for starter motors. and inverter/chargers.

As mentioned above in the “Ampere Interrupt Capacity” section, whenever an electric current is interrupted by a switch, and particularly to any kind of electric motor or transformer, there is an electric arc at the contacts as the contacts part (open up). These arcs cause symptoms of overheating and voltage drop across the relay, necessitating replacement.  In similar manner, circuit breakers used as “on”/”off” switches in power distribution circuits have a “nominal” 15 – 20 year service life. Use of relays as a control circuit technique allows for physically larger metallic contacts, which lengthens service life and allows for modestly-sized, more delicate user controls. The modern car has probably 30-50 small relays that control all functions of the car, from windshield wipers to horns to marker and turn signal lights to tank sensors to windshield washer levels. The relays are inexpensive electrical commodities, and produce service lives in the 15 to 25 year range.

Some devices on cars and boats have relays/solenoids that are actually incorporated into them. One typical example is the fuel solenoid, which opens a mechanical valve to allow fuel to flow and closes the valve to stop fuel from flowing. The valve is moved by the magnetic field of the solenoid.  Solenoid-operated valves that require power to open them are actually fairly sophisticated devices.  A fuel solenoid that requires DC power to open it has to be able to operate reliably while the starter motor is cranking, a period of time when battery terminal voltage is lowered, and perhaps cranking for a prolonged period of time.  Battery terminal voltage falls during engine cranking, typically from 12.5V to 10.5V, sometimes less, and the fuel solenoid must operate with that lower available system voltage.  So the solenoid is built with two coils; one to “pick” it and another to “hold” it open.  The “pick” coil will close the solenoid, but takes a great deal of power to do so, and produces a lot of heat.  The “hold” coil will kick in after the solenoid valve has shuttled into the open position, and “hold” the valve open.  There is an internal microswitch that energizes the “hold” coil and disconnects the “pick” coil when the valve is fully open.  Fuel solenoids can be found in models that require 12V/24V DC power to keep them open and in models that require 12V/24V power to close them.

Relays/solenoids provide the electrical system designer with a great variety of ways to implement electrical system functional flexibility for boat owners.

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Galvanic isolator

A passive electronic device that is installed in the boat’s main AC Safety Ground Conductor where the ground conductor enters the boat at the shore power inlet. (Note: this does not apply to boats fit with an isolation transformer.)  The Galvanic Isolator contains a diode pack that blocks small DC voltages and DC currents formed by the interaction of underwater metals of the boat in salt water. The purpose of the device is to prevent metal corrosion by “galvanic” currents.

Galvanic Isolators are after-market add-on components for boats.  GIs have been available for many years.  In that time, they have gone through several technical design iterations (generations).  In 2021, the ABYC-required version is called “Fail Safe.” If buying for a new or replacement installation, make sure to only buy the “Fail Safe” model.  This device is intended to maintain the integrity of the ground connection in cases where spikes in the shore power system may have damaged the diode pack.  The “Fail Safe” rated devices are “more likely” to survive a nearby lightening strike. Nothing will survive a direct hit lightening strike.

🎓More information in Galvanic Isolators

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  Sacrificial Anodes

In Chemistry, “zinc” is one of the earth’s fundamental metallic elements; chemical symbol “Zn.”  The word “Zincs” is also generic “lingo” that refers to a corrosion control component more properly called a “Sacrificial Anode.”  Zinc is one metal element that is used as a “sacrificial anode.” Zinc is most appropriate for use in Salt and brackish waters. Aluminum is used as a sacrificial anode material and it’s suited to all waters. Magnesium is used as sacrificial anode material in fresh waters.

Sacrificial anodes protect more valuable metals by participating in self-destructive electro-chemical reactions so that the more valuable underwater metals of the boat do not deteriorate.

🎓More information in Metal Corrosion and Zinc Wasting

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A mechanical device that converts mechanical energy into electrical energy.  Usually on boats, a device with a fossil fuel engine driving a rotating electrical generator.  The generating end can be either a device that produces AC (single phase or three phase) or a device that produces DC.

AC generators must spin at speeds that result in 60Hz AC output (two-pole machines spin at 3600 RPM, four-pole machines spin at 1800 RPM).  Generally, the slower the machine spins, the quieter and less vibration is likely.  Typical output voltages from boat AC generators are 120V/240V, which is directly usable by conventional 120V/240V appliances.  AC generators are typically less efficient than their DC cousins because they must spin at that fixed high rotational speed regardless of load. That requirement also leads to the mechanical complexity of the speed governor.  Safe AC generator installations on boats require sophisticated electrical switching techniques.

DC generators are generally used as battery chargers.  Used in that way, Inverters are used to create 120V/240V AC for use on the boat. DC machines are generally more efficient because they do not have the high constant rotational speed requirement or the need for precise RPM regulation.  DC generators are often not made in the large output (kW) sizes found in AC generators because in battery charging applications, that is not necessary.

Both AC and DC generators are viable for powering boat electricity needs.

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An electronic device that converts DC to AC; commonly, 12V or 24V DC from battery banks to 120V or 120V/240V AC electricity.  Inverters allow boaters to use 120V home appliances on boats.  Most typically, refrigerators, freezers, ice makers, mixers, microwaves, crock pots, toasters, TVs, SOHO computer equipment and iGadgets.  Inverters are available in a wide range of capacities to support 120V/240V appliances.  These devices usually require large capacity battery banks.

🎓More information in Inverters On Boats

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A propulsion device that causes a boat to move in the water. A single bow or stern thruster causes the boat to pivot around the natural center-of-rotation of the hull. Pairs of thrusters (bow and stern) cause the boat to move “sideways” in the water. Thrusters are used as an aid to maneuvering in close quarters.

Thrusters can be electrical, powered from batteries, or hydraulic, powered by the propulsion engine or an auxiliary engine. Electric thrusters are made with reversible DC motors having the mechanical style and electrical demand profile of engine starter motors. To produce needed thrust, they require large amounts of electric current, large diameter feed cables, and short cable runs. An electric solenoid is used to reverse the polarity of the DC power applied thruster in order to reverse its direction of rotation.

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Terminal Block / Bussbar

A component in an electrical distribution system, either AC or DC.  Bussbars are specifically designed and used as a means of connecting conductors together.  Buss bars provide great flexibility for electrical system designers to achieve cost effective and safe electrical connections in boat electrical distribution systems.

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“Dielectric Grease”

A “dielectric” is a substance or material that DOES NOT conduct electricity; an “insulator.”

Dielectric grease is NON-CONDUCTIVE.  As a liquid, it can be sprayed (Corrosion-X, Boeshield T-9) directly into 120V/240V AC receptacles.  As a grease, it can be applied into terminal barrels and used to coat conductor ends before assembly.

When coated plug blades are inserted into a receptacle, they make mechanical contact.  As they are inserted, the dielectric is “wiped away” at the points of contact, creating a clean electrical connection. The dielectric then surrounds the area of mechanical contact, and prevents moisture and oxygen-bearing air from getting to the mechanical connection and causing oxidation (metal corrosion).

I use and recommend Dow-Corning “High Vacuum Grease” (developed for NASA for Space Shuttle electrical connections) for assembling 2/0 and 4/0 wire-to-terminal connections. I pack the barrel of the terminal connector with dielectric grease, and coat the wire with grease.  The electrical connection is formed at the crimp, where the conductors have been compressed mechanically together.  At that joint, all of the dielectric will have been pushed away and clear of the metal contact areas of the mechanical compression joint.  The crimped assembly will consist of a water tight, air tight, sealed chamber that will never corrode.  There will be waste in the form of grease “squeeze out.”

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Bulk / Absorb / Float

The names of the three stages of charging for lead-acid batteries, based on the most energy efficient manner of charging lead-acid cells and the best manner of charging for maximizing in-service life of the batteries.

A lead-acid battery that is significantly discharged will take charge quite rapidly. During that period, the charger operates in “Bulk” mode to give the battery the largest amount of charge (in Amps) that the battery can accept.  As the battery charges, the rate at which it can accept charge goes down and the terminal voltage rises. At about 85% State-Of-Charge (SOC), the battery electrolyte will begin to break down and produce hydrogen gas. At that point, the charger changes into “Absorb” mode. Absorb mode slows the rate of current flow to eliminate the hydrogen off-gassing, and allows the battery to become more completely charged. Finally, when the rate of current flow is less that 2% of the battery’s charge acceptance rate, the charger will drop into “Float” Mode. Float Mode is designed to offset the effects of “Self-Discharge.”

🎓More information in Batteries: Charging and Care

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Boat monitor

An electronic device that has the capability to monitor the state of various boat systems and report real-time status back to absentee owners. The unit I have installed on Sanctuary monitors for the presence of Shore Power, monitors Salon Temperature, monitors Bilge Pump Cycles and High Bilge Warning Alarm, and monitors Refrigerator, Freezer and Engine Room Temperatures.  The device contains a GPS, and so it knows where it is and reports to me when the boat moves more than about 100 feet (“Geofence;” programmable option).  The device is web-based.  It reports via cellular connection to a central server.  When there is an alarm or change-of-state that I have told the system I want to be notified about, I get a text from the host manufacturer’s server farm with the information I requested.  I also get a daily summary email from the system if I ask for that.

The device has “saved our bacon” several times.  When traveling away from the boat, we know if shore power has been lost, and I can call to have the facility investigate and take corrective action.  I know if important temperatures are out-of-spec. Under way, I can see bilge pump cycles as they occur and I can monitor bilge pump run frequency.  This is a huge “Peace of mind” item.

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Wire Gauge

A term that refers to the diameter of wire conductors. There are two “international” scales with which boaters come into contact:

  1. American Wire Gauge (AWG)
  2. Society of Automotive Engineers (SAE)

Both scales specify the safe amount of electric current that diameter wire can carry, called “Ampacity.” Both scales are widely used in boating.  SAE wire sizes are about 10% smaller than AWG wire sizes, so it’s important to be certain to know which scale is being cited, and adjust if larger wire is needed.  Wire “ampacity” ratings are also affected by the temperature rating of the insulation protecting the wire.

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Voltage Sensing Relay (VSR)
Automatic Charging Relay (ACR)

Terms used to describe electrical devices, each different types of “Solenoids.” Combiners interconnect two circuits in parallel. Often, they connect battery banks. In battery banks, in automatic operation, when one bank is being charged, at a certain point in the charging cycle, a second bank is added in by the “combiner” to be charged by the same battery charging equipment. These devices can also be set up to allow the boater to manually interconnect the battery banks. Depending on the manufacturer’s ratings of the specific device, the rated amperage capacity may be limited.

These devices are “dumb” devices that connect two circuits and allow current to flow in either direction.  The current flow is uncontrolled and dependent on conditions in other parts or the electrical system.

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DC-to-DC converter

An electrical device; can be used as a “combiner” in a “master”/”slave” relationship, can be used as a regulated DC power supply to change voltage (up from 12V to 24V, or down from 24V to 12V, or other combinations), or can be used simply as a regulated DC power supply (for instruments, for example) that is independent of the regulation quality of the source of its input power. DC-to-DC converters are “smart” devices (programmable, automatic). When use to transfer power, they transfer that power in only one direction (from a “master” source to a “slave” circuit or component).

DC-to-DC converters are preferred over VSRs for interconnecting battery banks of different chemistries.

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Heat Pump
Reverse Cycle

Terms that relate to HVAC equipment (Heating, Ventilation and Air Conditioning).

“Heat Pumps” transfer heat from one type medium to another; on boats, back and forth from water-to-air or air-to-water.  The heat transfer occurs through a chemical refrigerant in a heat exchanger.  The chemical refrigerant is forced by a compressor to change state from gas-to-liquid or liquid-to-gas.  the change of state is what determines the direction of flow of the heat that is transferred.  Heat pumps have two heat exchangers:

  • Condenser
  • Evaporator

In “Cooling” mode (air conditioning mode), heat pumps transfer heat from warm air in living spaces to circulating raw water. The heat that is transferred into the circulating raw water is “dumped” into the boat basin.  In “Heating” mode, heat is transferred from the basin water to the air in interior living spaces.  When they are operating in space heating mode, they are in their “Reverse Cycle” mode.  “Reverse Cycle” units contain valves that change operating mode by changing the direction of circulation of the refrigerant in the unit.  This also reverses the functional relationship of the “coils” in the unit between the “cooling” cycle and the “heating” cycle.  In the “cooling” cycle, air is cooled at the coil functioning as the refrigerant’s “evaporator.”  In the “heating” cycle, the air is heated by the same physical coil, but this time functioning as the refrigerant’s “condenser.”

Not all marine air conditioning equipment is designed to operate as described above; i.e., in “reverse cycle” mode. Some boat HVAC units have single-mode refrigerant systems that perform the “air conditioning” cooling function only, and are fit with electric heating elements that perform the “heating” function. The electric heating elements perform the function of an “electric furnace.” Installation ventilation requirements for these units can differ from units that are operated by refrigerant and compressors only. This is simply a design alternative.

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Self-discharge is a phenomena mostly of lead-acid batteries. Self-discharge is what happens as batteries stand unused (usually disconnected) “on the shelf,” out of service. Electrons and ions migrate within the battery and the result is loss of charge. This process is affected by temperature, much increased as temperature increases. The process occurs at a greater rate with flooded wet cells, less so for AGM and Gel, and least for Carbon Foam.

Battery chargers must be sized for the bank they will support. A medium-sized bank – say 700 amp hours – will have a larger self-discharge load than a smaller bank, and a 1200 amp hour bank of the same battery type will have greater self-discharge.

Self-discharge rates range from 3.5%/month for wet cells at ambient temps to lesser numbers for other technologies. Curves of self-discharge characteristics are available on the Internet.

Chargers use the “Float” stage of battery charging to manage self-discharge to keep lead-acid cells at full charge. Float-charging currents are 1.5% to 2% of Charge Acceptance Rates (CAR), so for flooded wet cells, maybe 2A for an 8D battery of 220 amp hours. Three 8Ds in parallel would be 660 amp hours, and “Float” current would range upwards from 6.5A, ±. Self-discharge percentage increases with age and battery condition, so could easily exceed 15A for a moderately sized bank that’s been in service 3 – 4 years.

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Charge Acceptance Rate (CAR)

Term that describes the rate at which batteries can accept charge from a charging source. Usually described at the ratio of battery capacity, in “Amp Hours,” to the nominal rate of charge acceptance current, in “Amps.” Batteries of different construction technique accept charge from charging sources at different rates. Nominally, flooded wet cells can accept a charge of 25% of the rated Amp Hour Capacity of the battery, in Amps. For example, a 100 Amp Hour wet cell can accept ~ 25A from the charger. Nominally, AGM and Gel cells can accept 40% of the rated Amp Hour capacity of the battery in Amps, so a 100 Amp Hour AGM could accept ~ 40A.

The discussion of Charge Acceptance Rate applies to partially discharged batteries. The maximum Charge Acceptance Rate is generally considered to occur when the battery is substantially discharged; i.e., 50% or less, State-of-Charge. As the battery State-of-Charge increases, the rate of charge acceptance decreases. Understanding Charge Acceptance Rate helps select battery charging equipment for best compatibility for the capacity of the batteries being charged.

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Power Quality

This term refers to the AC sinusoidal voltage waveform in an AC system being both 1) free of distortion at 2) within the rated voltage and frequency (60 Hz throughout North America) specified for the system.

The following are common forms of voltage and frequency events that lead to customer equipment failures:

The following are forms of voltage waveform distortion “anomalies,” which can be present one-at-a-time or in random combination of several-at-a-time.  These waveform anomalies can cause customer equipment to fail to operate correctly or to fail to operate at all.

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Switch Mode Power Supply (SMPS)

A switch mode power supply is a power “converter” that utilizes solid state switching components to continuously switch internal circuit power “on” and “off” at high frequency. The switched power is fed to energy storage devices (capacitors and inductors) to supply power during the non-conduction state of the switching device.

SMPS power supplies have efficiencies approaching 90%; much “better” than traditional linear mode power supplies which run around 40%-45%.  SMPS supplies are small in size and have become widely used in computers and other sensitive electronic equipment.

The basic SMPS design variations are categorized based on supply input and output voltage. The four principle groups are:

  • AC to DC – DC power supply
  • DC to DC – Converter
  • DC to AC – Inverter
  • AC to AC – Cyclo-converter or “frequency changer.”

The main components of an SMPS are:

  • Input rectifier and filter
  • Inverter consisting of a high frequency signal and switching devices
  • Power transformer
  • Output rectifier
  • Feedback system and circuit control

Following is a block diagram of the main components of an SMPS. The red circles highlight the voltage waveforms that pass from block to block.  The pulses fed to the transformer are created by a “Pulse Width Modulation” (PWM) circuit in the bottom, center block.  

Advantages of SMPS:

  • More compact and use smaller transformers. Smaller size and lighter weight is an advantage for electronic device with limited space
  • Regulated and reliable outputs regardless of variations in input supply voltage
  • High efficiency: 68% to 90%
  • The transformer-isolated supplies have stable outputs independent of the input supply voltage
  • High power density

Disadvantages of SMPS:

  • Generates EMI and electrical noise (waveform distortion and harmonics)
  • Complex design
  • More components → expensive compared to linear supplies

🎓More technical information/description: SMPS

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A generic term for all types of disconnecting devices (fuses, circuit breakers, switches, power panels, relays, solenoids). This term is used across the electrical power industry from generating station to transformer yard to residential location.  It usually applies to large disconnect stations, but is not exclusive to that use.

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Conduit (EMT)
Spiral Wrap
Split Wrap

Different terms for types of protective wire containment in a building or boat “through which”/”in which” wiring is run in order to penetrate partitions or bulkheads, run through open spaces or otherwise achieve access to distant locations. The purpose of these techniques is to protect wiring from accidental physical damage.

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Solid Core

Wire in the form of a single, solid, metallic conductor of a particular metal and a specific AWG or SAE diameter; solid copper wire in useful AWG diameters is routinely used in residential and commercial buildings.  Solid core wire is very susceptible to metal fatigue due to bending, flexing and vibration, so is NEVER, NEVER, NEVER appropriate for use on boats.  It is explicitly “forbidden” for boats by the ABYC electrical standard, E-11.

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Stranded Core

Wire in which a conductor of useful finished AWG or SAE diameter in comprised of many smaller-diameter filaments of copper which are spun together spirally into a single finished conductor. “Type I” strands are quite heavy, and are used on residential and light commercial outdoor settings, deep well power cables, and other applications where some small amount of motion is expected in normal operation. “Type II” stranded filaments are medium-fine, and are typically found in automotive stores for aftermarket accessory wiring and speaker cable applications; “Type III” strands are extra-fine, and produce a finished cable that is very flexible and resistant to metal fatigue, and is recommended by ABYC for use in all wiring applications aboard boats.

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Home Run

A wiring installation technique on boats that is different from cars. ABYC requires that metal boat hulls NOT be utilized as electrical circuit conductors for any reason, and FRP (Fiberglass Reinforced Plastic) hulls are not electrically conductive in any case, so the “home-run” wiring technique is required for all electrical appliances on boats.

In DC systems on motor vehicles, the metal frame of the vehicle is used as the return conductor for appliances like lights, accessories and control systems. The negative terminal of the battery picks up the chassis, and the chassis becomes the return conductor. This techniques has several negative side effects, and can’t be used on FRP boats, so a separate, physical return conductor (B-) is required to every appliance on the boat.

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A manufacturing process where individual bare copper wire filaments are pulled through a bath or molten lead/tin solder. This coats the copper wire filaments prior to being spun into a finished stranded wire of AWG or SAE diameter. Tinning is done to allow the conductor’s surface to resist forming surface oxidation and corrosion deterioration, so extends useful service life following original installation.

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The non-conductive, protective material that encapsulates electrical conductors; the means or protecting the electrical conductor from environmental contaminants and unwanted cross-connections, and protecting people, pets and wildlife from accidental contact and potential electric shock hazards.

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Insulation Properties

Manufacturer-provided rating specifications for temperature, flexibility, resistance to chemicals, resistance to moisture, resistance to abrasion that characterize the protection provided by the insulating material.  The ratings come from, and depend upon, the chemical composition and production techniques of the insulating material.

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Boat Cable

UL1426 is the standard definition of boat cable:

1.0 SCOPE:

1.1 These requirements cover electrical cables for boats. The cables are intended for use in marine pleasure craft and consist of a single insulated conductor without a jacket or of two or more insulated conductors with or without an overall nonmetallic jacket. Each boat cable is rated as follows: 600 V; 60°C (140°F), 75°C (167°F), or 90°C (194°F) wet; and 60°C (140°F), 75°C (167°F), 90°C (194°F), or 105°C (221°F) dry. Boat cable dry-rated 125°C (257°F) or 200°C (392°F) may be investigated. A boat cable so marked has insulation (and jacket if a jacket is used) that is for use where exposed to oil at 60°C (140°F) and lower temperatures. Boat cables employ stranded copper conductors that are 18 – 4/0 AWG for multiple conductors and 16 – 4/0 AWG for single conductors.

1.2 The ampacity of a boat cable shall be as stated in the US Coast Guard regulations Title 33, Chapter I, Parts 183.430 and 183.435 of the CFR.

Also, a specific product (Type BC5W2) made to withstand the harsh physical environment and vibration found on boats. BC5W2 has 105℃ as well as physical and chemical resistant insulation. The high temperature-rated insulation used on boat cable allows wire of a given AWG size to be rated for use with higher ampacity ratings than lower temperature-rated conductors of the same size.  For example, #10 AWG Boat Cable used outside an engine space is rated at 60A; Inside an engine room, it’s rated at 51A.  Type NM residential solid core wire of #10 AWG size is rated at only 30A.

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Primary Wire

A single conductor, insulation-bearing, electrical conductor. The insulation itself can be of any desired color. Commercial roles of primary wire are available in a wide variety of insulation colors.

ABYC publishes a chart of recommended DC wire colors based on the function of the circuit in which the wire will be used, as follows:

National electric codes also define wire colors for AC applications. Boats found in the US may have been built elsewhere, and may be found with non-US variations. The following chart shows some alternatives boaters may encounter:

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Duplex Wire

Any length of two, insulation-bearing electrical conductors, in turn wrapped together within a surrounding jacket of insulation. The individual conductors should have contrasting-color insulation or otherwise contain colored traces of different color so they can be individually identified at the remote end. Duplex wires for boats are usually used in DC applications, and will have individual conductors bearing red or black insulation (perhaps red and yellow insulation) and will in turn be wrapped in a white exterior jacket.

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Triplex Wire

Any length of three, insulation-bearing electrical conductors, in turn wrapped together within a surrounding jacket of insulation. The individual conductors should have insulation of contrasting colors or colored tracers to allow the conductors to be identified at the remote end.

Triplex cables for boats are used for 120V AC circuits, and in that application, will have individual wires bearing black, white and green insulation, and will usually be wrapped in a white exterior jacket.

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Multiplex Cable

Any length of four or more, individual, insulation-bearing electrical conductors, in turn wrapped together within a surrounding jacket of insulation. The individual conductors should have insulation of contrasting colors or colored tracers to allow the conductors to be identified at the remote end.

On boats, four-wire multiplex cable is used for 120V/240V AC circuits, and in this application will have individual wires bearing black, red, white and green insulation, in turn wrapped in a white exterior capsule.

Another application for multi-conductor multiplex cable is in the wiring of engine gauge and control circuits from engine room to salon/pilot house nav station to flybridge help station.  In this application, many wires of different color are all wrapped together in an exterior capsule.  Multiples wire is commonly used with all kinds of navigation equipment.  With these applications, the individual colors may not convey equivalent meaning.  Particularly from manufacturer to manufacturer, color meanings are often different, and extreme diligence is required to connect these cables correctly.

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Wire “Type”

Refers to an NEMA and electrical industry-wide standard classification scheme that describes the properties and construction of the conductor itself, and of the insulation found encasing the wire. Typical wire types recommended for use on boats for AC and DC distribution use and branch circuit load wiring include Types SOW, STW, THW, THHN, XHHW, MTW, AWM and UL1426 (Boat Cable). Boat Cable (Type BC-5W2) is made by a number of manufactures including Ancor, Pacer and others. All of these different Type-ratings are associated with different physical properties (temperature, flexibility, chemical and physical endurance properties, and “ampacity”) of the wire and insulation.

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A device used to protect the insulation of individual primary wiring and multiplex wiring against chafe and cuts as it passes through holes punched in metal enclosures or drilled in bulkheads or decks of boats; usually made of rubber, nitrile, neoprene or other soft insulating material.

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Wire Tie

An adjustable plastic strap or plastic or metallic retainer used to secure and retain individual wires into a single group or “package;” the grouped conductors, as a unit, are referred to as a “cable.”

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This term has two meanings, depending in context:

    • as above, a collection of primary and multiplex wires, grouped together, running together, which may or may not support the same equipment, and
    • as a reference to individual, large-diameter conductors at batteries, windlasses, thrusters, and similar high current, low voltage equipment.

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Terminal End

The electrical connector affixed to the ends of a length of wire or cable; enables the wire to be attached to components of an electrical system, such as a battery post, or a switch, solenoid, radio, etc, fit with screw connectors; usually installed with a crimping tool.

Types of terminal ends include:

  1. Ring terminals,
  2. Captive Spades,
  3. Snap (Bullet) and
  4. Blade

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All sound electrical connections are made by tightly compressing together the current carrying elements of the connection; usually.  It is the firm, intentional, mechanical contact of the metal elements that makes an electrical connection of high-integrity and low resistance. Strong and tight mechanical connections can be accomplished by securing ring terminals under a screw, bolting connections together, crimping a sleeve over conductor ends, or of course, with soldering techniques.

Crimping is a simple and cost-effective method of affixing a terminal end to a conductor to make a high-integrity, low resistance electrical connection.  The metal collar of a terminal end or butt connector is physically crushed-in-place by a compression tool in such a way that the collar grasps the properly prepared bare end of a conductor.  With closed-end tubular terminals, it’s a good idea to slather the conductor’s prepared, bare end with dielectric grease before inserting the conductor into the terminal end and making the crimp.  The grease is very reliable way to ensure that air, moisture and dust is kept out of, and away from, the actual electrical connection.

One pliers-style hand crimp tool for smaller gauge conductors looks like this:

The pliers style tools aren’t practical for larger gauge conductors, so a “hammer crimper” can be used in DIY work. ALWAYS WEAR SAFETY GLASSES WHEN USING HAMMER SYTLE CRIMPERS.

There are many styles of crimp tools. The above are simple, small, portable and low cost. Mechanics professional tools used by working pro electrical technicians are available, including hydraulic crimp tools for larger conductors, but they are physically larger, heavier and have much greater cost.  Most recreational boaters won’t get the payback from these tools, but they are appropriate for serious DIYers.

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Heat Shrink

A tubular insulating material used to reduce exposed metal area to reduce the possibility of accidental contact and prevent air and moisture intrusion of the crimp. Available in many sizes and colors. Sold by the foot or in convenience packages of pre-cut length. Installed by slipping over the attachment point between the wire end and the crimped terminal body.

High quality heat shrink has a heat activated glue that seals the connection from air, humidity, water and other environmental contamination. It is permanently installed by applying heat with a heat gun. The heat causes the tubing to shrink tightly around the wire and terminal body and activates the glue.

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(Noun) The point of connection between two conductors of the same or different wire gauges.  Splices should be of the crimp type, and are done with either

  1. Butt Connector or
  2. Reducing Butt Connector

(Verb) the act of connecting two pieces of wire together to make up a single, longer wire.

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Clamp-on Ammeter

A test tool that allows the measurement of current flowing in a conductor, or the net current flowing in a group of conductors. Extremely useful in monitoring boat systems and in performing basic electrical diagnosis.  Shown below monitoring the shore power pedestal connection to a 120V/240V, 50A boat power cord.

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Battery Charge/Discharge Cycle

The process of removing and replacing electrical energy from a battery.  Batteries produce DC electricity as part of the chemical interaction of battery components.  Over time, the chemical reactions alter the physical composition of the battery’s lead plates, decreasing the energy storage capacity of the battery cells. and limiting the service life of the battery. Battery service life is advertised in terms of probable lifetime charge/discharge cycles.

One individual charge discharge cycle of a 12V flooded wet cell is shown in the chart below:

In this chart, on the left hand side, the battery bank was fully charged and being held at its float voltage of 13.3V. We anchored at around 1800 hours, and shut the engine off, so the battery was no longer being charged.  System voltage went to 12.5V, the terminal voltage of a fully charged 12V lead-acid battery. Overnight, the battery terminal voltage slowly fell off as the bank discharged into our house loads, reaching about 12.0V after coffee in the morning.

At about 0700 hours, we started the engine to get underway, and the bank began recharging in Bulk Mode (Constant Current into the battery bank).  In Constant Current mode, the voltage is slowly adjusted upward to hold the current flowing into the battery to the max that the batteries can accept (Charge Acceptance Rate).  If the charging source (engine alternator in this case) cannot product the max current the batteries can accept, it will produce its rated maximum output, controlled by the voltage regulator.  Those steps in voltage increase appear between 0700 hours and 1000 hours on the chart.

At about 1000 hours, the alternator went into Absorb Mode (Constant Voltage across the battery bank).  During the Constant Voltage charging stage, current falls off. As the battery achieves a higher and higher state of charge, the Charge Acceptance Rate falls. This stage is visible on the chart as the flat area at a voltage of around 14.4V.

Finally at about 1200 hours, the bank was fully recharged and the alternator’s voltage regulator dropped into float mode at 13.3V, awaiting another night at anchor and a repeat of the charge/discharge cycle.

The deep voltage dip to 11.8V occurred in the morning when the coffee maker was run (via the inverter).  The coffee maker draws 5A-6A AC, so at a ratio of at least 10:1, ours drew 67A DC from the battery bank.  With a partially discharged battery bank, it’s normal for the terminal voltage to fall off under load, but the terminal voltage recovers immediately when the large current drain stops and load level returns to normal.  If the engine had been running at that time, the dip would have been less, because the electric energy needed by the coffee maker would have come from the alternator instead of the battery bank.

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A point in an electrical circuit where conductors are connected together (bonded, joined, paralleled) by whatever means. The behavior of electric current at a node in a circuit is described by Kirchhoff’s Law.

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“Islanding” is a state-of-operation that occurs when a boat’s inverter generates AC electricity from the boat’s battery bank.  This usually occurs when shore power is down or unavailable.

In the mid-20-teens, several manufacturers (Victron, Outback, Magnum) began producing inverters that could synchronize with a local power grid and supplement the amount of shore power available from a dock.  This was called “Power Assist.”  At a dock where only a residential 15A outlet is available to a 30A boat, that’s adequate for keeping batteries topped off and refrigeration, but not enough to include microwaves, toasters, cooktops and coffee makers.

With an inverter designed for Power Assist, the inverter can be limited to draw no more than 12A from shore power, thus not tripping a shore power circuit breaker, but the inverter can supplement that  shore power with additional AC power.  So during dinner preparation, the microwave and cooktop could be used.  Power for additional load comes in part from shore power and in part from the boat’s battery bank, both tied together and the AC wave synchronized at 60 hZ by the inverter electronics.

In Power Assist mode, the power created from the boat’s battery bank is in parallel with the electric utility power grid. An extremely important safety consideration in this arrangement is, what does the inverter do if shore power is lost?  Backfeeding power into the power grid from the inverter (or generator) would be dangerous to personnel working on downed wires, to say nothing of the electrical load it would place on the inverter.

Inverters that are designed to provide “power assist” must also be designed for incorporate “anti-islanding” protections.  These inverters MUST automatically disconnect from the incoming power grid mains if shore power is lost, and there are safety specs for how fast that must happen. Inverters designed to be safe in these situations are listed to UL 1741.  Buyers are responsible to ensure equipment they install is suitable and compatible for use, and safe.

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Trade Association for the North American battery industry Battery Council International.  BCI represents members on issues with the potential to affect battery manufacturers and recyclers at the local, state and Federal levels; including legislation and state and federal regulatory activities. BCI promulgates “industry standard” physical package sizes and technical definitions for battery-related technical topics.  BCI members commit to safe, responsible battery manufacturing, recycling and resource utilization practices.

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Lithium Batteries On Boats – Part 2

8/13/2021: Initial post


Many boaters have been lead to think that lithium chemistry batteries are just an efficient energy storage technology that can easily replace existing lead acid batteries in a boat.  For 100+ years, boat DC “electrical systems” have evolved to be inextricably compatible with the characteristics and behavior of lead-acid batteries.  Lithium chemistry batteries are protected and controlled by internal, “semi-intelligent” BMS circuitry which is not inherently compatible with lead-acid “electrical system” designs.  Incompatibilities expose some reliability gaps with traditional “electrical system” designs which become concerns when lithium chemistry batteries simply “replace” lead-acid batteries.  The guiding principle in battery replacement must be that a reliable electrical system is essential to crew and vessel safety on any boat.  “Electrical system” designs that are fully compatible with lithium chemistry battery characteristics are yet to be fully developed, tested, certified and adopted in order to assure “electrical systemreliability with LFP battery technology.  Issues and concerns are not about the batteries; it’s about the “electrical systems” into which the batteries are fit!

This article, like the previous article, grew quickly and became quite lengthy.  These twin articles are an “orientation” to using lithium chemistry technology on boats that just skim technical design detail.  The scope of this subject fills engineering textbooks.  Just as all of the organs of the body have their own complex internal functions, each must work harmoniously with all of the others for the health and wellbeing of the human host.  Doctors work with hearts, lungs, bones, nerves and muscles as component parts that make up a complete, living body.  Engineers, service technicians and skilled DIYers work with batteries, wires, switches, motors, appliances and instruments; all of the parts that make up a complete boat “electrical system.”   For the system to be healthy and RELIABLE, all of the parts must work harmoniously together!

Profile of Today’s Recreational Vessels:

There are three principal factory configurations for the DC electrics on cruising boats manufactured in the last 20 -30 years.  Most recreational boats in the 30’ – 65’ range will be in one of these cases:

    1. boats built with a single, central battery bank that “does everything.”
    2. boats built with two battery banks, one that starts the engine (“Start”) and does nothing else, and a second one that does everything else (“House”), or
    3. boats as in two, above, but fit with a third battery bank dedicated to onboard inverter(s) which in turn power some or all onboard AC loads.

Variations of the above are certainly found;

    1. separate “start” banks for multiple engines
    2. “house bank” split into two parts, Port and STBD; fore and aft,
    3. separate “not start bank,” “not house bank” for thrusters or windlass or winch.

Most commonly, battery banks on mid-sized boats built for lead-acid batteries are 12V or 24V configurations.  In some cases, both 12V and 24v banks will be found on the same boat.  Infrequently, 32V system may be seen.  Below, following, 12V systems will be the baseline of discussion.

Case 1: Central Battery Bank

The DC portion of the “electrical system” aboard our Sanctuary has a single “central bank” as shown in Figure 1.  This approach maximize the utilization, efficiency and service life of our single battery bank composed of six lead-acid flooded wet cell, 6V golf cart batteries (2S3P) and spreads normal aging and lifetime loss-of-capacity progression evenly across all of our individual batteries.

This single 12V bank runs our house loads, inverter loads and starts our propulsion engine.  This configuration is efficient, uncomplicated and economical to own and operate.  It requires a single AC charger to charge batteries from shore power or our onboard 120V genset.  Our AC charger is a Magnum MS2012 inverter/charger rated at 100A DC charging current.

Sanctuary is a single-engine trawler with one engine-driven alternator.  When underway, the alternator provides 12V B+ power to our DC system and charges our batteries.  The alternator is a 12V, 110A Balmar (Prestolite) small-frame machine fit with an external Balmar 3-stage (bulk, absorb and float) voltage regulator.  Sanctuary is also fit with a Magnum Model BMK Coulomb-counting battery status monitor to enable tracking of battery voltage, load current and battery State-of-Charge (SOC).

Since there is only one central battery bank aboard, there is no “1-2-both-off” battery switch, no duplication of power distribution wiring, and no concern for resistive losses in the DC cabling that makes up the battery bank power distribution circuits.  Monitoring and maintenance are easy.

Over the years, I’ve recommend the above approach for its simplicity, utility and reliability.

The principle “perceived weakness” to Sanctuary’s consolidated, central battery bank is “lack of redundancy.”  The concern centers on the idea that if our single bank were to fail or become sufficiently over-discharged, the boat could be left unable to start its propulsion engine.  Since we have an onboard genset, that would be only a minor inconvenience, but in an inadequately monitored and managed system without a genset, over-discharge could happen.  To a boat manufacturer selling into a generic consumer market, that risk is a valid consideration for the design of a boat “electrical system,” so a second “start” bank is sometimes added by boat builders.

Case 2: Separate “Start” and “House” Banks

In Figure 2, the DC portion of the  “electrical system” shows one battery bank that is dedicated to starting the propulsion engine and a second battery bank that is dedicated to all other DC loads.  The “start” bank only powers the engine’s starter motor and the engine’s control circuits, sensors and gauges (fuel solenoid, coolant temperature, oil pressure,  exhaust high temperature alarm and gauge illumination), but nothing else on the boat.  In the lead-acid world, “start service” batteries are specifically intended to give up the energy they store in very large, short-duration bursts.  “Start service” batteries do not store much total energy.

“House” loads comprise “everything else DC” on the boat (bilge, water and waste pumps, deck washdown pump, navigation instruments, windlass, thruster, inverter/charger, refrigeration, computer and entertainment equipment and lighting).  In the lead-acid world, “house” batteries are “deep cycle” batteries intended to hold a great deal of energy, but to give it up over long periods of time.

Redundancy paths are depicted in Figure 2 by the dashed blue lines, showing there are two possible DC sources that can alternatively (redundantly) be selected to power the engine’s starter motor.  If either path were to be unavailable/fail, the other would be available as backup.  Distribution wiring and switching detail is not shown in this “conceptual diagram.”  In return for the “operational redundancy” that this dual bank system is seen to provide, the addition of a separate “start” bank adds cost in the form of batteries, additional switching and cabling, and charging equipment.  Additional batteries and equipment also adds ongoing monitoring complexity and maintenance for the boat owner.

Case 3: Independent “Start,” “House” and “Inverter” Banks

The DC portion of the “electrical system” in Figure 3 shows separate, independent battery banks for:

  1. “starting” the propulsion engine,
  2. “house” DC loads, and
  3. AC “inverter” or “inverter/charger.”

This approach provides additional power redundancy (blue dashed lines), but brings with it additional equipment cost, wiring and switching complexity, owner monitoring and maintenance overhead and the greatest service life compromises for evenly balanced battery utilization.

The benefit of a separate battery bank to power an inverter(s) is because AC loads (microwaves, cooktops, toaster ovens, washer/dryers, ice makers, freezers, watermakers, AC davit winches, etc) create very large DC amp draw from the batteries that feed the inverter.  In 12V systems (ignoring inverter efficiency), the ratio of 120V AC to 12V DC current is 10:1.  So 10 Amps AC drawn from the inverter to power a toaster will exceed 100 Amps DC drawn from “inverter” bank batteries.

High-demand transients cause lead-acid battery terminal voltage to “sag.”  High demand loads also invoke “Peukert’s Law,” which describes the phenomena where lead-acid batteries that are discharged faster than their design rate (20-hours in North America) will not return as much of the energy as they could return if discharged at or more slowly than their design discharge rate.

A separate battery bank for the AC inverter and its AC loads isolates DC “house loads” from DC voltage sag caused by AC equipment cycling “on” and “off” at random intervals.

Electrical System Compatibility Considerations

Lithium chemistry batteries rely on a BMS to keep their electrochemistry operating within safe parameters.  The BMS is an autonomous, semi-intelligent component inside the battery’s case that is able to independently, suddenly and without warning “decide” to disconnect attached DC loads.  The BMS will electrically disconnect the batteries from the circuits they are intended to power in the event of internal electrochemical or thermal threats with continued battery operation.  As a result, “electrical system” designers face previously nonexistent compatibility concerns with lithium chemistry power.

With lithium chemistry batteries, it must be decided early in the process of designing the “electrical system” which DC loads can tolerate a sudden power loss and which loads cannot tolerate such a power loss without potential safety impact to crew and/or vessel.  This analysis affects everything about the layout and composition of the electrical equipment on the finished boat, as well as the electrical cabling needed in the boat-build process.

In the past, boat designers have not had to perform this analysis.  In lead-acid “electrical systems,” it is entirely acceptable to assume that all DC loads will have DC power all of the time.  In lithium chemistry systems with mission-critical loads that cannot lose DC power without risk to the safety of crew and vessel, alternative means of assuring continuous power availability to mission-critical circuits must be provided through the intentional design and implementation of the “electrical system.”

When converting from lead-acid to lithium-chemistry batteries aboard a boat, this same safety analysis  must be done early in the conversion project plan.  Since the boat already exists, and is fit with an existing lead-acid based DC system, a retrospective engineering safety evaluation is needed, starting with an inventory of all of the electrical equipment on the host.  Once the inventory is complete, priorities based on importance to crew and vessel safety can be assigned.

The evaluation begins by listing ALL of the equipment aboard the boat.  Then, considered judgements about the safety role and resulting operational priority for each specific listed item are made.  This evaluation includes consideration of how the boat is actually used by its owner: ex., single-hander vs crew aboard, live-aboard full-time cruiser vs periodic weekend and vacation vs marina maven, frequency of low viz ops, night ops, international offshore ops (ex: Miami to the Bahamas), near-shore offshore ops (avoid crowded conditions on the A-ICW), seasonal layup periods, regional weather patterns, seasonal river flooding levels, or other factors that apply to the safety of crew and vessel.

By way of example only, Sanctuary’s equipment list and prioritization might look like this:

Circuit  Priority   Notes
Salon compass illumination 3 Multiple sources of console illumination available
Anchor light 2 Multiple portable backups
Navigation lights 1 Dusk-to-dawn, low viz
Deck Lights (downlight) 3 Portable lighting available
LPG Control Valve 3 Not used when underway
Engine Room lights 1 Must work
Cabin Lights 1 Must work
Aft Cabin Lights 2 Not essential
Windshield Wipers 2 Very rarely used
Horn 1 Must work
Bilge Pump 1 Automatic; must work
Bilge Pump 3 manual
“High Bilge” Alarm 1 Indicates potential emergency
Sump Pump 1 Automatic – doubles as bilge pump
Sump Pump 3 manual
Potable Water Pressure Pump 1 Manual pump not installed
Anchor/Deck Washdown Pump 1 Doubles as a fire-fighting option
Refrigerator/Freezer 1 12V/120V model
12VDC utility outlets (flybridge) 3 iPhone, Sirius/XM radio
12VDC utility outlets (salon) 3 TV Antenna
Head(s) 3 Electric macerator with manual backup
Salon AM/FM Radio 3
Windlass 3 Manual ops available, feasible
DC-to-AC Inverter 1 3 Nav table items: Printer, TV, DVR, DVD
DC-to-AC Inverter 2 1 All boat AC Outlets (computer, router, USB Hub, iPad, iPhone, AIS Transceiver, space lighting, crockpot, coffee maker, microwave)
GPS 1 (Garmin) 1 Flybridge (DSC 1)
GPS 2 (Raymarine) 1 Salon (DSC 2)
VHF Transceiver 1 (DSC 1) 1 Flybridge (Foghorn)
VHF Transceiver 2 (DSC 2) 1 Salon (redundancy)
Depth Sounder 1 Flybridge
Chart Plotter 1 (Autopilot) 1 Flybridge
Chart Plotter 2 (RADAR) 1 Flybridge
Chart Plotter 3 3 Salon; redundant iPad, iPhone apps
AIS Receiver 3 non-essential
NMEA Data Multiplexor 1 Supports iPad, iPhone apps
Compass 3 Analog; multiple redundancies
Propulsion Engine Systems 1 Key switch, alarm sounders, gauges, temp & oil pressure sensors, console instrument illumination
Thruster Control System 3 Not used underway; manual maneuvering feasible
N2K Instrument Backbone 1 Autopilot ops, incl display heads
Autopilot components: ECU/CCU and Hydraulic Pump 1 Autopilot ops
Amateur Radio Transceiver 3
SSB Antenna Tuner 3
Boat Monitor 1 Bilge pump cycles; battery voltage; fridge/freezer/salon/engine room temps

About LiFePO4 Batteries:

Buyer beware!  Caveat Emptor!

Virtually all LFP battery components originate in China.  It is well known that product quality from the various cell manufacturers varies widely.  The primary market for lithium chemistry batteries world-wide is not recreational boating in developed countries.  Recreational boating is very much a secondary market.  The cells sold into secondary markets are often “seconds” and “blems” that do not meet the quality, consistency and capacity standards requirements of the primary markets.

Any “Made in the USA” claim by ANY lithium battery manufacturer/seller is unlikely to be “the whole truth and nothing but the truth.”  LFP cells and BMS circuit boards are made in SE Asia and shipped to the US.  For some brands, it’s acceptable to see “Assembled in the USA.”  Warrantees can be misleading; a 10-year warranty only covers “manufacturing defects,” NOT “cycle life” promises.

Manufacturer’s battery specifications are very, very important.  “The devil is in the details!”

There are no formal safety testing standards specifically targeted to LFP batteries.  One UL/cUL Standard in the North America that includes LFP batteries within its scope is UL1973.  Equipment fails.  Mistakes and accidents happen.  LFP batteries are no exception.  UL does aggressive testing, including vibration, severe over-charge and over-discharge, and short-circuit testing.  Certification testing of this kind is very expensive for manufacturers, so very few LFP batteries carry UL’s listing mark.  Retail pricing for those that do reflects testing costs.  Batteries listed to UL1973 are unlikely to suffer catastrophic consequences caused by human error or component failure.

LFP Battery Characteristics:

A mandatory task in evaluating lithium chemistry batteries is to carefully review and consider the manufacturer’s performance characteristics of the batteries, themselves.  This task is quite challenging, even for trained and experienced electrical practitioners, because laboratory technical reports often differ in significant ways from each other and from the claims made by battery manufacturers in brand-specific promotional literature.  Further, the claims made by any one individual battery manufacturer are often at variance from the claims of industry competitors.  It’s often necessary to review multiple information sources, and “average it out,” based on “conservative best guesstimate.”

The following table is a compilation of data assembled from many sources, but not an absolute list of all LFP technical characteristics.  Individual manufacturers often claim different values for comparable operational factors.  For example, the safe operating amp and temperature ranges for charging LFP batteries is significantly more narrow than the safe operating amp and temperature ranges for discharging them.  Comparisons are difficult, sometimes imprecise, but there is enough in this table to help identify and consider some important “electrical system” design questions and choices.

There is a great deal of technical detail behind all of these comparison factors, and it’s neither practical nor reasonable to expect lay buyers to understand them all.  But they’re there, and they do affect risk, reliability, lifetime cost-of-ownership and value.  To avoid cost, disappointment and risk, a pre-conversion understanding of the major technical details is important.

One reason commonly cited for the apparent desirability of LFP batteries is the comparison of amp hour capacity of lead-acid batteries to LFP batteries by usable energy percentage, weight and volume.  The comparison implies that buyers can allocate less weight and total space to the more energy dense LFP batteries.  True?  Maybe.  In this discussion, buyers must diligently avoid “wishful hearing.”  The above conclusion works for “amp hour” capacity, but there’s more to batteries than amp hours.  Consider: “amps.”

Amp Hours and Amps ARE NOT the same thing.  “Amp hours” is a measure of energy storage capacity (ex: a fuel tank able to hold some fixed number of quarts or gallons of fuel; it’s “capacity”).  “Amps” is the measurement of the rate at which energy is consumed (ex: 6.0 gallons-per-hour of fuel consumed at “hull speed” vs 28 GPH consumed at “planing speed”).  Terminal voltage of lead-acid batteries “sags” when the battery is giving up high amps.  Terminal voltage of LFP batteries doesn’t sag, but thermal heating occurs as high amp draws consume energy, with the possibility the BMS will trip and disconnect the battery from its load.

PbSO4                                                   LiFePO4            
1 Technology Wet Cells AGM & Gel Carbon Foam enhanced AGM Lithium Ferro-Phosphate; LFP
2 Weight Heavy 1/3 weight of PbSO4 per aHr of C-Rating
3 Orientation dependent Yes; liquid electrolyte No – sealed No
4 Maintenance Needs “watering” and equalization None – captive electrolyte None (with BMS)
5 Service Life Baseline of 1.0X assumed; ±300 charge/ discharge cycles in lab conditions; ~150 in real life 4X – 10X PbSO4; shorter cycle life if deeper avg discharge
6 BMS HVCO n/a 3.65V/cell
7 BMS LVCO 2.50V/cell
8 BMS HTCO >135℉ charging1;

>160 ℉ discharging

9 BMS LTCO <25℉ charging; <-4℉ dischg
10 Energy Density Lowest per unit weight & volume;

35 wH/kG

Slightly better than wet cells;

40 wH/kG

Slightly better than AGM 3X >PBSO4 by weight & volume;

115 WH/KG

11 Recommended Discharge – SOC 35% – 40% (Pessimistic?) 50% (Industry rule-of-thumb) 65% (optimistic?) 80% – 90%
12 Max discharge (volts/cell) 1.75V Consult Manufacturer 2.5V
13 Max discharge amps n/a 0.5C – 1.0C
14 Cost per aHr Lowest per aHr returned 2X – 3X wet cells 1.5X – 2.0X AGM 3X PbSO4; TCO is < PbSO4 over rated cycle lifetime
15 Voltage Curve Sags with ↓ SOC Sags slightly < wet cells Sags < AGMs Nearly flat from 100% ↓ 15% SOC
16 Charge Time Baseline 1.3X faster 1.6X faster 4X faster
17 Equalization necessary Yes Some can be No No; “voltage balancing” done by BMS
18 Subject to sulfation Yes No
19 Can be stored partially charged No Yes; prefers 60% SOC for storage
20 Needs “Floating” Yes Consult Manufacturer No
21 Self Discharge Moderate Low Very Low
22 Sensitive to over-charging Least Slight – AGM; Yes – Gel Consult manufacturer Yes; BMS required
23 Sensitive to over-discharging Yes Least Yes; BMS required
24 Capacity  (C)  vs Discharge Rate 100% at 20 hr rate
80% at 4 hr rate
60% at 1 hr rate
Consult manufacturer No Peukert effect
25 Charge Temperature Correction (“Slope”)2        Deka: -3mV/°C/cell
Crown: -3mV/°C/cell
Surette: -4mV/°C/cell
Exide: -5.5mV/°C/cell
Ref: IEEE 450-2010
Consult manufacturer None
26 Conversion Costs (PbSO4 → LiFePO4) n/a Highly variable; depends on choice of platform, incl modifications & reconfiguration of existing host electrical system
27 Releases Hydrogen when charging4) Yes Sealed – VRLA No
28 Releases Chlorine gas if submerged Yes Sealed

Table Note 1: May be too low to accommodate placing the battery bank in the engine room.

Table Note 2: The industry-wide baseline reference temp for lead-acid batteries for temperature correction is 25°C (77°F); the normal, permissible operating temperature  range is considered to be 5°F – 130°F.

Load Profile – Start Bank:

All starter motors draw very high DC currents.  Sanctuary’s 4-cylinder, 150hP diesel has a 12V Delco starter motor with an inrush current of 380A and a steady-state running current of 250A when cranking over the engine.  Bigger engines may have greater starter motor power demands.   Newer reduction gear starter motors may have lesser amp draw demand.  Cold engines crank longer than warm engines.  Engines with glow plugs may need additional DC power to “light the glow plugs.”

To simply start a healthy engine, starter motors don’t have to crank very long in normal operation, but periodically – as when re-priming a diesel fuel system after major maintenance – they do need to crank for prolonged periods.  Lead-acid start service batteries can meet all of these needs because they can release high amp draws very quickly without concern for overheating.

LFP batteries don’t come in “start service” and “house service” models.  LFP batteries are rated in amp hour capacity (often, watt-hour/kilogram, or Wh/kG), also known as “C-Rating.”  LFP batteries may or may not be a suitable choice for high-current starter motors.  Generally, the “industry guideline” for LFP batteries is to limit amp draw to 0.5C.  Many marine manufacturers limit their second quality or other-than-new LFP cells to 0.3C – 0.4C.  To prevent BMS disconnects or cell damage, and maximize cycle life, a lithium battery with a 100 amp hour (aHr) C-rating should be limited to loads of no more than 40A – 50A.   An LFP battery rated at 300 aHr should be limited to loads of no more than 150A.

The math of “amp-hours” confirms that it doesn’t take a lot of total energy to start an engine.  Figure 4 shows an actual measurement of Sanctuary’s starter motor inrush current, in Amps.  Our engine takes less than one aHr of energy capacity to start it (one Amp Hour being one amp drawn for one hour).  This math very conservatively assumes 380A inrush is sustained throughout a 5-second-long duration starter motor cranking time; most healthy engines start in 2 seconds, ±:

aHr = (A x Hr)
= (380A x 5 seconds)
= (380A x 5/3600 Hr)
= 0.53 aHr.

It’s clear Sanctuary doesn’t use much total stored energy, but does need 380A to spin the motor against the mechanical load of cranking the engine.

In consideration of the above, can the lithium chemistry battery tolerate prolonged engine cranking load?  Answer: maybe.  It depends on the ratio of LFP battery C-Rating to starter motor amp demand; that is, “how much larger is the starter motor amp draw than the discharge rating of the LFP battery/battery bank?”

Sanctuary needs less than 300 aHr of usable energy for a winter overnight (14 hours of darkness) at anchor, but she needs 380 inrush amps and 250 running amps to crank the engine.  With 400aHr of LFP battery capacity, there would be plenty of energy for us to overnight on the hook, but when cranking, would be well in excess of the 0.3C – 0.5C current draw rating of most LFP batteries.  All LFP batteries will have a short-duration “overload margin;” perhaps 1C overload for 30 seconds, 2C overload for 5 seconds or 10C overload for 500 mS, so it’s tempting to conclude that a 400 aHr LFP battery would be acceptable for Sanctuary‘s starting needs.  But, the next time the fuel system needs to be re-primed after changing the secondary fuel filter or servicing the injectors, we might be faced with a nasty surprise.  That service activity would require prolonged periods of engine cranking load, and surfaces as an obvious “electrical system” reliability concern.

Pro Tip: the larger the discharge rating of the battery in proportion to the amp demand of the starter motor, the better its suitability for use as a “start” battery.  In considering this design question, keep in mind that there will be times when cranking will be required for continuous minutes.  That happens only infrequently, so that’s the time when inadequate design is most likely to surface as an issue.

Balancing the Cell Packs

One further consideration is how the needed amp hour capacity will be obtained:

    1. a custom-designed high C-rating battery with a BMS matched to the needs of that assembly (i.e., 600 aHr, 3P4S configuration of 3.2V, 200 aHr cells), or
    2. a parallel collection of 3 x 200 aHr “drop-in” LFP independent units.

Option 1 may be adequate to handle the starter motor, but option 2 may not.  “Drop-in” batteries are more susceptible to being individually overloaded when arranged in a high-load parallel circuit where small differences in cable length and connection integrity are typical, introduce unequal resistances, and result in unbalanced currents in the individual battery contribution to the total load.  The net is, absent significant “electrical system” design, starter motors are not well suited to being powered by lithium chemistry batteries of modest or conservative C-rating ratios.

As with inverters, a design alternative is to upgrade “start” bank operating voltage.   Doubling the voltage of the bank from 12V to 24V cuts the amp draw requirements in half to get the same kW of energy.  That choice isn’t always practical.  For example, there would have to be a 24V starter motor that would fit on the existing engine.  Engine controls like fuel solenoid, oil and temperature sensors, gauges and instrument illumination would have to be considered; these devices could be changed to 24V or remain wired for 12V.  Conversion cost details then arise about how to charge the 24V battery.

Pro Tip: Electrically, windlasses, winches and thrusters all contain DC motors with the load profile of starter motors.  They draw very high currents for variable, but generally short-duration, bursts.  A thruster running for 20 – 30 seconds may be harder on LFP battery internal heating than a much larger engine starter motor running for only 2-3 seconds.  But, 1C for 30 seconds might not cover the runtime needs of a foul windlass, or a thruster in abnormal conditions of wind or current, or a winch with a dinghy full of gear or rainwater.  Boat owners must decide if these are conditions where loss of battery by BMS Disconnect would be acceptable.

Load Profile – House Bank:

“House” is a “catch-all” term that tends to mean “everything else not starter motor.”  That has its origins in the differences between lead-acid “start service” batteries and lead-acid “deep cycle” batteries.  “House loads” are DC loads of more modest magnitude and longer duration, supported by “deep cycle” batteries.

Toward the goal of designing a lithium chemistry system (a reliable electrical system that is safe for crew and vessel), equipment that makes up the “house loads” has previously been inventoried and individual loads have been classified as “high,” “medium” or “low” priority for needing to be continuously powered.  High priority equipment needs battery systems that are 100% reliable.

With lead-acid batteries as the underlying design assumption, all of the “house stuff” was traditionally powered from one, common B+ buss.  Conversion to LFP batteries may require installation of net new B+ buss(es) to power a high priority load group(s), and/or may require re-configuration (subdivision) and re-wiring of some existing circuits.  In evaluating existing B+ power wiring, owners need to consider whether some very high priority equipment shares a circuit breaker that ALSO POWERS lesser priority equipment.  When Sanctuary was built, the electrical cables that brought DC from her battery bank to her power panel and from her power panel to her branch circuit load locations never considered anything other than lead-acid batteries.  As a result, our high priority flybridge and salon nav instrument B+ feeds also include lesser priority device attachments.

The obvious “mission-critical” equipment on an existing “house” bank is the Navigation Electronics (depth sounder, chart plotter, VHF Radio, autopilot, RADAR, AIS and “fly-by-wire” steering and throttle controls).  Secondary loads like navigation lights, horns, windshield wipers and refrigeration are important, but perhaps not “mission critical.”  Tertiary loads like LPG gas valves, inverter/chargers, water makers, fresh water pumps, waste pumps, washdown pumps, space lighting, deck lighting and accent lighting are lifestyle and convenience items, but certainly not “mission critical.”

Load Profile – Inverter Bank

Typically, AC equipment on a boat includes things like induction cook tops, microwaves, coffee makers, toasters, instapots, crockpots, blenders, ice makers, fans, TVs and other entertainment equipment and computer equipment.  Mostly, this equipment is not “mission critical” to safety; lifestyle, convenience and comfort, certainly; but, not safety.

What about AC medical equipment like BPAP/CPAP, WoundVac or infusion pump?  Medical equipment isn’t mission critical to the vessel, but it certainly can be to affected crew member(s).  If there are “mission critical” AC appliances aboard, they must have 100% reliable AC power.

Pro Tip: Stand-alone inverter battery banks can be upgraded to 24V, reducing battery cable size requirements and moderating battery voltage sag related to transient AC loads.

Pro Tip: AC power can optionally be accomplished with a collection of smaller inverters distributed around the boat rather than one, large central inverter.

Pro Tip: AC powe large inverter battery bank may be able to support both a large, central inverter/charger and other DC equipment deemed lower priority, like LP Gas valves, washdown pumps, water makers, and most other pumps.  Sudden loss of this equipment is certainly, but not unsafe to crew and vessel.

Battery Charging

Just as lithium-chemistry batteries discharge differently from lead-acid batteries, they also charge differently; very differently.  It is quite common to see lead-acid and lithium batteries characterized as “dumb” vs “smart” batteries.  Perhaps readers have seen magazine or Internet articles suggesting:

    1. lead-acid batteries are “dumb batteries” that require “smart chargers,” and/or
    2. LFP batteries are “smart batteries” (because of the BMS) and can be charged by “dumb chargers.”

These are misleading battery characterizations, based on the presence of what is assumed to be an “intelligent” BMS.  The purpose of the BMS is to ensure the battery stays withing safe operating limits, not to “manage” battery charging.  In “drop in” batteries, a cheap Asian-made BMS may not trip off until an internal event triggering limit has already been exceeded.  Using the “disconnect capability” of the BMS to disconnect a battery from a charger when the battery reaches “full charge” is possible, but IS NOT good design practice on a boat, either for safety or for charge-discharge cycle lifespan.

Unlike lead-acid chemistry, partially discharged lithium chemistry batteries hold their maximum Charge Acceptance Rate (CAR) right up to the point of being fully charged.  A “dumb” AC (shore power or genset) battery charger doesn’t know or care when that fully charged point is imminent or actually reached.  That “dumb” charger just pumps amps into the battery.  In this scenario, the system designer is depending on the BMS to disconnect the battery from the charger, not on the charger to realize the battery is full.  The result is, when this battery reaches full charge (or over-charge), the BMS will disconnect the battery from its loads, possibly damaging the charger, and certainly leaving the charger itself as the only source of power for DC attached equipment.

After an overnight on the hook, the “house” battery bank is partially discharged.  The crew gets underway as usual.  The engine alternator has the task of recharging the bank.  As the morning hours wax towards noon, the battery bank accepts charge.  When the bank reaches full charge, the BMS disconnects it from the system.  That sudden disconnect event brings with it all of the attendant potential problems that have been previously discussed.

Lithium Chemistry batteries do not need to be “floated,” and some manufacturers say not to float them.  The issue is, a “float” voltage set too high risks over-charging an LFP battery, so the float voltage must be very carefully set to be compatible with the LFP manufacturer’s specifications.  But as an “electrical system” design consideration, a lead-acid battery charger in “float” mode has two functions.  One is to maintain the lead-acid battery against the effects of self-discharge and the other is to provide DC power to DC equipment.  In a lead-acid system with the battery charger in “float” mode, DC B+ power for loads comes from the battery charger, NOT the batteries.  With an LFP battery, the self-discharge mission has gone away, but the need to provide B+ power to house loads remains.

Best practice:

    1. lead-acid batteries are “best charged” by a multi-stage (“smart”) charger;
    2. LFP batteries are “best charged” by a constant-current capable charger sized to the charge current specification of the battery, and set to discontinue charging at less than 100% SOC;
    3. a modern lead-acid “smart charger” with both lead-acid and LFP charging programs will charge both battery chemistries just fine.
    4. LFP batteries should not be “floated,” lest they over-charge and trip their BMS; but if “floated,” they should be “floated” at or below the voltage recommended by their manufacturer.

Pro Tip: In a lithium chemistry system, it matters how the battery charging systems are designed.  It is unwise to let an engine alternator with a “dumb” internal voltage regulator drive the LFP battery bank to its absolute full charge.  Alternators should be fit with a “smart” external voltage regulator that can discontinue constant current charge when the LFP target bank is at 80% SOC instead of 100% SOC.

Pro Tip: Very careful design consideration MUST BE GIVEN to ANY SYSTEM where the BMS will be relied on to terminate charging.  In any system of that design, the LFP battery will be disconnected from it’s load, and that means the load may lose DC power.  In most boat applications, that IS NOT desirable.

Charging multiple banks:

With shore power (or genset) AC, it’s easy to install a battery charger, or multiple battery chargers, to charge multiple, different battery banks.  Charging with engine-driven alternators, which don’t lend themselves to being “stacked” in multiples onto an engine, requires more analysis.  Alternator configuration depends on the number of engines and presence of any available auxiliary alternator mounting locations.  With single engine boats, alternator output capacity is a paramount design consideration.  With multiple engines, additional charging options do present themselves.  Each owner must consider this requirement in the context of presently existing equipment.

Particularly on boats with multiple lead-acid battery banks, it is common to find Voltage Sensing Relays (VSRs) used to isolate existing lead-acid “start” and “house” battery banks from one another when not charging, and to connect them together for charging.  A VSR is just a dumb solenoid.  Current can flow in either direction between the two banks.  For a lead-acid “start” bank to be paralleled to a lead-acid “house” bank with a VSR, the “house” battery/bank should be permanently connected directly to the alternator.  The VSR would “parallel-in” the “start” battery/bank.

After an overnight on the hook, upon “engine start”, the alternator voltage regulator initiates “bulk mode” to bring the terminal voltage of a partially discharged “house” battery up to it’s 14.5V bulk target voltage.  As terminal voltage ramps up, the VSR senses the “connect” voltage and “parallels-in” the “start” bank.  At VSR-connect time, the “house” and “start” batteries are close to each other, and so the inrush current exchanged between the two battery banks through the VSR is low.  When the VSR solenoid is closed, the two battery banks are electrically in parallel and charge together, as one.

Because of high inrush currents, VSRs are less desirable for paralleling a lead-acid battery and an LFP battery, particularly in the case where the banks are NOT both fully charged.  In the case of a solenoid connecting a partially charged LFP “house” battery to a mostly charged lead-acid “start” battery, there will be a huge mismatch in the charge and equivalent resistances of the two, and there will be a huge inrush current to the partially discharged LFP “house” battery from the mostly charged lead-acid “start” battery.  That large inrush current looks like a starter motor load to the “start” battery.  It discharges the “start” battery (undesirable) and may look like a short circuit to the LFP BMS.  If so, it will cause the BMS to disconnect (undesirable).

Assuming the configuration survives the VSR-connect, the LFP BMS will disconnect its battery when any one of the internal cells reaches an over-voltage SOC.  In order to avoid that unwanted BMS disconnect, the above VSR configuration would require the charging source behind the battery pair to be “smart;” that is, “programmable,” so that the overall charging process can be stopped at a voltage preset low enough to avoid having the integrated BMS disconnect the battery.  That also helps protect the alternator from overload and over-heating and from throwing a voltage spike at BMS Disconnect time.  That can be accomplished with an external, programmable voltage regulator fit to the alternator.

Pro Tip: For interconnecting lead-acid and lithium chemistry batteries at different states-of-charge, consider use of DC-to-DC Converters in place of VSRs.  Logically, these two devices are equivalent; both “parallel-in” another battery bank for charging and both isolate banks from one another when not charging.  But unlike “on” or “off” VSRs, DC-to-DC converters isolate the two battery banks and pass current in only one direction.  Depending on features, DC-to-DC converters can appear to their load side as “smart chargers,” controlling the voltage at the load and limiting the current supplied to the load.  The DC-to-DC converter can accept either “smart” or “dumb” input sources.  Upgrading the system from VSRs to DC-to-DC Converters is a recommended conversion-cost in switching to LFP batteries.

Solar Systems:

This article is already quite long, so I will just note here that I haven’t forgotten solar and wind charging sources, but have intentionally omitted including them in this discussion.  The “electrical system” compatibility concepts are all the same.

Conversion Costs:

The obvious initial consideration of conversion costs from lead-acid to LFP batteries is the cost of the batteries themselves.  LFP batteries can be discharged to a larger percentage of capacity than lead-acid batteries.  Thus, if 450 aHr of usable energy is needed to support a boat’s cruising needs, ~1000 aHr of lead-acid batteries would be needed to get that capacity, but only ~500 aHr of LFP would be needed to get the same usable capacity.  The lead-acid solution prices out to less than $1000.  The LFP solution prices to 3x – 5x that of lead-acid; there are highly variable unit costs depending on the quality of the LFP cells being purchased.   In this calculus, assumed service life projection (charge/discharge cycles) appear to drive the value proposition of conversion positive.

As discussed above, there are other potential costs related to conversion.  Every boat is unique.  The more existing batteries and battery banks there are aboard, the more charging equipment and monitoring complexity comes with conversion to lithium chemistry power.

This is particularly true if there are different battery banks of different voltages (mixed 12V and 24V) that require different charging equipment.  Take a moment now to carefully review Figures 1 – 3.  Identify the figure that most closely approximates what you have on the boat you currently own.  Figure 5 shows a system with both 12V and 24V house battery banks.

These drawings should help to visualize the scope of an actual conversion project.  With them, identify what areas might need reconfiguration, and begin to consider the steps that might be involved in a conversion project plan.

For Sanctuary, re-wiring branch circuits to separately power “high priority,” “medium priority” and “low priority” branch circuits would be very challenging given the OEM cabinet and bulkhead arrangement and the locations of the existing batteries, OEM electrical panels, OEM cabling and existing installed equipment.  Without major interior remodeling, batteries would have to remain located in the engine room, where cooling the batteries is already problematic and additional cooling would definitely be necessary.  Addition of new B+ power buss would require at least a revision to the power panel configuration.  New cabling would need to be installed in in already crowded wire chases and raceways.  Conversion to a technology that is more sensitive to ambient temperatures and involves significant remodeling of the living space on the boat is unjustifiable, both economically and in terms of operating risk.  Every owner has to look at this from their own, unique perspective.  Conversion and reconfiguration cost to obtain “a reliable electrical system that is safe for crew and vessel” is unique to each vessel and the needs and desires of each owner, but will not be insignificant.  Read that as, more than expected and more than planned.

Considering all of the preceding, I’ll again ask the boat owner who is considering lithium chemistry batteries the following questions:

    1. How do you use your boat?  Do you cruise from marina-to-marina?  If you have a “shore power lifestyle,” do expensive batteries and complex “electrical systems” still have value?
    2. Do you live aboard, enjoy continuous cruising, endure a short boating season, encounter long idle periods, lengthy summer/winter layup?  Idle periods and layup periods require different planning for LFP batteries than lead-acid batteries; cold weather vs hot weather layup requirements are different.
    3. How long have you owned your boat, and how long do you intend to own it?  Financial ROI for lithium chemistry batteries is 3 – 4 lifecycle turnovers of lead-acid batteries (minimum of 10 – 12 years), much longer if initial “electrical system” conversion costs are included in the calculus.
    4. What’s the boat’s present age and present market value?  Conversion value will be lost if you should decide to sell in the near-term future.  It’s unlikely conversion to lithium would add market value to the boat.
    5. Are there insurance or insurability implications to conversion to lithium chemistry?  LFP is a very stable chemistry, but if it does burn, there’s no extinguishing it.  A lithium fire is an “abandon ship” event.
    6. Why is it worth it to you to convert to lithium chemistry batteries?  What are the value benefits to you of having lithium chemistry batteries?  Will a conversion of battery chemistry fix something that isn’t working in the existing electrical system on your boat?  Are you sure?
    7. Given the rapid present evolution of battery, system and component technology, does the boat owner have the skills, resources and will to repetitively redesign and upgrade the electrical system?  Does the owner have the skills to keep the system current with improvements in equipment?  DIY electrical skills have huge Net Present Value (NPV) in the conversion calculus.  Commercially hired, contracted electrical skills in 3Q2021 will price out at $140/hour, ±, plus equipment/materials markup.  Often, the labor cost component will exceed material and equipment costs.
    8. Does the owner of a cruising boat have the DIY skills to troubleshoot electrical system issues that occur “while cruising in exotic places?”  If not, when something fails, “where in the world” (literally) will the boat be when a failure occurs?  Will parts, replacement equipment and technician skills be available when and where they are needed?
    9. Does the boat owner have temperament and tolerance for possible “system outages” and cruising delays?   Spouse also temperamentally suited to outages?  What anxiety levels are OK, and when do anxieties become overwhelming?

At this point in this article, we have considered:

    1. how the owner uses the boat, in cruising and in layup,
    2. how much longer the owner plans to own the boat,
    3. analysis of the existing electrical system in place aboard the boat,
    4. prioritization of the safety importance of DC circuits aboard the boat
    5. analysis of owner electrical skills and conversion costs,
    6. characteristics, strengths, weaknesses and maturity of current LFP solutions, and
    7. owner tolerance for outages, including how outages might impact upon our key crew members.

Matching Batteries to Applications:

Start Service:” Battery C-Ratings must be matched to starter motor current draw magnitude and draw duration.  It’s not enough to limit capacity considerations to starting a properly adjusted, properly tuned, properly running engine under ordinary, normal conditions.  Consideration must be given to the cranking needs of the engine under in-service and post-service conditions, where prolonged periods of cranking load will occur.  Except with high C-Rated LFP batteries controlled by a single, robust BMS, lead-acid batteries are best suited to “start” service applications, including windlass, winches and thrusters.  Groups of “drop-in” LFP batteries, each individual with its own BMS, may not be well suited to “start service” applications.

House Service:” DC “House” loads judged to be “high priority” need to be matched with highly reliable battery sources.  This is an application where hybrid banks of mixed lead-acid and LFP components are well-suited, and were discussed in this linked article.   The LFP component of the hybrid provides the benefits of voltage stability to loads, while the lead-acid component of the hybrid provides the rarely needed but critically important alternator protection, continuous availability and high reliability in the event of sudden BMS disconnect.  “Low priority” house loads where intermittent transients are few, and amp draws are modest, are well suited to LFP banks without the lead-acid component.  One fully integrated, 400 aHr C-Rated LFP battery with robust BMS is a better option than four 100 aHr “drop-in” batteries, each with separate BMS, arranged in parallel.

Inverter Service:” “Inverter” loads are generally not mission-critical loads.  Inverter banks do have some transient high amp demand characteristics which need quantification.  Some AC loads create very high, short term transient demands in DC battery loading.   A bank with a microwave, coffee maker, toaster oven or induction cooktop will create dramatic transient loading fluctuations in normal use, and the C-Rating of the inverter bank must be capable of handling the maximum transient load for its entire duration.  If cooking pasta in a microwave takes 28 minutes at 50% microwave power, the BMS over-current setting must be able to support the corresponding battery DC amp draw duty cycle for its full duration.  As a design choice, doubling the bank voltage (12V → 24V) cuts amp demand in half, but simultaneously obsoletes 12V equipment, necessitating upgrades, and so adds to conversion cost.

Commercially available “system” solutions:

I do not feel LFP-based platforms are, as yet, “install and forget” solutions suitable to a general consumer boating market or owners without at least moderate electrical technical background.  Today in 3Q2021, the hardware solutions being rolled out by leading equipment manufacturers in support of lithium chemistry applications are in varying stages of technical maturity.  The majority of today’s buyers enjoy DIY projects, are technically proficient (because they have to be), and fall into the category of “early technology adopters.”  Reliable technical advice is not yet widely available.  Battery and equipment manufacturers generally understand the state-of-maturity of their products, and are working hard to close system reliability gaps.  New equipment versions are rolling out in cycles of deployment of 6 – 9 months per manufacturer.  Expect rapid hardware evolutions and rapid product obsolescence over the next several years.  Also expect that at least some manufacturer names that are well known in 2021 may not be here at all in 10 years, putting warranty promises in doubt.


A lifetime of engineering project development experience suggests that as insights are gained from early adopters and system failures, incremental improvements will be staged into existing products and net new, increasingly feature rich products will become available.  User options will expand, efficiencies will improve, and costs (capital, conversion and maintenance) will come down.  That is where I believe we are today, in what is essentially a proof-of-concept and beta test period with the rollout of lithium chemistry batteries and system solutions to informed early adopters.  There remain important, unanswered design and safety questions.  It’s quite likely that the equipment beings sold today as “leading edge” and “ground breaking” will be viewed as “primitive” and “inadequate” in 7 – 10 years, so should be considered by buyers as “interim solutions.”  Given the very long ROI timeframe of a conversion project, it’s likely batteries and systems will be far advanced well before cost recovery for equipment installed today has been realized.  There’s not much room for manufacturers to obtain ROI on product investment in this small and expensive market space, so expect high prices and availability constraints for the best quality equipment.

Today, lithium chemistry battery solutions do have a small market for which they are well suited: that is, folks who meet all of the following criteria.

    1. DIYers possessed of “advanced” or “expert” electrical skills,
    2. living-aboard boats that remain in continuous service year-’round,
    3. fit with large solar systems (800W or greater), and
    4. cruising in remote and “off-grid” places, away from shore power.

That profile does not describe a very large cross-section of recreational vessels or recreational vessel owners.  For most of us, lead-acid batteries remain the simplest, best and most cost-effective battery solution; granted, not glitzy, but serviceable, utilitarian and affordable.

There is a very aggressive research and development effort going on in the lead-acid sector, and if it realizes anywhere near its potential promise, will change existing technology and financial equations yet again.  (Well, did you really think lead-acid interests would go off into the corner and die?)  It is unlikely that the currently available lithium chemistry batteries will live up to their “hype” in the real world.  Lithium chemistry batteries are not going into boats with a Net Present Value below about $250K.   Presumably that means they’re going to people who can both afford the conversion expense and absorb the hidden costs of failed or inadequate conversion assumptions and design decisions.

In the medium term, those boaters enduring “major lithium chemistry envy” should consider carbon foam lead-acid batteries.  Carbon foam may offer a very acceptable incremental compromise toward the overall advantages of lithium-chemistry, but without the BMS-related risks.

This article has discussed LFP batteries in the context of the “electrical systems” in which these batteries function.  It has not discussed the technologies of construction of the battery cells, BMS electronics or BMS design shortcomings, electrical safety components not yet available in a lithium platform system or the economics of the current manufacturing infrastructure.  It’s fair to say this discussion “glosses over” much of the technical detail related to the batteries themselves.  But, hey!  Boat owners don’t want to have to know that stuff anyway, right?!  I refer those interested in a deeper dive into the technical detail to Rod Collins’ wonderful website for his excellent article entitled, “LiFePO4 Batteries On Boats.”

Lithium Chemistry Batteries on Boats

Initial Post: April 22, 2021

Topics on “Lithium Ion Batteries” fill textbooks, but I know there is great interest in the boating community.  This article is intended as a “brief” survey of the possibilities and challenges associated with Lithium chemistry batteries.  My focus throughout is on “electrical system reliability.”  The length of this article grew very quickly, so I’ve limited my content to areas that often lack adequate consideration.  “How do I make this simple for readers to understand?”  That is my primary goal and has been a real struggle.  To limit length, I’ve had to assume that readers are already somewhat familiar with the advantages, operating characteristics and risks associated with lithium chemistry batteries.  I think that describes boaters actually contemplating adoption of lithium batteries.

A reliable electrical system is essential to crew and vessel safety on any boat.  Lithium chemistry battery articles typically focus on lithium technology and its benefits.  Such articles rarely discuss the implications to the overall reliability of the vessel’s electrical system.  Modern boats are often in immediate peril (physical risk, operator anxiety, inconvenience, lost time, and cost) if any part of the vessel’s electrical system incurs a fault or outage.  This article considers the possible – not “inevitable,” perhaps “infrequent,” but certainly “possible” – reliability impacts of lithium chemistry batteries to the “Electrical System” of boats involved in cruising.

Lithium chemistry battery systems involve more technical complexity than their lead-acid cousins.  Lithium chemistry requires a “control system” to keep the battery’s internal electro-chemistry within limits that are safe on boats.  Design details required of these control systems are maturing rapidly, but in 1Q2021, new requirements are still emerging from the good and bad experience of early adopters and the experience gained from early system implementations rolled out by leading equipment manufacturers.  Regarding lithium chemistry batteries, we cruisers are in a period best described as one huge “beta test.”  Some designs are proving valuable and others are proving unsuitable.

The Boat “Electrical System”

I have long “preached” that boaters need to think about boat electrical systems “as an integrated system” and not as “individual components” hooked together.  Engines with DC fuel solenoids/electronic fuel injection, alternator(s), start batteries, house batteries, battery charger(s), inverter(s), water and waste pump(s), bilge pumps, windlass, thrusters, windshield wipers, horns, DC nav equipment, nav lights, solar panels, DC generators, AC generators, hot water heaters, HVAC equipment, raw-water circulator pumps, watermakers, coffee makers, space lighting, washer/dryers, refrigerators, microwaves and other galley equipment, SOHO routers, computers, entertainment electronics, video cameras, boat monitors; all are “components” of the “electrical system.”  While each has unique needs, reliability means all must work cooperatively together, as a whole; never has that been more true than with lithium technology batteries.

Figure 1 is a simplified drawing of a “typical” cruiser’s Electrical System.  The AC sub-division is shown in green, and the DC sub-division in red.  The interface layer between the AC and DC sub-divisions, in which power flow is “bi-directional,” is blue.  This is a “direction-of-power-flow” diagram (source-to-load), not a wiring diagram.  In this “model,” the DC sub-division emerges as the “soul” of the boat’s overall electrical system, and the house battery bank is the “beating heart” of the DC sub-division.  Most of the “stuff” that comprises the AC sub-division is “optional;” i.e., discretionary to the lifestyle owners want to live while aboard.  The “stuff” that comprises the DC sub-division isn’t optional; without that “stuff,” we don’t have a safe, operational vessel.

Above, the DC sub-division “battery bank,” as shown, can be viewed as either 1) a “central” bank that “does everything,” including engine starting, or 2) a “House” bank with an auxiliary “Start” bank “hung off the side.”   Both views have at least two battery charging sources: 1) an AC-powered charging source and 2) an engine alternator.  There are multiple approaches to battery charging, and many different combinations of charging equipment.

Batteries provide DC power to all of the vessel’s DC loads.  Some boats are set up to have a third, entirely separate and independent bank that supports the inverter and its AC loads.  However battery charging is done, in the end, all battery banks must be charged.  The more banks, the more equipment and monitoring complexity, and maintenance, is involved in managing charging.  The current design platform (Unique Figure 1 configuration) of a boat’s “electrical system” will have enormous impacts on how to best adopt a suitable, compatible lithium chemistry solution.

Lithium “Energy” Cells

Lithium battery material is a complex “chemical soup;” in this article, 3.2V cells of “Lithium-Iron-Phosphate,” or “Lithium Ferro-Phosphate,” or “LiFePO4,” or just the “LFP” chemical compound.  This lithium technology is the safest of the lithium technologies, but it’s not as safe as lead-acid.

Figure 2 shows the two principle ways that commercial lithium cells are packaged; i.e., “prismatic” and “cylindrical” construction.  The word “cell” refers to any package with its own positive and negative electrical terminals.  Cylindrical cells are cylindrical in shape, within their own case.  Cylinders are a physically strong shape and are relatively easy to temperature regulate.  Prismatic cells are composed of thin layers of “soup” folded and sandwiched together and packaged into an aluminum or steel container.  Prismatic cells are easy to handle and take less space than packs of cylindrical construction of the same aHr capacity.  In packs made up of cylindrical cells, if one cell fails, the pack will continue its useful service, albeit at a lesser reduced capacity.  If one cell inside a prismatic package fails, that failed cell may “take out” sibling cells and the entire prismatic package may fail.  Cylindrical cells are less expensive to manufacture and are more durable than prismatic cells in some applications.  In automotive, RV and marine applications, where the platform is subject to constant mechanical shock and vibration, cylindrical cells are more durable than prismatic cells.  Prismatic cells can be utilized effectively in these applications, but measures must be taken to mitigate and absorb mechanical shocks and vibration.

Battery Terminal Voltage and Amp Hour (aHr) Capacity

Figure 3 shows that the more chemical “soup” in a pack of cells, by volume, the more energy that pack can store: i.e., “physically bigger pack, more Amp Hour (aHr) capacity.”   LiFePO4 “soup” contains 220 Wh/L of energy storage capacity, so liters equate to stored energy.   In describing battery construction, the battery industry describes the number of cells-in-series, followed by the number of cells-in-parallel; i.e., “2S2P.”  Four 3.2V packs stacked in series (4S) yield a nominal “12V” battery.  Packs can be stacked in series to create 12V, 24V, 48V, and greater nominal voltages, suitable for use in a wide range of applications.

Four 3.2V, 100 aHr cylindrical-cell packs (or prismatic cells) stacked in series (4S) makes a 12V battery of 100 aHr capacity.  Eight 3.2V cylindrical-cell packs (or prismatic cells) configured with two in parallel (2P) and four pairs in series (4S) results in a 12V battery of 200 aHr (4S2P).  So, in a 4S4P configuration of 100 aHr cells, the result would be a 12V battery of 400 aHr capacity, and in a 4S6P configuration of 100 aHr cells, the result would be a 12V battery of 600 aHr capacity.

Lithium Chemistry Batteries

Compared to lead-acid chemistry batteries, lithium chemistry batteries are:

  1. more volatile in response to internal temperatures, so are:
    • extremely sensitive to high and low ambient temperature conditions,
    • more sensitive to rate-of-charge and rate-of-discharge,
    • more sensitive to “thermal runaway” events;
  2. more susceptible to permanent damage caused by a very deep state-of-discharge;
  3. more subject to permanent damage from a “short circuit” accident;
  4. more subject to voltage drift between cell-packs of the battery, so
    • subject to both High Voltage Events (HVE) and Low Voltage Events (LVE);
  5. have unique/different charge and discharge characteristics and needs, and are
  6. more susceptible to physical shock and vibration

Figure 4 depicts the construction of a completed lithium chemistry “battery,” shown surrounded by the heavy blue border, box-labeled “Battery Assembly.”

All Lithium Ion batteries need a BATTERY MANAGEMENT SYSTEM (BMS) to manage their electrochemical behavior.  The BMS is a package of electronic sensors and circuits that ensure the chemical cells remain within safe operating limits.  The BMS monitors charge and discharge currents, internal temperature, and voltage variations between packs. The BMS manages voltage equalization between packs.  With lithium batteries, owners must be aware of what the BMS is doing, and why it’s needed.

The battery “assembly” shown in Figure 4 is representative of a single, large-capacity, lithium chemistry battery composed of four prismatic cells.  This option is practical for cruising boats in 2021.  A single 600 aHr capacity lithium battery (cylindrical or prismatic) is approximately equivalent to the usable capacity of a 1000 aHr lead-acid battery bank.  Prismatic 3.2V cells are widely-available, commercially-manufactured components.  The BMS is available as a commercially-manufactured electronic component, which optionally may come equipped with an Alarm Warning feature.  The disconnect solenoid (the BMS Disconnect switch) is a commercially-available component.  The battery “case” must provide mechanical protection to the packs, but can be as simple as a plastic milk carton with closed-cell foam cushioning or range to a custom-made fiberglass container.

This overall “solution” amounts to a “kit.” (Yea, “Heathkit!”) The kit’s designer (and general contractor) must specify the bill-of-materials, source the components, order them and install them.  The kit installer can be a boat builder, boatyard mechanic, independent consultant or DIY owner.

Pro Tip: When sizing batteries to match the “electrical system” power requirements of a boat, remember that both lead-acid and lithium chemistry batteries lose aHr capacity as they age.  That loss can be as much as 20% of the rated capacity of the batteries (0.2C).  Plan accordingly.

Balancing the Cell Packs

In the “kit” described above, nominal 3.2V packs are made up of paralleled cells.  “New” packs from the supplier can be at a different SOC, so the actual pack voltage can range anywhere along the nominal LFP voltage curve.  Pack voltages must be “balanced” before being placed in service, to equalize voltages variations.  Packs can be either “Top Balanced” or “Bottom Balanced.”

Figure 5 shows the open-circuit voltage curve of a 3.2V lithium chemistry cell at different SOCs.  LFP cell voltage remains very flat from maximum-charge to maximum-discharge, and Peukert’s Factor has little effect at high levels of discharge current.  For the “kit” builder, the “out-of-the-box” task is measuring the terminal voltage of each pack when delivered.  New cells should be nearly the same voltage; ex: 3.2V ± 100mV.

Because the voltage discharge curve of Lithium chemistry cells is very “flat” from “full” to “empty,” any “brand new pack” could be at 70% SOC and any other “brand new pack” at almost the same voltage could be at 30% SOC.   Early in “kit assembly,” the individual packs would not yet have a BMS to protect them.  The assembly-time risk is permanent damage to a brand new pack by accidental over-charging or over-discharging.  This IS NOT a warranty claim; manufacturer’s consider it “abuse.”  It’s a very costly “user error.”

Figure 6 is another view of the pack-to-pack voltage variation, with four nominal 3.2V cells at slightly different SOC voltages.  Wired in series while charging or discharging, these unevenly balanced pack voltages would rise and fall by the same proportional amounts, in response to the current flowing through them.  The relative voltage differences would remain as pack SOC increased or decreased.  Charging, when the lowest voltage cell reached full charge, the three higher voltage cells would be in the over-voltage danger zone (overheating, O2 outgassing).  Discharging, when the highest voltage cell reached its discharge low-voltage limit, the lower three cells would be in the under-voltage danger area (cathode disintegration).  To avoid over-voltage or under-voltage cell damage, “kit build” packs must be “balanced” during assembly.  In consideration of the length of this article, I’ll send readers interested in the details of the balancing process to Youtube.  This is a link to a Youtube video I think explains balancing quite well (Russian accent excepted).

The “Dark Ship” Moment

Lithium chemistry batteries are all fit with a BMS, which is intended to protect the battery.  If the BMS senses a high or low temperature, a High Voltage Event (HVE), a Low Voltage Event (LVE) or an over-current event, it should ideally emit an Alarm Warning Signal.  After a brief delay, the BMS will disconnect the battery from the host DC system.  Reliability of the host “electrical system” is critically important!  The BMS is there to protect the battery; it does not consider potential adverse impacts to the host vessel in its disconnect decisions.  Depending on the configuration of the individual boat’s DC electrical sub-division, a BMS Disconnect can be an “everything just went dark” moment.  Knowing the impacts of a BMS Disconnect event to the host DC “electrical system” is the responsibility of the system designer (and boat owner).

Figure 7 depicts the situation where the BMS detected a safety threat to the battery.  The BMS initiated a BMS Disconnect event.  The Disconnect Solenoid is an internal part of the battery assembly.  When it opens, everything powered by the battery simultaneously loses DC power.  Many engines will shut down, and their instrument cluster lights and gauges fail!  The RADAR, Chart Plotter, Autopilot, Depth Sounder, VHF, SSB, AIS, electronic throttle and steering controls (fly-by-wire) all fail.  The horn, foghorn, windshield wipers and inverter are in-op!  Space lighting failed.  The silence is deafening!  The lithium chemistry battery pack is safely protected, but the ship is dead in the water.  The rudder position is whatever the rudder position was.  “Dark Ship” events are possible, not inevitable!  They don’t happen often, but they can and do happen.  Good “Electrical System” design is what moderates/prevents them.

Pro Tip: A separate, stand-alone lead-acid bank that provides DC for critical Nav equipment can moderate the “Dark Ship” impact, but also adds complexity to the vessel “electrical system.”

Lithium chemistry batteries made up of ganged prismatic 3.2V packs, including BMS, are usually “repairable” after a failure via parts-replacement.

Pro Tip: “cheap” is not a good word to describe a BMS.  Cheap electronic parts are cheap for a reason; they’re made of cheap components.  A BMS failure is a big deal.  Buy for quality!  The longer the warranty, the better.  Remember, the BMS is internal to a “drop-in” battery’s case.

“Drop-in” Lithium Chemistry Batteries

Figure 8 shows three individual “drop-in” lithium chemistry battery assemblies wired in parallel to provide DC power to the boat.  These “Drop-in” batteries are composed of cylindrical cells made up into 3.2V packs.

This paralleled LFP configuration is analogous to multiple, paralleled, deep cycle lead-acid batteries.  Today (1Q2021), commercially-available 12V “drop-in lithium chemistry solutions,” are available in the market.  “Drop-in” batteries are made by several companies, in the size and shape (“form factor”) of Group 27, Group 31, and even 8D 12V lead-acid batteries.

Assuming each of these lithium chemistry “drop-in” batteries has a rated capacity of 200 aHr, this bank would have a total capacity of 600 aHr, equivalent to the earlier prismatic example.  If one BMS detects a fault, that individual battery would drop itself offline.  Depending on the cause of the fault, it may or may not be possible to return that battery assembly to service.  The individual components of “drop-in” batteries are not separately serviceable.  If internal BMS electronics or other internal battery component fails, that battery assembly would become a toxic waste “brick.”

In preparing this article, several manufacturer websites were queried for information on currently-available “drop-in” batteries and system solutions.  Some highlights:

      • VictronEnergy® seems by far the most “thorough and complete” of all the manufacturers in lithium battery SYSTEM technology.  They provide a complete solution, including the batteries, BMS, monitoring components and battery charging options.  In this solution, several monitoring system components are widely distributed throughout the host “electrical system.”  These components were not previously necessary in lead-acid systems.  Victron® appears compliant with the current content of the ABYC TE-13 Technical Report, and their BMS does provide a pre-alarm prior to BMS shutdown.  The pre-alarm is utilized by their own add-on components via a proprietary networking arrangement.   The possibility of a “dark ship” event is not eliminated in this system’s solution design.  This solution is rolled out as individual parts and is currently in “beta test.”
      • Battle Born® (DragonFlyTM) is not TE-13 compliant at this time (1Q2021).  They provide only the “drop-in” batteries, and not system-wide monitoring equipment or solutions where multiple batteries would be co-resident.  They point users at Victron® accessory components in some areas, but for monitoring, not automatic control of their “drop-in” units.  Controls are still manual, the owner/operator’s responsibility.  They say they are working towards compliance “in the near future.”
      • Xantrex® seems to have at least a visual alarm on the “drop-in” battery case, consisting of rapidly blinking LEDs.  They have an external monitor that can be attached to multiple Xantrex batteries (doesn’t say how many, but at least two).  There is no indication that a battery warning from one battery is available for use in externally connected equipment or that it can be utilized by non-Xantrex equipment.
      • Renogy® does not mention any type of warning alarm in advance of a BMS Disconnect. These batteries cannot be connected together in series (two 12V batteries in series to create a 24V system).

Protecting Alternators

For the designer/installer/owner of a lithium chemistry battery, it’s important to realize that one of the most essential components in the entire “electrical system” is the engine’s alternator(s).  Alternators do their work through the magic of magnetic fields.  The field winding of the alternator is a mechanically-driven, spinning electromagnet.  The electromagnet gets its electrical strength from a voltage regulator.  As the electromagnet spins, it induces an electric current in the alternator’s stator windings.  The induced 3-phase AC voltage is converted to DC output by an internal solid-state rectifier diode pack.  The primary electrical load on a working alternator is the battery/bank that the alternator is charging.  LFP battery load (charging current) can be very large, so the magnetic field generated in the stator winding is very significant.

Pro Tip: it is always a good idea to install alternators fit with external voltage regulators on boats.  This simple configuration alternative is appropriate to both lead-acid and lithium chemistry batteries.  External regulators have user-selectable, technology-specific, multi-stage charging programs, and are able to monitor alternator case temperatures.  In response to high case temperature, the voltage regulator modulates field drive to protect the alternator from overheating.

Lithium chemistry batteries accept very large charging currents, so “electrical system” designers assume that alternators will be working towards their maximum capacity throughout most of the battery-charging cycle.  Lithium chemistry batteries charge in “Bulk” mode to 99% SOC.

Pro Tip: charging programs for shore power chargers, engine alternator voltage regulators and solar controllers must be set so the maximum B+ voltage stays beneath the HVE voltage disconnect specification for attached BMSs.  Lithium chemistry (LFP) has a High Voltage Cutoff of ~14.6V.  An HVE can occur because of internal pack drift (pack-to-pack voltage variations) or it can be caused by an externally impressed charging voltage coming from an external charging source.  Obviously, the charging source itself must not become the cause an HVE disconnect.

Alternators must never be disconnected from their load while they are producing output.  Sudden loss of the battery connection to an alternator is usually catastrophic to the alternator.  A BMS Disconnect event is exactly that: a “sudden alternator disconnect” in which the alternator output current is suddenly interrupted.  This results in the sudden collapse of the magnetic field in the alternator’s stator windings.  The collapsing magnetic field results in a huge voltage spike in the alternator windings.  Disconnected from its load by the now open BMS Disconnect solenoid, there is no place for the spike energy to go to be safely absorbed.  That spike voltage either destroys the alternator’s rectifier diodes or damages the insulation of the alternator’s stator windings, or both.  And that’s not all of the bad things that can happen!

In a typical boat “electrical system,” the alternator is one of many device attachments to the B+ buss fed by the battery/bank.  Also attached to the B+ buss is all of the rest of the boat’s DC equipment which was simultaneously in use at the moment of BMS Disconnect.  Some of that equipment is “delicate electronic equipment” (RADAR, Chart Plotter, Autopilot, depth sounder, VHF, AIS, inverter, solar controller, etc).  The spike that kills the alternator is also impressed onto the boat’s B+ Buss, so all equipment attached to that buss is also at risk of damage.

Pro Tip: it is always good to install a surge bypassing diode at any alternator on any boat.  Surge Protective Devices (SPD), also called Transient Voltage Suppressors (TVS), are used to shunt high voltage spike energy to ground, thus also protecting equipment connected to the buss.  While “good” to do, this strategy is not a 100% reliable damage mitigation solution.  Depending on a range of variables, the spike voltage can exceed the capacity of commercial DC SPD products.  SPDs that do successfully suppress transient spikes can be left in a partially or totally “damaged” state by the event, and may then not function correctly, or at all, in a future event.  So this strategy, while useful, is not “sufficient” to the overall goal of protecting alternators.

Figure 9 is a simplified view of a typical alternator application which includes an SPD.  The alternator field coil gets its “exciting” DC voltage from a voltage regulator, shown here as external to the alternator case.

A BMS disconnect scenario in this large-capacity, single battery bank would start with a problem detected by the BMS.  The BMS would send out an alarm signal, indicating an imminently pending disconnect event.  Good system design would then use that warning alarm signal to shut down the alternator’s field drive by shutting down the voltage regulator.  That design removes field drive from the alternator, so it shuts down gracefully, without damage to the alternator, the protective SPD or any sensitive buss-attached equipment.

After a momentary delay, the BMS Disconnect solenoid would open, and the vessel would sustain a “Dark Ship” event.  Bad enough, true, but at least the alternator and electronics attached to the B+ buss wouldn’t be damaged as a result of the disconnect event.

A key assumption in this scenario is that the BMS has the alarm warning feature.  Not all BMS designs have that capability, so the DIYer specifying a BMS for a “kit build” has to know its available, needed and, indeed, essential to the integrity of the “electrical system.

Solar Controllers

The preceding discussion, “Protecting Alternators,” applies equally to “electrical systems” containing solar controllers.  Solar controllers receive a high and moment-to-moment variable input voltage from solar panels and convert that input voltage to “battery charging voltage.”  In doing so, they act as chargers for attached batteries.  A sudden BMS Disconnect disconnects the battery from the B+ buss, but leaves all other DC equipment attached.  If the battery – in this case, the load on a charging source –  is suddenly disconnected, solar controllers can be damaged in a manner similar to the way alternators are damaged.  This can happen in two ways.  One, by an alternator spike event.  But also, by sudden voltage imbalances within the controller caused by the sudden loss of load.  Mitigation of this potential problem is important and needs to be considered in the overall design of the “electrical system.”  This is another case where a BMS Alarm Warning system can be used to gracefully shut solar controllers down without damage.

Pro Tip: in the solar industry, the guideline is to cycle lithium batteries between 20% and 80% SOC to improve battery longevity.  That is also a useful strategy on boats, which carries a potentially huge hidden benefit.  When charging less than max-high, and discharging less than max-low limits, it’s much less likely to encounter an HVE or LVE.  Even if one of the packs of the battery (or one of a “drop-in” set) is, or has become, unevenly balanced, its much less likely that condition will result in a HVC or LVC BMS Disconnect.  With an adjustable voltage charger, for 12V lithium chemistry batteries, select max-charge and max-discharge voltages to cycle the battery (battery bank) between 20% and 80%  (This is also suitable for boats in long-term layup.)

Electrical System Reliability With “Drop-in” Lithium Chemistry Batteries

Figure 10 shows three individual lithium chemistry “drop-in” batteries wired in parallel to form a single bank of suitable capacity to power a cruiser’s boat.  This configuration also carries the need to protect the alternator.  This multiple-battery configuration may not be able to meet that goal.  It does not have the means to shut down the alternator in a graceful and protective way, and there are other concerns with this approach as well.

Individual “drop-in” batteries have their own internal BMS, but today, only Victron Energy® has a “whole system” management solution that controls an entire battery bank of multiple “drop-in” batteries.  The “marketplace” for “drop-ins” is “all over the place” in design maturity.  Promotional literature is sometimes/often misleading.  Buyers MUST do technical “due diligence!”

Considering “system reliability:”

In a 12V system configured as shown in Figure 1, both nav equipment and inverter are powered from the same battery bank.  With LFP “drop-ins,” wholesale replacement of lead-acid batteries reprises the “Dark Ship” scenario.  Moderating that scenario requires DC sub-system reconfiguration prior to conversion.

In a deeply-discharged 12V configuration, one disconnected “drop-in” battery will increase DC load on its peers and can cause others to disconnect in a cascade-type scenario.  Would the owner/operator know that just one paralleled battery had dropped out?  How?  At a minimum, the bank would lose aHr capacity.  What is the system reliability impact of reduced aHr capacity to the system, even over a short time?  Many uncertainties and design concerns.

With “drop-in” batteries, the concept of “voltage balancing” still applies.  Consider a 24V system with 12V “drop-in” batteries in series.  New “drop-in” batteries can vary in SOC.  What’s going to happen if series batteries are slightly uneven, or slightly under-capacity for the installation?  If just one of several “drop-in” batteries in any configuration were to suffer a BMS disconnect event, there would be some impact to the host “electrical system.”

Cruising boats do not have uncomplicated, fast, simple escape and assistance scenarios.  Boat needs must be evaluated based on the nature of boats operating underway, at some distance from land.  Today, there aren’t good solution designs for the use of multiple, series-parallel “drop-in” batteries in a large capacity bank that meet the reliability needs of the “electrical system” on cruising boats.  The impact to the host “electrical system” of any one individual battery having a BMS disconnect is indeterminate, unpredictable, unique to each boat’s “as-is” wiring configuration.  BMS Disconnect events always come associated with risks.

An Alternative Way To Protect the Electrical System

Figure 11 shows a battery bank with a lead-acid chemistry battery wired permanently in parallel with one or more lithium chemistry batteries.   These two dissimilar chemistry batteries become one, single bank, charged and discharged as one.  They are NEVER separated in service (except for periodic maintenance activity where they need to be physically removed from service).  Operationally, they are the right and left hands of the same individual organism.

Good “electrical system” design is that the lead-acid battery remains connected to the B+ buss following any BMS Disconnect to “absorb” any voltage spike.  This design actually results in several significant “system reliability” benefits:

    1. The larger the capacity rating of the lead-acid sibling, the better spike energy would be absorbed to mitigate other severe side effects.
    2. Even if an attached SPD were driven into conduction, the chances of it surviving to fight another day are improved by the absorption behavior of the lead-acid sibling.
    3. With a permanently paralleled lead-acid battery as part of the bank, there would be at least some available “DC reserve power” to seamlessly continue powering the vessel’s engine electronics, nav electronics, fly-by-wire controls, lighting and safety equipment following a BMS Disconnect event.  That clear benefit does mitigate the negative consequences of a total “Dark Ship” event.


    Pro Tip: The “hybrid” schema shown in Figure 11 can be used with either a single, large-capacity lithium chemistry battery or with a battery bank made up of multiple lithium chemistry “drop-in” batteries arranged in parallel.  If the boat already has serviceable lead-acid batteries, keep them.  Simply install lithium “drop-in” batteries in parallel with the existing lead-acid set.

    How A “Hybrid” Battery Bank Works

    Note:  Later in this discussion, readers are referred to a Youtube video further describing this “hybrid solution.”  To align the two discussions, I’m using “typical” voltages as used in the video; i.e., the “equilibrium state” of a normally charged LFP produces a terminal voltage of ~13.2V, and the “equilibrium state” of a normally charged PbSO4 produces a terminal voltage of ~12.6V.

    The “natural resting voltage” of a lead-acid battery is measured 24 hours after the battery has been fully charged and fully disconnected from all load circuits and from its siblings in a bank (four hours for LiFePO4 chemistry batteries).  That 24 hour “period of total isolation” gives electrons and ions (charged atomic particles) time to reach equilibrium.  The term “natural resting voltage” describes an electrochemical state where the sub-atomic charges in the battery materials have all “found a home” in the atomic structure of their host chemical compounds.  At the atomic level, they are “in equilibrium;” “resting,” not floating about, and not “flowing” anywhere.

    Observe, then, that lithium chemistry batteries in the normal range of charge have an operational terminal voltage that is very close to the nominal “float voltage” (13.3V) of lead-acid batteries.  Once connected together into a bank, absent fault events, a lead-acid sibling rarely has any work to do in the bank.  Compared to the lead-acid sibling, the voltage discharge curve of the LFP sibling is flatter and stays at a higher voltage level throughout most of its usable discharge cycle.  It simply lives there, symbiotically, neither contributing nor burdening.  The lithium chemistry sibling “does the work” of providing DC power to the boat.  The lead-acid sibling will enjoy a relatively long service life because to it, there are rarely any discharge cycles, and its LFP sibling(s) continuously maintains it at its “float” voltage.

    Pro Tip: this lithium/lead-acid hybrid pair can be charged in the same way – and with the same charging equipment and charging voltage profile – as a lead-acid AGM battery.

    Making the Connection

    Connecting lead-acid and lithium chemistry batteries together in parallel can be a non-trivial technical chore which requires some explanation.

    When paralleling two lead-acid batteries, a small spark may occur when the final connection is made.  Fully charged, their terminal voltages and internal charges are nearly identical.  In that case, there will be no significant difference in the electrical charge between them.  Paralleling fully-charged lithium chemistry batteries is similarly non-dramatic.

    When paralleling a LiFePO4 chemistry (13.2V) and a PbSO4 chemistry (12.6V), the difference between the two natural resting voltages isn’t large (600mV), but the amount of electrical charge it takes to “equalize” that difference is actually very significant.  Simply connecting them will result in a dramatic, possibly damaging, spark and a prolonged high current flow from lithium to lead-acid.  To the lithium sibling, the outrush current looks like a low voltage “short circuit.”  The goal here is to get these two dissimilar chemistry batteries paralleled together so that a “surface charge” is built up on the plates of the lead-acid sibling.

    “Surface charge” is a very well understood phenomena with lead-acid batteries.  With fully-charged lead-acid batteries, “surface charge” occurs because there is no longer an available vacancy for free electrons to occupy within the atomic lattice of the lead plates.   When any external charge source is connected to a fully-charged lead-acid battery, a “surface charge” of electrons collects on the surface of the negative plate.  “Surface charge,” alters the “natural” resting equilibrium of the fully-charged battery to a new, and “unstable,” state-of-equilibrium, caused by the presence of the externally impressed voltage applied by the charging source.  In the automotive world, this is exactly what a portable lithium “jump start” pack does to a “dead” lead-acid car battery.  That is, the charging pack delivers electrons to the dead battery faster than the battery can absorb them, and the electrons collect on the surface of the lead plates.

    So, when initially connected together, the lithium chemistry “charging source” instantaneously tries to pump a large current into the lead-acid “load.”  That “outrush current” flowing from the lithium to the lead-acid battery is seen by the BMS as a very large current, and the BMS may trip “off” to protect the lithium battery from damage.  Whence the BMS resets itself, that cycle may repeat.  The result is that the two dissimilar batteries couldn’t be successfully paralleled.

    Enter now, the humble power resistor, such as this one from
    Think of this resistor as an “installation tool;” a “tool” used at initial setup time.  Like all tools, it’s taken out when needed, used, and put away until it’s needed again.

    The purpose of the resistor is to limit the “equalizing current” so the BMS doesn’t trip. When initially connected, Lithium at 13.2V gives up electrons to its lead-acid sibling at 12.6V. The resistor is placed “in series” in the conductor connecting the two batteries to “limit” that flow of current to a level acceptable to the lithium electron-donor’s BMS.  As arriving electrons build up “surface charge” on the lead-acid plates, the terminal voltage of the lead-acid sibling rises, the voltage difference between the two different chemistry batteries falls, and so the current creating the surface charge in the lead-acid sibling simultaneously falls.

    The surface charge on the lead-acid chemistry battery does not dissipate instantly when the charging source is disconnected.  Since, at initial connection time, there is no electrical system connected to the bank, there is no path into which “surface charge” is able to “bleed.”  So the installer of these connections has enough working time to disconnect and remove the resistor and connect the batteries directly in parallel while that surface charge remains intact.  Since the normal terminal voltages of the lithium sibling (~13.2V) and the lead-acid sibling now with its surface charge established (~13.2V) are equalized at the same (or very nearly the same) voltage, the batteries can be connected directly together without drama or fireworks.

    To initially connect lead-acid batteries into a lithium chemistry bank, the installer will need a large wattage resistor; in the range of 50Ω, 100W.  The bigger the resistance, the lower the current (Ohm’s Law), so the longer the equalization process will take, but the “more controlled” it will be.  Warning: the resistor may get hot to the touch!  The resistor would be needed any time the two dissimilar chemistry batteries need to be connected/reconnected together, so for example:

      1. when first assembled together, or
      2. reconnecting them after a BMS disconnect event, or
      3. when emergency cross-connecting another bank, such as a lead-acid “start” bank, or
      4. after any maintenance activity on the battery bank, like cell/battery replacement.

    For more information on hybrid banks and connecting dissimilar chemistry batteries, review this Youtube video.  The presenter talks through the above entire scenario in detail.  Clark Willix is a boater with many years of liveaboard experience and an engaging speaking style.  He has a 1500 aHr lead-acid bank of L-16s, so way, way bigger than most of us.  But that said, he presents sound conceptual descriptions.

    Pro Tip: whether the above procedure is necessary at all will depend on the relative charge capacities (aHr) of the lithium and lead-acid siblings.  A relatively smaller capacity lithium sibling connecting to a relatively larger capacity lead-acid sibling is more likely to trip the BMS.  A relatively larger capacity lithium sibling connecting to a relatively smaller capacity lead-acid sibling could work without significant drama with a suitably-rated battery paralleling switch.  Arc suppression at the switch will prolong the service life of the mating surfaces of solenoid or switch contactor points.

    Once connected together, there is a permanent, small circulating current (the IFloat current for the lead-acid sibling).  The dissimilar natural voltages are “equalized” between the two dissimilar chemistries, and the lead-acid float current load is very low between them.  That IFloat current is a “Parasitic Load” to the lithium chemistry battery.  Parasitic loads consume power and are oft forgotten.  On boats, “always on” equipment can be viewed as a “parasitic load,” such as digital radio station memories, digital clocks, smoke alarms, intrusion alarms, doorbell cameras, CO/COdetectors, propane gas detectors, bilge pumps, etc.  Parasitic loads can “kill” batteries.  Readers are encouraged to view this “float current parasitic load” as a small price to pay in return for a potentially huge reliability benefit (reward!) provided to the boat’s “Electrical System.”

    Pro Tip: a useful device to install on all boats is a “boat monitor,” such as the Siren Marine “MTC,” or equivalent.  The device notifies boat owners by text message and/or email the moment shore power is lost.  They monitor system B+ voltage, and can notify owners of low house battery voltage.  These devices can be viewed as “parasitic loads,” because they are “always on,” and their presence forgotten, even when batteries are not being charged.  However, these devices can save thousands of dollars in damaged batteries.  A text that reports lost shore power indicates there’s a problem at the boat, and a timely phone call to the boatyard or marina can avoid/save great inconvenience and unnecessary costs.

    Pro Tip: plan ahead for lengthy dockside stays and long-term boat storage lay-up periods.  The lithium industry does not have good solutions for managing “float” charging on shore power.  Lithium chemistry batteries do not like to be left fully charged unless that is part of ongoing discharge and charge cycling, and they are subject to self-discharge when chargers are discontinued.  Owners of boats hauled and left in storage for some months, or otherwise unattended, must make provisions for the care of lithium chemistry batteries.  Lithium solutions in 1Q2021 are for cruisers, not marina mavens.  Plan to leave LFPs between 50% and 70% charged, and NOT on a full-voltage trickle charger.  With proper equipment (adjustable-voltage charger), one can treat lithium chemistry batteries as in an off-grid solar application, cycling them between 50% and 70% SOC.  Maybe with a solar panel and suitably adjusted solar controller…

    Pro Tip: a design requirement for any BMS is that the BMS itself must be disconnected from the battery in the case of an LVC event.  If the BMS stays connected to the battery AFTER an LVC, the BMS itself becomes a parasitic load.  In an unattended situation, like summer layup, that small continuing parasitic load will, itself, lead to permanent damage to (destruction of) the host battery.

    Technical Guidance on Lithium Chemistry Batteries

    ABYC has published a “Technical Report” entitled “Lithium Ion Batteries” (TE-13, July, 2020). That document is not yet an “ABYC standard” to which boat builders, equipment builders, technicians, and surveyors are expected to adhere, but it’s an important guidance document for those interested in lithium chemistry battery solution design.  Some quotes:

    13.5.6 If a shutdown condition is approaching, a BMS should notify the operator with a visual and/or audible alarm prior to disconnecting the battery from the DC system.

    13.9.4 HVE/HVC/LVE/LVC – A BMS should protect the lithium ion battery cells in response to an HVE and an LVE. Means of protection should not disconnect critical loads without prior warning and should not stop the charging source in a manner that causes damage to the charging device.

    EMPHASIS ADDED in is the authors.  “How” this good guidance is accomplished today remains at the discretion and responsibility of the designer of the  “electrical system,” through specification of equipment in the technical specification of the bill-of-materials.

    My interpretation is, if there is an impending High Voltage Event (High Voltage Cutoff) or Low Voltage Event (Low Voltage Cutoff) – or if there were an impending High Temperature Cutoff – the BMS SHOULD provide a warning in advance of a BMS Disconnect.  The ABYC document does not specify the lead time in advance of the disconnect or the duration of the pre-alarm signal, but this would be something to look for in the manufacturer’s spec sheet before buying a BMS or before buying any of the commercially available “drop-in” batteries.

    There is no specific guidance in the ABYC document on a need for a single “Operation Over-Watch” or “Operation Overlord” BMS intended to protect lithium chemistry systems made up of multiple individual 12V/24V “drop-in” units.  But, here is what the document does say (again, emphasis is mine):

    3.5.1 All lithium ion battery systems should have a BMS installed to prevent damage to the battery and provide for battery shutoff if potentially dangerous conditions exist.
                NOTE: BMS can be external or internal to the battery.

    Clearly, a battery bank composed of multiple “drop-in” LFP batteries does constitute a “lithium ion battery system.”  In 1Q2021, the BMS component(s) needed to accomplish 13.5.1 with a bank of “drop-in” batteries, and the engineering of the “drop-in” batteries themselves, is/are not widely available in the marine market as commercially-manufactured offerings.

    Article Summary

    Lithium-Iron-Phosphate battery alternatives (using prismatic cells) are practical on boats in 2021.  “Drop-in” LFP alternatives exist, but systems to control them are immature and incomplete.

    This article has focused on boats used for long-distance cruising, characterized by frequent, repetitive battery cycling profiles (charge/discharge cycles) and a very high dependency on reliable, always-available “electrical systems.”  The complexity of LFP conversion and operation isn’t technically justified in 2021 for those who’s preferred lifestyle involves traveling from marina to marine or anchors out only occasionally.  The industry is still trying to figure out the best way to handle extended time on shore power; i.e., boats in infrequent, day-use or prolonged periods at dockside on shore power (boats used as seasonal condos).

    Readers that have reached this summary have seen that lithium chemistry battery installations are complex and do have some potentially negative technical impacts.  The net is, as of 1Q2021, there is not yet “expert consensus” on the “optimum design” of a lithium-based SYSTEM; that is, configuration variants that need to be supported and how those variants can, should and need to be controlled and protected for their safety and for the overall SAFETY and RELIABILITY of the host vessel and its crew.  A “dark ship” event is a super big deal, even on a sunny afternoon in uncrowded waters.  It would be a particularly unwelcome event coming into a lock on the Tenn-Tom Waterway or the Waterford Flight on the Erie Canal, or coming into Barnegat Inlet or Ft. Pierce inlet at max ebb (running from a thunderstorm), or crossing the Columbia River bar any time!   We boaters do not yet have the control systems and equipment that will be the “necessary, normal, safe and reliable” platform in just 5 – 10 years time.

    Lithium chemistry systems are, in 1Q2021, an “emerging technology;” NOT YET an “install and forget” platform.  Engineers, manufacturers, boatyards and electrical technicians continue to learn daily from the failure events experienced by “early adopters.”  There are still no accepted industry standards for lithium chemistry “systems,” and the existing UL Standards for lithium battery cells refer to batteries with very small amounts of lithium metal (<5 grams), as found in cell phones and tablet PCs.  There is still way too much “technical stuff” for individual boat owners to have to know.  There are still too few technicians trained to work on lithium systems.  Costs are still very high.  Mistakes are very expensive.  Lithium batteries are subject to permanent damage if “abused.”  Long service lifetime claims – although better than lead-acid batteries – are not borne out in “real life” by “early adopters.”   LFPs should not be installed in engine rooms (heat).  These systems will probably not fail where they were built and installed, so it’s likely the owners, or someone the owner has to locate and hire, will have to deal with any future failure event without benefit of the original builder’s design assumptions, drawings, documentation or assistance; a scenario which leads to “system re-design on the fly…    …at lots of additional cost…”

    A quote from an experienced friend who’s made this transition: “1) Don’t do it just because you can, and 2) if you do, be damn sure you understand how to do it right.”

    The net is, it’s on you as the buyer/owner/operator to ensure your lithium chemistry platform is as reliable from a design point-of-view as it can be before accepting it from the builder.  That means you need to know a lot about lithium technology and “electrical systems.”  It’s also on you to have current electrical diagrams and manufacturer names, part numbers, user manuals and contact information, on-board and available, for all components of the entire system.  If that system needs “repair” at some future time in some exotic cruising locale, you’ll at least have the reference documentation you’ll need to get help.

    For the Serious DIYer

    Serious DIYers interested in comparing lithium batteries to lead-acid batteries should view this analysis.  A very clear lesson that emerges from the data presented here is the cost effectiveness of flooded wet cells compared to AGMs.

    For those who are truly interested in lithium chemistry batteries, and also have SIGNIFICANT DIY ELECTRICAL SKILLS, TECHNICAL CURIOSITY, TOLERANCE FOR POSSIBLE DISRUPTION, A CRUISING BOAT USE PROFILE and at least SEMI-DEEP POCKETS, here is a reasonable 1Q2021 lithium chemistry platform option for you.  In my opinion, this is the most cost-effective way to move to lithium technology in 1Q2021.  I would only undertake this conversion if I planned to own the boat for 10 – 12 more years.  In less time, there is essentially no opportunity for ROI, and the remaining advantages of lithium chemistry over lead-acid are less appealing without ROI as a reward.  Lithium technology has great potential; but that potential is still a few year away for the needs/wants/desires of the “install and forget” buyer/owner.

Weather – To “Go” or “Not To Go”

11/22/2020: Initial post
10/23/2021: Editing/formatting

Whether the weather is hot or whether the weather cold,
Whether the weather is wet or whether the weather dry,
Whether the weather is windy or whether the weather is calm,
Whether the weather is nice or whether the weather is snot,
There will be weather, whether or not.

The US East Coast ICW from Maine to Florida, or the Great Loop cruise, are long cruises that span significant geography in eastern North America. Weather conditions encountered by long distance cruisers will range over time from “delightful” to “severe.”  Annual El Niño and La Niña conditions in the eastern tropical Pacific Ocean (ENSO), and the Madden-Jullian Oscillation (MJO) in the Western Pacific and Indian Oceans, can lead to significant year-to-year variations in overall North American weather patters.  Regional patterns in the Southern US are different than in the Northern US and Canada.  Year-to-year variations are a fact of life.  There are dry years and wet years. There are calm years and stormy years.

Summer T’storm Out!ow Boundary produced 50-plus mph gusting winds for 40 minutes. Could not see the shoreline, barely could see the blue-roofed MYC Pavilion from our boat. Very exciting!

In general, I discourage the all-to-common notion that a fast boat can “run away from” – or, “outrun” – developing thunderstorms.  This is NOT a good assumption on large bodies of water where there are limited “ditch-out” options to get off the water.  It is NOT a good option for running offshore or running large Bays and Sounds.  The air masses that produce thunderstorms can be hundreds of miles across.  Very large air masses can become unstable (cross the “wet lapse rate”) in minutes, and build across wide areas very quickly.  We have had severe weather blow up for 30 to 50 miles all around us in a matter of 15 – 30 minutes.  Being exposed to high winds, heavy rain and lightening in a boat on the water IS NOT part of our definition of “havin’ fun.”  We suggest prudent avoidance is much better than managing an unpleasant – or dangerous – heavy weather encounter.

Hazy, hot, humid summer afternoon on a mooring ball in the US Northeast.

We suggest that each individual cruiser establish in advance – a written criteria for the conditions that are acceptable for their routine daily departures from safe harbor.  One size does not f!t all, and no single criteria fits all cruisers. Different boat designs ride rough seas quite differently from one another. Dfferent individuals on boats of the same make and model may have very different attitudes and sensitivities (tolerance) about what constitutes “acceptable” travel conditions to them. Steelhulled (all metal hulled) boats are more safe in lightening than Fiberglass-Reinforced Plastic (FRP). Boats with active stabilizers often handle rough seas better than boats without stabilizers. Individualized vessel departure criteria must address the needs of the captain and crew, pets, any guests (children) and the vessel itself.

The US East Coast and Gulf Coast are prone to hurricanes from mid-August to mid-November.  Less of a time-span in El Niño years, more in La Niña years.  With just a few exceptions, we live in a time where weather forecasts are reasonably good indicators of future reality.  During hurricane season, we watch the Atlantic Basin and Gulf of Mexico daily.  When storms are forming and tracks are developing, we don’t depart from a safe harbor unless we know we can get to the next safe harbor well before encountering the storm.

Your individual departure criteria must ensure the safety, and consider the comfort, of all aboard.  Boat’s are generally tougher than people. We find little enjoyment in being beaten up on the water. Seasick or terri!ed guests are undesirable. A happy crew and a happy marriage depend on getting this criteria right for your crew and your boat.

Finally, the prudent captain will review the weather forecast for the daily cruise area against his or her departure criteria.  A beautiful early morning can deteriorate into a lousy afternoon. Sanctuary’s departure criteria follows:

Sanctuary’s “go”/”no go” criteria:

  • Bright sun to periodic, light rain;
  • visibility >3 StM;
  • Seas <2 ft from any quarter;
  • winds <15 kts;
  • air temps >60℉.
  • Periods of rain, no or “isolated” t’storms;
  • visibility >1<3 StM;
  • seas 2-4 ft if following, 2-3 ft if ahead, abeam or abaft of abeam;
  • winds 20- 25 kts;
  • air temps >45℉<60℉;
  • these conditions forecast to be stable or with an improving forecast.
  • T’storms;
  • strong squalls;
  • persistent rain;
  • visibility <1 StM;
  • winds >25 kts;
  • air temps <45℉;
  • deteriorating forecast.
 Additional              Considerations:
  • Direction of surface feature steering winds at altitude 18K;
  • travel on protected inland river/ICW vs. open water;
  • distance from  “safe harbor;”
  • if offshore, period and direction of ocean swells;
  • air temps;
  • hours-of-daylight;
  • seasonal wx patterns of the region;
  • availability of traveling companion (“Buddy Boat”);
  • availability of Tow Boat if needed.

Some useful guidelines in the US Northeast and mid-Atlantic states for Barometer status and trends:

  1. barometer rising & Westerly wind → good weather
  2. barometer falling & Easterly wind → perform “180° turn;” return to safe harbor
  3. barometer slowly falling & steady breeze → unsettled, likely wet, weather
  4. barometer rising → best for boating
  5. barometer falling → stay alert & watchful

Some useful resources for tracking local weather and weather forecasts include:

  1. Sirius/XM satellite “Master Mariner” subscription; provides real-time weather conditions displayed on new generation chart plotters and via WxWorx-on-the-Water on Windows-based computers. Sanctuary uses WxWorx-on-the-water.
  2. Link: Marv’s Weather Service
  3. Link: Atlantic Marine Zones
  4. Link: National Weather Buoy Data
  5. Link: Great Lakes Weather
  6. Link: Chesapeake Bay Weather
  7. Link: Severe weather
  8. Link: Local weather forecasts
  9. Link: Local weather forecasts
  10. Link: Local Weather forecasts

iOS (iPhone & iPad) weather apps I personally use and like:

  1. Storm Radar
  2. Dark Sky
  3. Marine Weather Forecast Pro
  4. Windy; pro upgrade gives several weather model wind forecasts that are particularly useful on large East Coast Bays and Sounds
  5. NOAA Buoys Live Marine Weather (Hurricane forecasts; not easy to use)

For hurricane tracks and track projections:

  1. Marv’s Weather Service
  3. Mike’s Weather Page

Prescription Meds and Personal Care Needs

11/21/2020 – Initial post

ON THE WATER, FOR ANYONE HAVING CHEST PAIN OR DIFFICULTY BREATHING, A “M’AIDEZ” CALL SHOULD BE MADE IMMEDIATELY.  LIKEWISE, MENTAL CONFUSION, SLURRING OF WORDS OR DROOPING OF FACIAL EXPRESSION, HEAD INJURIES WITH OR WITHOUT CONFUSION AND SERIOUS FALLS REQUIRE IMMEDIATE, URGENT EVALUATION AND CARE.  Plan and drill ahead of time for the possibility of a “m’aidez” call.  Know what you will need to be able to tell the USCG and rescue personnel about your location and the nature of your emergency.  Emergencies don’t care about the weather or seastate, so consider how you would handle the boat while caring for your spouse’s chest pain BEFORE an emergency happens.  Plan and drill for the sudden and unexpected possibility of being “Suddenly Alone.”  Plan for sudden disability of the Captain; plan for sudden disability of the First Mate or visiting crew member.

Prescription Medications and Medical Devices:  Always do  your own due diligence!  My Admiral and I take over-the-counter vitamins and aspirin, and prescription statin anti-lipid drugs.  I take prescription eye drops, and she takes several cardiac medications.  Well in advance of cruise departure, order a 3-month supply of all prescription medications and personal care needs.  With the proper documentation, customs and immigration officers (Bahamas, Canada) will clear them.  Without documentation, check-in delays are possible, if not likely.  It can be a problem to ship some medications across international borders, so medications that you absolutely cannot be without must be transported with you.

In the past several years, we have dealt with several different mail-order pharmacies.  They are not very good at supporting travelers.  (No, it’s worse than that; they are a superb pain in the posterior.)  We know that they can accommodate travelers, so stay calm, but be persistent.  Order early, and order only when you’re going to be in one place for long enough to wait them out.  Different medications will come from different warehouses in different parts of the country.  Be sure to update your delivery address a couple of days before you actually place the medication order.

Expect surprises.  We have gotten to the point that we call the pharmacy company a day or two ahead of placing the order just to update our shipping address.  That allows time for the updated delivery address to propagate across the pharmacy company’s computer server farm. We have had parts of our medication orders go to several different, random delivery addresses.  The pharmacy companies will always blame that on you.  They will want to use up one of your refills and will threaten to charge you full price for a re-do of the order.  We can scramble through all that, but it’s time-consuming and annoying.  If you change your permanent address before you place the order, there is a better chance they’ll get fulfillment and delivery right.  But, it’s on you to manage the process and your pharmacy company.

When cruising, be sure to have documentation aboard for any injectable medications and injection supplies that you transport.  What particularly comes to mind is diabetes medications and needles, and epinephrine injectors for anaphylactic allergic reactions.  Sanctuary’s Admiral uses a medical device called a “TENS unit.”  It’s electrical; it runs on batteries and needs electrode wires.  Carry spare batteries, and if you need/use rechargeable batteries, carry a spare charger.  Carry spare electrodes.  Get a copy of the prescription from the doctor to keep in your onboard file.  Consumable supplies for CPAP, BPAP, infusion pumps, catheters and ostomy supplies, glucose test strips and INR measurement test strips should be secured in advance of cruise departure.

Talk with your doctor before departure about having a 10-day supply of antibiotics and pain killers aboard for use in emergent situations.  If you’re cruising in some remote place, it may be a day or two or three before you can get to see a nurse, let alone a doctor.  Assuming no allergies to the antibiotic, starting an antibiotic right away may be/can be the right thing to do.  Your doctor can advise products suitable to your personal needs.  We were traveling in the salt marshes of the US southeast when the Admiral came down with an abscessed tooth.  Having a pain killer at hand definitely made us both more comfortable until we could locate a dentist.  I nicked a finger with a screwdriver, and within 18 hours, the finger was three times normal size and very painful.  I caught the wheel of a shopping cart at Walmart with my ankle.  By the next morning, my ankle was several times normal size.  Things happen.  Having a supply of Cephalexin/Ciprofloxacin was a really wise exercise in planning ahead, but it does not replace seeing a doctor as soon as you can.

Emergency Care: many pharmacies offer emergent care services intended to manage relatively minor health incidents.  They also administer flu, pneumonia, tetanus and shingles shots (shingles with a prescription, of course).  Walgreens and CVS are two national pharmacy chains we have used successfully.  These facilities are typically  staffed by a Nurse Practitioner.  The ARNP will be able to prescribe antibiotics, and handle simple conditions like skin rashes, bruises, splinters, cuts with simple stitches, burns, sprains, and similar types of injuries.  Depending on state law, an ARNP may not be able to prescribe opiate pain medications.  For initial assessment of an emerging situation, these facilities are very helpful, if available.

Free-standing emergency care facilities are also available in many areas.  These usually have on-site staff physicians.  Obviously, they can handle a wider range of health problems, including x-rays, simple blood and urine labs, and EKGs.

If you are taking responsibility for minor children (grandchildren, for example), make sure you understand any medications they need to take and whatever condition(s) those medications are intended to treat!  For minor children in the absence of their parents/legal guardians, have written and notarized parental permission to seek and request medical treatment if that should become necessary.  As we Boy Scouts are fond of saying: Be Prepared!

Earthing and Grounding

11/6/2020: Initial Post
11/16/2020: Text and Graphics added
2/9/2021: Graphics added; minor edits


This is an introductory article, written to provide a basic understanding of a complex aspect of AC electric systems to an audience with little or no prior background in electricity. This subject is fundamental to AC system wiring in buildings and on boats, and is a prominent underlying part of the discussion in many other articles about AC Shore Power found on this website.

The concepts around “earthing” and “grounding” are at the very core of making electrical systems as safe as possible to people, pets, farm animals and wildlife. But, “earthing” and “grounding” may or may not mean the same thing when used in conversations and when used without context. These subtle concepts and the terminology they involve can be new and confusing to people without prior electrical backgrounds, and are among the most important to electrical safety. “Grounds” and “grounding” are topics that embrace multiple related ideas. “Earthing” and “Grounding” have different implications in residential single-family house settings than they do on boats, and residential electricians often are not aware of issues that apply to electrical safety on boats. Context is very important to understanding these issues, and as always in electricity, there are many “language shortcuts” that occur in group discussions on docks. Boaters will benefit from an understanding of these topics.

Static Electricity/Lightening

In nature, there is a form of electricity called “static electricity.” A major characteristic of static electricity is that it “flows” outside of wired electrical circuits, through the air. Its flow is intermittent and spontaneous. Static electricity is caused by the friction of two surfaces moving across one another. Static electricity results from the accumulation of electrons on one object (“negative charge”) and a deficit of electrons on another object (“positive charge”). It occurs where friction between surfaces creates a negative charge on the surface with excess electrons and a positive charge on the surface from which electrons were taken.

Residents of low-humidity, cold climates are familiar with static electricity. Little static shocks result from walking across a carpeted room in wool socks, petting the cat or dog, putting on a sweater or overcoat, and then contacting a doorknob, another person, or a car door (or any number of similar life activities).

“St. Elmo’s Fire” is a visible ionized corona; a static electricity “charge” that occurs when conditions are still and humid. In St. Elmo’s Fire, a sphere of blue or purple ionized plasma forms at the sharp points of outdoor structures, such as electric utility towers, church spires, chimney’s, masts, spars, flag poles, weather vanes, etc.  In the far upper atmosphere, a similar phenomena is responsible for the “Northern Lights.”

In clouds, warm, rising water droplets collide with cold, descending ice crystals, causing static charge to accumulate and eventually result in lightening.  The “discharge” of static electricity is a visible flash – an “electric arc” – composed of electrons flowing through ionized air. Electrons “flowing” between two points is the definition of an “electric current.” With static electricity, a voltage difference (electric charge) between the two poles of the static system becomes instantaneously great enough that the insulating characteristic of the normally nonconductive air gap breaks down and conducts. In household situations, the arc is mainly a nuisance, although it can damage modern semiconductor electronics and the “shock,” together with an occasionally audible “snap,” can scare/surprise its animal and human victims.

Lightening is by far the most impressive static electricity discharge phenomena with which we are all familiar. Lightening is static electricity with a massive visible arc composed of many, many thousands of amps. That arc current creates many thousands of degrees of instantaneous temperature rise in the surrounding air, resulting in thunder. Lightening releases massive amounts of energy (mega-joules) and often results in severe damage at its earth contact point. Lightening is more than capable of killing animals and people.

The arc of a static discharge “neutralizes” the accumulated positive and negative atomic charge of the oppositely charged poles of the static “system.” Protecting building electrical systems from being damaged by the discharge arc of lightening involves creating a means to get the arc current to flow AROUND, rather than THROUGH, the electrical system of the building, or its structural components. To protect a building, metallic “air terminals” are placed high, on roofs. A network of heavy electrical conductors connect the air terminals to rods driven into the earth. Large communications towers, bridges and high rise buildings often utilize their own metallic structure as a safe path for guiding discharge currents into the earth. Farm structures (barns, grain elevators, windmill pumps, etc), industrial sites (refineries, chemical plants, chimneys, etc, etc), and hospitals are protected with air terminals and metallic paths to earth ground. These protective devices are apparently considered unsightly and undesirable in suburbia, because they are rarely found on single-family residential buildings. When we lived in Indiana, our neighbor across the street had the chimney blown off his house by a lightening strike to that unprotected structure.

Lightening protection for boats is a separate and complex study; inexact, expensive to install, and impossible to properly retrofit if not built into the initial design at the construction phase of the boat’s life. Boats struck by lightening almost always experience severe electrical system damage and extensive damage to electronic equipment aboard. Lightening can literally blow a hole in a boat’s hull on its way to earth ground.

See my article on “Faraday Cages” for ways to protect sensitive electronic gadgets from lightening; for example, hand-held VHF radios, hand-held GPS, computers, back-up hard drives and cellular telephones.

Residential Electric Circuit “Wire” Naming and Identification

All operational electric circuits require two conductors (wires); one outbound from the source to the load, and one returning from the load to the source. The pair of conductors that lead current to and from the source of power are both called “Current Carrying Conductors.”

In DC circuits on boats, the conductor carrying the positive charge is called “B+,” and can also called the “plus” or “positive” conductor. By conventional agreement, the positive DC conductor is red in color. The conductor that returns current from the load to the source is called the “B-,” or “negative” conductor. By conventional agreement, the negative conductor (in 2020) is yellow in color. Until recent years, DC negative conductors on boats had black insulation, and many such systems are still in service today. In boats with both DC and AC systems installed, the black DC negative wire was easily confused with the black AC energized wire, so the DC color code was changed to “yellow” to eliminate the safety implications of confusing those two wires. In DC situations, the “negative,” or “B-” conductor is sometimes referred to as a “ground,” although that is usually (almost always) not technically correct, since “ground” wires are not intended to carry current in normally operating systems (reasons explained later).

In AC circuits in buildings, the power on the conductors is alternately positive and negative, so the DC nomenclature “B+” and “B-” doesn’t work. In single phase 120V circuits in North America, the two conductors are named for their role in the circuit. The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor. By code and convention in North America, “L1” is black in color. The other conductor in a 120V circuit is considered to be the return conductor. It is called the “Grounded Conductor” (for reasons explained later), or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.” They are commonly called “Line 1” and “Line 2.” “L1” is black, and “L2” is red. In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

In electrical engineering, “earth” is the single reference point in an electrical system from which voltages are measured and which provides a direct physical connection to the earth. Since the 1950s, the National Electric Code for AC distribution circuits in buildings has required “Equipment Bonding Conductors” and an “Equipment Grounding Conductor.” In the NEC, Article 250 is the standard for “grounding and bonding.” Each individual conductor that is an individual component that comprises the network of conductors that make up the “ground system” has its own specific name. For the purpose of understanding concepts, the term used here will be the “ground conductor,” or “safety ground.”

The NEC, Article 100, defines an “Effective Ground-Fault Path” as an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origination point of a ground-fault in a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault sensors. The purpose of the safety ground is to create an “effective ground-fault path.” That low resistance path is intended to function as a fault-clearing path; for that single “emergency use” only. “Fault-clearing” means that the circuit breaker feeding that faulting circuit will trip to remove power from the circuit. Under normal conditions in a properly wired electrical system, the safety ground conductors (including the bonding system network on a boat) DO NOT/MUST NOT carry current in normal, routine operation. The safety ground conductors are intended to ONLY carry current when there is a “fault” in the system. In buildings on land, the ground conductor is typically bare copper wire. On boats and in appliances, the ground wire is insulated, and solid green in color, or green with a yellow stripe.

Subtle take-away: the DC “negative” conductor has the same role in a DC circuit that the AC “neutral” conductor has in a residential/boat AC circuit. That is, the DC “Negative” conductor returns current from the load to the power source (battery). In a “grounded DC electrical system” (which is uncommon), there is a third conductor that is part of the DC circuit, just as there is a safety ground in a 120V AC system. The B- conductor is a “Current-Carrying, Grounded Conductor,” and is entirely separate from the actual ground conductor.

NEVER, NEVER use wires of the wrong color for the wrong purpose in a circuit. In new and repair work, always install the correct color of primary wire. Personnel safety and equipment safety depends on colors being correct! It is code-legal to “change” the color of a conductor in cases where that cannot be avoided. Changing the color of a wire is accomplished by wrapping electrical tape of the proper functional color for a distance of several inches at BOTH ENDS of the wire having its color changed. If ever that is found in existing work, DO NOT DISTURB that wrap of tape. The NEC does not allow green safety ground wiring to be changed. Safety ground wiring must be green, and green must not be used for any other purpose.

Note: on some but not all boats built overseas, AC wire colors may be different than the North American standard (NEC and ABYC) colors cited above. On some boats, like some Grand Banks trawlers, one 120V “hot” conductor (L1) is black, but the other (L2) is brown, not red; and the AC neutral conductors are blue, not white. This difference is also common on boats built overseas, because they follow the European color standards. If “strange colors” are found aboard a boat, BE PARTICULARLY CAREFUL to determine how that wiring is used to ensure equipment, fire and personnel safety.

However tedious this discussion seems, an understanding of wiring terminology and color conventions is important to understanding electrical installation instructions for many different types of electrical equipment on boats, and to understanding the host electrical systems, themselves.

Electrical Circuits

Core concept: opposite to the situation with static electricity, in man-made electrical circuits, the electricity originates at a point that is know to be its “source.” This can be a battery, a solar cell, a fuel cell, a generator, or a point-of-connection to the electrical grid. An electric “circuit” is said to exist when an electric “current” has a path that enables electrons to flow out of the source on a conductor, travel through a load to do useful work, and then return to its source on another conductor. The “source” can be DC or AC. Whether DC or AC, a “voltage” can appear at the output terminals of a source (like a battery or generator), but a “circuit” does not exist unless electrons can flow out of the source, through a load, and back into the source. A “circuit” consists of is a round trip of continuous conductive wiring for current flow out of a source and back into the source. A “switch” is any electrical device that “opens” a circuit to prevent electron flow as a matter of convenience and/or function; a relay is a device that interrupts current flow in the circuit that it controls; a “fuse” or “circuit breaker” is a device that “opens” a circuit to protect conductor insulation or remove power as a matter of fire prevention and/or personnel safety; and, a “severed” (“broken”) wire is a “malfunction” (“fault”) that “opens” a circuit so that there is, in effect, no round-trip circuit for electrons.

Fundamental Physics of Electric Circuits

Rule 1: Electric currents MUST RETURN TO THEIR OWN SOURCE.
Rule 2: Electric current will return to its source on ALL AVAILABLE PATHS. Corollary: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path.
Rule 3: An electric “circuit” does not exist UNLESS current has a continuous conductive path on which to flow from source back to source.

Readers will come back to these fundamental rules of electrical behavior over-and-over again when dealing with electrical systems and the concept of electrical faults. The more complex the electrical system, the more numerous and complex the issues, but electrical safety always comes back to the physics that underlies the behavior of electric currents.

The National Electric Code (NEC) and the American Boat and Yacht Council (ABYC) electrical standard, E-11, provide design and installation requirements that define system controls that manage how voltages will be safely removed and currents will be safely stopped (disconnected) in response to faults of various kinds that may occur in an electrical system. It is actually quite easy “to get something to work.” It is much more complicated and much more important to control electricity when something isn’t right. Disconnecting power, and disconnecting power safely, is the only way to prevent fires and electric shock risks to personnel.

Electric Code Grounding Categories

Finally, we get to “earthing” and “grounding.” There are two contexts for electrical “grounding” as required by the NEC.

  1. System Grounding
  2. Equipment Grounding (Bonding)

Residential System Grounding

“Ground” is the standard reference point for measurement of voltages. The NEC, Article 100, defines the crust of our beloved home planet as “Ground.” Ergo, Sir Knight, the electrical potential (natural voltage) of the earth’s “soil” is defined to be “zero volts.” All voltages are measured from an earth ground reference point.

The crust of the earth is electrically conductive. The earth’s crust contains many minerals and mineral salts which provide “free electrons.” In response to an impressed voltage, electrons will flow from point-to-point around and within the earth’s crust. An important corollary is that currents flowing in the crust of the earth follow the fundamental rules of electro-physics, including “Ohm’s Law” and “Kirchhoff’s Law.” In order to create a residential electrical system connection to “earth ground,” one or more interconnected metallic rods (often copper) are driven into the earth.

In the North American residential AC system model, three conductors arise from the utility power transformer at the street. All three are “Current Carrying Conductors.” Two of those conductors are considered, by conventional agreement, to be “energized” (“L1” and “L2”) and one is the neutral line (“N”). This is known as a “Single Phase, Center Tapped, Three-Pole,” system. The “Neutral is the transformer’s center-tap connection. As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V is present between “L1” and “N” and between “L2” and “N.” Note, however, that at the street, these voltages “float” with respect to their external environmental surroundings. They are not connected to anything. This situation is referred to as a “floating neutral system,” and in a “floating neutral system,” the voltage between the neutral and earth ground is unlikely to be “zero.”

If these three lines were connected to a distribution panel in a residence, all electrical appliances would work correctly. All of the necessary operating voltages inside the building would be correct. But, measured against a ground reference, it’s entirely likely the neutral would be at some perhaps large voltage difference with respect to the metal sink where food is prepared, or the metal bathtub when the baby gets bathed, or the metal faucets in the family shower. Clearly, a shock hazard would exist. To eliminate that hazard, the “Neutral” is electrically “tied” (connected) to an earth-ground reference point.

To create a system referenced to a known. zero-volt earth ground, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. This connection results in an earth-ground, “grounded neutral” system, throughout the premises. In a grounded neutral system, the voltage between the neutral conductor and the safety ground conductor is “zero,” or should be very close to “zero.”

While it’s true that the earth is electrically conductive, the earth is not a good conductor. Even at its best, “dirt” is not as good at conducting electricity as aluminum and copper wire (and also not as good as salt water). But rest assured, Ohm’s Law is a fixed “law” of physics, and it does apply to currents flowing in the earth. So while “dirt” may not be a great conductor, it is a very large-diameter conductor, with an infinite number of parallel paths, and with virtually unlimited ampacity. Just how well any local parcel of “dirt” conducts electricity depends on many things, including mineral and moisture content. The NEC requires that ground rods have a minimum contact resistance of 25Ω to earth. Sometimes, that can be achieved with a single 10′ rod driven into the soil; sometimes it requires a long rod driven 40′ – 50′ into the ground; and, sometimes it requires an entire network of long ground rods, all driven deep, and all connected together in parallel.

The essential point here is that “earth ground” is a universal reference point for all terrestrial power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the absence of any voltage. This works well because in a properly functioning, properly wired system, no current flows on the grounding system. Since no current flows, the voltage at the contact point with the copper grounding rods stays reliably at zero volts (as predicted by Ohm’s Law). Electrical faults (discussed later) create vastly different, sometime dangerous conditions.

Important to realize in this discussion, the earth ground alone DOES NOT protect against electric shock. It is merely a reference point against which system voltages are stabilized at “zero.” Earth ground IS NOT a reference for protective devices (fuses, circuit breakers) to trip to remove power when an electrical fault condition occurs. The Earth IS NOT the “source” for any DC or AC electrical energy. Remember Rule 1: “All electric currents MUST RETURN TO THEIR OWN SOURCE.” Electrical currents in residential and boat electrical systems DO NOT originate in the earth, and so, do not return to the earth. However, under some kinds of fault conditions, current can and does return to its source by traveling through the “dirt;” or, through the water in which a boat is floating!

Well then, why do we have the “Earthing” connection? Well, “Earthing/Grounding” in this context is a single-point-of-connection (one point and ONLY ONE POINT) to the earth for the purposes of mitigating:

  1. Static build-up (wind induced),
  2. System voltage instability, including:
    ▪ Unintentional physical contact with a higher voltage system (automobile accident or severe weather incident involving “hot” utility services),
    ▪ Repetitive intermittent short circuits (dispatched to first responders as “trees on wires, burning!”), and
    ▪ Utility switchyard and distribution system switching surges (spikes).
  3. Nearby vicinity lightning splash, and
  4. Transient interference (from static discharge and local RF emissions).

Not all of these exceptional conditions apply equally to all residential premises systems, but because some do apply in all areas, the National Electric Code treats all alike.


Consider a building’s main electrical service panel as the “source” of AC power (volts and amps) for the building and all of its branch circuits. In a household AC electrical system, current from that source emerges from a wall outlet on one appliance conductor and returns to the wall outlet on the other appliance conductor. Refer to Rule 1: “Electric currents MUST RETURN TO THEIR OWN SOURCE.”

Now consider a hot water heater, washing machine, trash compactor, dish washer, garbage disposal, microwave or toaster oven, each constructed with a metal exterior cabinet. The appliance is an electrical “load.” Electricity is provided to it from the wall and returns from it to the wall. With just two conductors (supply and return), the appliance can work normally. But, what happens if there is a frayed or cut wire inside the cabinet, and in physical contact with the metal cabinet of the appliance? In that event, the cabinet will have a non-zero “touch potential” (voltage) on it’s metal enclosure, and that voltage could easily be a shock hazard to residents. This is exactly how houses were wired before the 1950s, and many people reading this will remember the “two prong” duplex outlets of that time. In those days, people did get shocks from household appliances, fans and table lamps. Sometimes even from the iron. (Did Granny’s iron have fraying cotton insulation at the plug end? Does anyone actually iron anymore?) And sometimes, the shocks were serious. These shocks were the results of “faults” in the circuit.

A “fault” is said to exist when:
1) an electric current does not flow when it should, or
2) an electric current flows in an unintended path to get back to its source.

Clearly, an electric shock – which is a path through a person’s body – is an unintended path. To avoid shocking experiences like this, a third electrical conductor (safety ground) was added to electrical systems in homes, garages, barns, workshops, supermarkets, retail stores, office buildings, malls, commercial offices, workplaces, etc. That is, anywhere people might come into contact with electricity.

Grounding Conductor

This brings us to the next major category of “grounds” and “grounding.” Not to the earth itself, although it is connected to the earth, but rather to a common point in the building’s main electric panel. In this context, the word “ground” is a useful – but misleading – concept, because the ground conductor does not live in the ground and it does not send fault current into the ground. The ground conductor is connected to the ground rods at the service entrance, so it is REFERENCED to ground. That way, that ground conductor is held at “zero” volts with respect to all other components in the electrical system.

The “equipment grounding conductor” (a/k/a the “safety ground”) in residential and boat electrical systems is designed and intended to cause fuses or circuit breakers to trip in order to DISCONNECT POWER in case of a fault. It is, in the words of the NEC, an “Effective Ground-Fault Current Path;” that is, an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origin point of a ground fault in a wiring system to the electrical supply’s source and that facilitates the operation of the overcurrent protective device or ground-fault sensors.

Disconnecting power is the ONLY WAY to protect against fire and personal injury caused by ground faults in an electrical system. Equipment grounding is the intentional (in fact, NEC Article 250.2 mandatory) act of providing a network of conductors that interconnects the metallic cases of all electrical equipment attached to an electrical distribution panel. The bare copper or green-insulated “grounding conductor” discussed earlier is connected to the metallic cabinets of all modern appliances, and to the round ground pin of North American 15A and 20A household electrical utility outlets. The wires that make up the network of grounding conductors in a home have several names, but “safety ground” is representative for this discussion. On a boat, this network of green grounds is called the “bonding system,” of which the AC Safety Ground is a key part.

Residential dwelling units in North America range from tiny houses to single family homes to compounds with outbuildings to multi-family buildings of all kinds. A “ground buss” is always located in the main service panel of a dwelling unit, and in any sub-panels that may be supplied from that main service panel. Ground conductors from all branch circuits in the panel are connected together at the panel’s “ground buss.” Sub-panel grounds are in turn brought back to the ground buss in the main service panel. Boats are wired as sub-panels, not as main service panels.

At ONE PLACE in the main electrical service panel of the building, the “Grounding Conductor” is electrically connected (bonded) to the “Neutral” “Current Carrying Conductor.” By code, there is ONLY ONE “Neutral-to-Ground” bond in a residential electrical system, and it is placed at the Main Service Panel – never in sub-panels. A boat is wired as a sub-panel, so there should NEVER be a neutral-to-ground bond aboard a boat connected to, and operating on, shore power. This mistake in wiring on a boat is a very common cause of boats tripping shore power ground fault sensors on docks.

Now consider the fault case where an internal fault of some amount tries to put a touch potential voltage on the metal cabinet of an appliance. Rule 2 applies; “electricity will return to the source on all available paths.” Since the grounding conductor is attached to that metal cabinet, the Grounding Conductor does two things. First, it holds the voltage of the appliance cabinet at zero volts (because it’s “grounded” at the main service panel to the network of ground rods), which protects people and pets from shock. Second, it provides a very low-resistance path back to the service panel, via the neutral-to-ground connection, which instantaneously draws a very large spike of current through the circuit breaker (or fuse). That instantaneous large overload trips the circuit breaker to REMOVE POWER from the faulting circuit. Removing power is how the system protects buildings against fire and protects people from electric shock.

Ground Faults

The earth’s crust is electrically conductive, so that creates two electrical system design and code issues.

Rule 1 again: Electric currents MUST RETURN TO THEIR OWN SOURCE; and
Rule 2 again: Electric current will return to its source on ALL AVAILABLE PATHS;

Enter, our Corollary to Rule 2:: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path. This law of physics is called Kirchhoff’s Law, which states that when there are multiple parallel paths back to the source, current will divide and some portion of the total will take each available path back to its source.

In both home appliances and boat appliances, the two most common causes of “ground faults” are aging water heater elements and aging motor/transformer windings. In a water heater, power can leak through the water in the heater between the energized heating element and the metallic case of the water heater. In a motor, over time, dust and other airborne contaminants build up in motor windings, and at the same time, heating and cooling cycles cause the winding’s insulation to break down and develop micro-pores. In these cases, the fault current isn’t enough to trip a circuit breaker, but small amounts of power can leak to the Grounding Conductor, and then back to their source at the main service entrance panel. This is a ground fault by definition, because ANY current flowing on the safety ground is flowing on an unintended path. In this case, the fault current flows back to the source on the Safety ground’s conductor. More in a couple of paragraphs, but first, some illustrations.

Here’s a homeowner scenario… Dad’s gonna trim up the lawn, trim some plants, and wash the car (he’s young and energetic, unlike myself). He runs a 100′ extension cord in order to power an electric hedge trimmer, grass trimmer, circular saw, reciprocating saw, radio, charcoal fire starter, polisher/buffer, whatever. The extension cord has a ground wire, but the “tools” attached to it by multi-outlet adapter either have only two wires, or the ground pin has been cut off as a “matter of portability convenience.” Tools that aren’t actively in use are lying on the ground, where they and their cords are in contact with the ground. Now there is a path for power to get back to its source through the soil, to the ground rod(s) serving the main electric panel, and back to the neutral in the main service panel. That is a ” ground fault” because it is clearly an unintended and unwanted electrical path through the soil (ground). And at some point in this scenario, Dad will pick up his tools and possibly have a shocking experience. Possibly even, a lethal shocking experience. Without a continuous “effective fault-clearing path,” there is no way to shut off the power to save Dad from a shocking experience

OK, here’s another scenario with which my daughter and I have direct, personal experience. One Halloween “Hell Night,” Kate came home in need of a shower to remove 17 cans of different brands of shaving creme and lord-only knows what else she had encountered while “out with friends.” She went off to the shower, whereupon Peg and I laughed at her state of dishevelment! Note here, one of our sons had just finished his shower from his night “out with friends.” After just a couple of minutes, there arouse a righteous and shrill scream from the upper reaches:

“Daddy! Turn the water back on!”

In my total, complete and absolute innocence, I grunted at Peg: “Huh?”

The house water pressure had disappeared to a dribble while Kate was all lathered up. Mid-shower! Springing into action, Mom was “off to the rescue,” and Dad was “off to the basement.” In the basement, all seemed OK, but alas, there was no house water pressure.

Plumbing leaks? No water on the floor!
Pressure in the well tank? No! Gauge reading “zero.”
Pump Circuit Breaker “on?” Yes; and not tripped.
Pump relay OK? Yes, relay “picked.”

“Uh oh!” “Darn it!” (or words to that effect)! “Must be the well pump!”

Our homestead in the Catskill Mountains – and all of our neighbors – had a private deep-well that supplied our drinking water.  Our well was 100′ deep, and the pump lived at the 90′ level (not very deep). As the pump started and stopped over many years, it twisted (torquing) on the end of 90′ of semi-flexible PVC hose. The wires running to the pump abraded against the earth and rock walls of the well, and eventually the wire’s insulation wore through. This created a ground fault connection from the exposed bare wire directly to the earth about 70′ down.

Deep well pumps are usually two-wire, 240V circuits. One conductor of ours was in direct contact with the wall of the well. If the point-of-contact had been within the cast iron portion of well casing, it’s likely the circuit breaker would have tripped, because that metal casing did have an equipment grounding conductor. But in our case the point-of-contact was with sediment or rock, the 240V circuit breaker indeed did not trip. That did, however, create a significant ground fault. The pump was trying to start, but didn’t get enough voltage to overcome the weight of a 90′ column of water. Power was flowing into the earth, but not enough to overload and trip the pump’s circuit breaker. Power divided where the bare wire touched the well’s wall. Some of the power going down that hole got to the pump and returned on the other current carrying conductor, but some of the power going down that hole flowed back to the panel through the earth, to our home’s ground rods, and back to the service panel’s neutral.

In these situations, a newly-installed (since 2002 or so) residential service panel would have been fit with “Ground Fault Circuit Interrupter” (GFCI) to remove power and terminate the ground fault condition. In the case of yard tools creating a shock hazard at the end of an extension cord, GFCI could literally save Dad’s life. In the case of the deep well fault, GFCI could have saved equipment from damage. Our deep-well pump got burned out by the prolonged stall created by the low supply voltage. Relate this to boats on docks with pedestals fit with 30mA “Equipment Protective Devices.” This is a case where a 30mA EPD on the well supply would have saved the well pump from damage, and would have provided a clear hint to the location and nature of the fault.

GFCIs and EPDs work by monitoring the outgoing and returning current on the two Current Carrying Conductors. The currents should balance equally between the two conductors. If not, there is a ground fault and the GFCI device trips power off. What happens if there is no GFCI, as was our case at that time? Well then, the ground fault condition continues, because power flows out from the source, but has multiple parallel return paths, one through the returning current carrying conductor and the other through the earth to the ground rods at the main service panel at the same time.

See my article on causes of ground faults on boats for information specific to that topic.

See my article on GFCIs for more detail on how these devices work.

Ground faults on boats behave in the same manner, but are very dangerous, because instead of flowing through dirt, which is largely inaccessible to people, pets and wildlife, ground faults on boats can and do flow through the water. People – especially children – pets and wildlife are sometimes found in the water.

See my article on “Electric Shock Drowning” to read about ground faults in the water.

Ground faults on land can be quite dangerous in another, subtly different way. Suppose a 240V mercury arc exterior driveway light has a ground fault at the pole base that is not large enough to trip an over-current circuit breaker. We all now know from my well scenario, above, that 240V in direct contact with the earth will probably not trip a circuit breaker. But in that condition, the soil surrounding the point-of-contact between the energized conductor and the soil itself is electrically “hot.” This condition sets up a “voltage gradient” on the surface soil surrounding the point-of-contact. Using 240V in this example, at the point-of-contact with the voltage, the voltage in the soil is the same as the supply voltage, so there is no DIFFERENCE in the pole voltage and the soil voltage. But Ohm’s Law applies here, and however much current is flowing into the ground and back to the service entrance panel is creating a voltage drop along the surface of the soil (or driveway). So, the resistance of the local soil matters. One electrical standard1 assumes that 25% of the total voltage drop due to path resistance will be found in the first foot of distance away from the point-of-contact. One foot away from the point-of-contact, the soil is at 163V of shock “step potential.” Three feet from the point-of-contact, the soil is at 202V. Five feet from the point-of-contact, the soil is at 206V. As you can see, straddling the voltage gradient of the surface soil can create dangerous “step potentials” in the soil. Imagine the potential for what could happen when Rover comes over to “mark his spot” at that light pole.

The same sort of voltage gradient forms in the water around the prop and rudder or a boat if there is an AC ground fault on the boat. That gradient is quite enough to get a diver’s undivided attention. If the fault itself is in a heat pump, and the diver is working on the boat when the heat pump cycles “on,” … Well, that diver would quickly know how Rover felt…

See my article on “Electric Shock Drowning” to read about ground fault voltage gradients in the water.

Ground faults can be very dangerous!

Do not defeat safety devices.

Install GFCI and ELCI on boats.


  1. ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) [4.1.1]

Inverters On Boats

7/20/2020: Initial Post

The ABYC definition of an inverter is “an electronic device, powered by batteries, designed primarily to provide AC current at a required voltage and frequency.”  In North America, inverters produce 120V AC (or 240V AC) at 60 Hz from energy stored in 12V or 24V batteries.  On boating forums that I follow, there have recently been many questions about selecting and installing inverters on boats, so in this article, the topic is “Inverters on Boats.”

There are two types of inverter installations found on boats.  The first case is the stand-alone inverter.  These are usually smaller inverters used for charging cell phone batteries or powering portable computers.  Larger stand-alone inverters can be installed alongside, but separate and isolated from, the built-in AC system of the host boat.  Stand-alone inverters are  limited in features, requiring manual intervention each time they are needed.  They are turned “on” manually when needed and turned “off” manually when no longer needed.  Their un-shared outlets are often mounted on the unit itself.

The second case is inverters installed within the host AC power system of a boat.  When installed fully-integrated within a boat’s AC power system, inverters offer boat owners a whole-boat “Uninterruptible Power Supply” (UPS), and commonly function as battery chargers while external AC power is available.  Inverters installed within the host electrical system must comply with cUL/UL-458 per the ABYC Electrical Standards E-11 and A-31.

In 2020, most inverters sold for installation on boats are Pure Sine Wave (PSW) devices.  Older inverters were Modified Sine Wave (MSW) devices.  Some 120V household devices did not work well, sometimes not at all, on MSW inverters.  Generally, PSW devices are to be preferred for overall compatibility with consumer electronics in household equipment and appliances.

Figure 1 shows a stand alone inverter.  Inverters in operation can demand a great deal of DC current from batteries. Regardless of stand-alone or fully integrated installation, the B+ and B- cables from the batteries to the inverter must be sized for the maximum current the inverter can draw from the battery.  The B+ feed must be fused to protect the cables, and should have a disconnect switch rated for continuous use at or exceeding the maximum demand of the inverter.  The device itself must be “grounded” to the grounding buss of the host boat.  Unfortunately, I too often see stand-alone inverters that do not meet these ABYC electrical standard requirements, which apply to all DC devices.

The ABYC electrical standard, E-11, “AC And DC Electrical Systems On Boats,” July, 2018, treats stand-alone inverters in the same way it treats any other DC device (windlass, winch, thruster, water pump, instruments, auto-pilot).  The AC output of a stand-alone inverter is entirely separate and isolated from the boat’s host AC power system.  Thus, there are no specific ABYC requirements for the AC output of a stand-alone inverter.  These devices are easy to install, relatively inexpensive, and can meet basic AC power needs.  Some stand-alone inverters do not comply with North American residential electrical system requirements (grounded-neutral).  Stand-alone inverters enable bad user practices, such as extension cords running across the floor of a boat, and wiring that is too small for the loads.  A common “operator error” is to forget to turn the stand-alone inverter “off” after use, which can damage or destroy batteries.  These “owner errors” are common as fire and personal safety concerns.

Figure 2 is a “simplified view” of a typical 120V AC shore power system as found on many cruising boats.  I have taken a shortcut to also show that this boat has a generator installed.

The ABYC E-11 electrical standard does apply to this AC system.  In a previous article, I discussed the E-11 Standard as it correlates to Sanctuary’s AC system.

There is an important US National Electric Code/Canadian Standards Association “rule” to remember about all end-user AC power systems in North America.  For fire and shock safety, AC power sources are grounded at their source.  The result is called a “grounded-neutral” system.  The neutral conductor itself is a current-carrying conductor that returns current from the load to its source.  To automatically disconnect electrical faults, the neutral conductor is held at zero volts by a connection between the neutral conductor and the facility’s ground conductor.  The connection is called the “neutral-to-ground bond,” or “System Bonding Jumper.”  So in Figure 2, the shore power neutral conductor is “bonded to” the shore power ground conductor before these conductors come onto the boat, in the electrical infrastructure of the marina/boatyard.  The neutral of the boat’s onboard generator is “bonded to” the boat’s AC safety ground network at the metal frame of the generator.

The “grounded-neutral” requirement is the reason the “energized” (“hot”) Line conductor AND the “grounded” Neutral conductor must BOTH be switched by the Generator Transfer Switch (GTS).  When the GTS is in the “Shore” position, the neutral-to-ground bond comes onto the boat from the shore facility, via the shore power cord.  When the GTS is in the “Generator” position, the neutral-to-ground bond is at the generator, as shown in Figure 2.  To eliminate a ground fault path, the generator’s neutral-to-ground bond CANNOT also be in the active circuit when shore power is feeding the boat.  So, it is switched “out” of the active circuit by the GTS, which switches both the hot and neutral conductors.

Figure 3 shows the case of an inverter that is fully-integrated into the host AC system of the boat.  In this case, the inverter is not stand-alone, as in Figure 1, but is installed within the host AC system, between any other AC power source(s) and the boat’s AC distribution panel.  Here, it can be operated manually, or it can operate automatically, changing modes as incoming AC power comes and goes.  Automatic operation is helpful when commercial power fails, or when a dock neighbor inadvertently turns “off” the pedestal breaker of another boat.

As shown in Figure 3, power from either shore or the onboard generator is supplied to the inverter’s AC input.  This cUL/UL-458 compliant design operates in one of two modes.

STANDBY mode – passes power that originates upstream of the inverter through to attached downstream loads (“passthru”); in Figure 3, all of the boat’s AC loads are fed via the inverter.

INVERT mode – draws energy from the onboard batteries in order to create AC output at the rated voltage (120V, 240V) and frequency (60Hz) to feed downstream loads.

Figure 4 shows a similar system, but here some loads are powered via the inverter and other loads are powered only by upstream AC sources.  On Sanctuary, our onboard utility outlets are powered via our inverter, but our hot water heater, genset battery charger and fridge only receive AC power from upstream sources.  That arrangement greatly conserves our available battery capacity.

Note that Figures 3 and 4 refer to the Underwriter’s Laboratory’s UL-458 Standard, which is entitled, “Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts.”  Recall that all AC power sources in North America must be grounded at the source (grounded-neutral), and so shore power is grounded in the facility infrastructure and the generator is grounded at the generator.  To accomplish automatic ground switching, inverters intended for use on mobile platforms (ambulances, trucks, airplanes, RVs and boats) MUST comply with cUL/UL-458.

This is a good time to digress for a moment to look at the ABYC portfolio of electrical safety standards.  These standards fall broadly into two categories.  The first is standards that apply to the design and construction of individual electrical components, such as:

    • A-16 Electric Navigation Lights
    • A-27 Alternating Current (AC) Generator Sets
    • A-28 Galvanic Isolators
    • A-31 Battery Chargers And Inverters
    • A-32 AC Power Conversion Equipment And Systems
    • E-10 Storage Batteries

The second is standards which apply to joining individual component parts together to work within a unified boat system, such as:

    • E-11 AC And DC Electrical Systems On Boats
    • E-30 Electric Propulsion Systems
    • H-22 Electric Bilge Pump Systems
    • TE-4 Lightening Protection
    • TE-12 Three Phase Electrical Systems On Boats

All of these standards make reference to other Industry Standard sources for detailed specification of performance requirements.  Typical outside references are to established by  industry standards organizations including IEEE, IEC, ISO, cUL/UL and eTL.

So as applies to inverters, there is an ABYC standard (A-31) that is specific to the design of the unit itself, and a second ABYC standard (E-11) governing the system into which the unit is installed.  For inverters, the design reference is UL-458 in the US (and CSA C22.2#107.1 in Canada).

When a UL-458 compliant inverter is in “Invert” mode, a relay inside the inverter automatically creates the inverter’s neutral-to-ground bond.  When the inverter is in “Standby” mode, that same relay automatically removes the inverter’s internal neutral-to-ground bond so both AC power and the source’s neutral-to-ground bond are “passed through” the inverter to the boat’s AC power panel.  Functionally, this is what a GTS does in the case of a generator; i.e., when the GTS is set to “Shore Power,” the neutral-to-ground bond at the generator is switched out of the system.  The GTS transfers both hot and neutral, and transferring the origin of the neutral is what changes the origin location of the Neutral-to-Ground bond.

Figure 5 shows a simplified drawing of a UL-458 inverter in “Standby” mode.  AC power passes through (“passthru”) the inverter from an external AC power source, whether that be shore power or generator.  The relay shown in the red circle is “energized” (“picked”) by the presence of external AC power, so it connects the incoming power hot and neutral conductors to the output load circuits.  The green circle shows the inverter’s ground connection, but since external power is present, the relay is “picked,” so the neutral-to-ground bond that is located at the incoming source is “passed through” the inverter to protect downstream branch circuits.

Figure 6 shows the same inverter operating in INVERT mode.  In this case, incoming AC power is absent, so the inverter’s internal relay (red circle) is de-energized (“down”).  Because the relay is “down,” AC output from the inverter is created by the inverter’s electronics from energy stored in the boat’s battery bank.

The green circle highlights the inverter’s internal neutral-to-ground bond, which in this mode is connected via the relay.  That connection is required because the inverter, in INVERT mode, is the actual “source” of the AC power being delivered to the boat.

Following in Figure 7 is a complete circuit diagram of the AC system aboard Sanctuary.  Our 120V, 30A, two inlet AC System is fairly common on boats of our size class, and consists of eight AC branch circuits serving the equipment on the boat.  Other than completeness, our system is just like the simplified view portrayed in Figure 4.  Boats with 240V, 50A shore power service (3-pole, 4-wire cords) will look slightly different on the front end, but 120V inverter installations will be the same as shown here.

Sanctuarys generator is in the upper-right corner of the drawing.  Note the generator’s neutral-to-ground bond, highlighted there in green.

Our fully-automatic, fully-integrated inverter/charger is in the lower left-center of the drawing, in the small red circle.

On the right middle, in the dotted red circle, is our house, “Shore 1,” AC distribution panel, containing the eight branch circuits.  The top four branch circuits are fed only from either shore or generator power, whichever is selected by the GTS.  The bottom four branch circuits are fed via “Invert” or “Standby (passthru)” power via the inverter.  Our inverter is always part of our outlet distribution circuit, 24x7x365-1/4.

In Sanctuary’s system, at inverter installation-time, the hot buss feeding the branch circuit breakers on the AC power panel had to be divided into two parts (blue ellipses) in order to accept two separate feeds from 1) external power and 2) the inverter.  Dividing the hot buss required modification of the OEM electrical panel.  Also at installation-time, the neutral buss (red ellipses) had to be divided in order to separate the neutrals of circuits that are not fed via the inverter from the neutrals of circuits that are fed via the inverter.

The need to separate the neutrals stems from the requirements of the 2011 NEC and 2012 ABYC E-11 standard, adopted in coordination to reduce/eliminate dangerous ground fault currents flowing into the water from docks and boats.  (See the article on Electric Shock Drowning for more information.)  If the neutrals are not separated, an unintended ground fault leakage path can be present.  The day the boat arrives at a marina or boatyard where pedestals are fit with ground fault sensing shore power breakers is the day that boat may trip the shore power breaker, and will not be able to get shore power.  The dock attendant will tell the unhappy boat owner that “there is an electrical problem on your boat.”  The unhappy boat owner will think, “but it’s been working for many years!”  Both statements are correct.  It had worked for many years, but there is “an electrical problem on the boat!”

A fundamental rule of all electricity is, current will flow on all available paths to get back to it’s source.  If the neutrals from one AC circuit on the boat are cross-connected to the neutrals of another AC circuit on the boat, power will divide at the cross-connection (neutral buss) and flow back to the source via all available paths.  That situation is, by definition, a ground fault.

Following are two relevant and important excerpts from ABYC E-11, July, 2018: Isolation of Sources – Individual circuits shall not be capable of being energized by more than one source of electrical power at a time.  Each shore power inlet, generator, or inverter is considered a separate source of power. Transfer of Power – The transfer of power to a circuit from one source to another shall be made by a means that opens all current-carrying conductors, including neutrals, before closing the alternate source circuit, to maintain isolation of power sources.

Ordinarily we think of cross-connected neutrals as a situation that affects boats fit with two 120V shore power inlets; indeed, the neutrals from those two inlet circuits must not be cross-connected on the boat.  But more subtly, the separation requirement also applies to distribution circuits fed from generators and inverters.  UL-458 is the design standard that specifies that the needed neutral-to-ground bond in an inverter be “established” and “removed” based on operating mode.  If the inverter neutrals and non-inverter neutrals are cross-connected (as, for example, all sharing a common neutral buss on the boat), the terms of may not be met, resulting in a short ground fault condition.  In that case, there can be a duplicate path, if only momentarily, for shore power to use to return to the pedestal.  The following events happen in a fraction of a second.  Just “milliseconds (mS).”

At the instant (time=0.000) shore power is applied to the boat, any AC current that comes onto the boat via the hot conductor should also return to the pedestal on the shore power neutral conductor, and ONLY the neutral conductor.  Period!  Full stop!  Fundamental rule!

But…   At the instant shore power is applied to the boat (t=0.000), the inverter is in “Invert” mode with its internal neutral-to-ground bond still in place.  For the time it takes the inverter to respond to shore power and transfer its internal relay from “Invert” mode to “Standby” mode, there are two paths for the newly applied shore power to take to get back to its source at the pedestal.  The first path is via the shore power neutral, as intended.  But with unseparated neutrals, there is also a second effective (ground fault) return path.  The ground fault path starts at the neutral buss, where the returning current divides.  Some current will return as intended, on the shore power neutral conductor, but some will divert to the shore power cord’s ground conductor, through the inverter’s as yet unbroken neutral-to-ground connection.  That diversion path is a true ground fault.  One half of the total current will flow in each path.  The pedestal ground fault sensor expects the outgoing and returning currents to balance (within 30mA), but in this case, that sensor will see much less current returning on the neutral conductor than what was delivered on the hot conductor.   The pedestal breaker will want to trip.  How fast will it take for the trip to happen?  Usually between 30mS (t=0.030) and 50mS (t=0.050), but in all cases, less than 100mS (t=0.100), the maximum specified for the pedestal circuit breaker to trip.

In any case, we now have a “race” condition.  The race “contestants” are 1) the inverter relay against 2) the ground fault sensor.  The intent is for the inverter relay to “win.”  My inverter’s spec for transfer time is 18mS (t=0.018).  But, if the time it takes for the shore power Ground Fault sensor to trip is less than the time it takes the inverter’s relay to transfer into “Standby” mode, the pedestal breaker will indeed trip.  Furthermore, turning the inverter “off” will not eliminate that ground fault condition because the inverter’s internal relay would still be de-energized (“down”), and therefore, even with the inverter set “off,” its internal neutral-to-ground bond would still be present, creating the ground fault path.  Regardless, if the neutrals are separated, no cross-connection, so “no problem!”  So yes, it really is necessary to separate the branch circuit neutrals of the inverter-fed circuits from the neutrals of circuits that are not fed from the inverter. Elimination of the cross-connection of these neutrals is what eliminates the unintended, unwanted ground fault path.

Although I have not implemented an Inverter Bypass Switch aboard Sanctuary, I have drawn up a circuit diagram for such a switch, for those interested.  In Figure 8, the bypass switch is shown in the “Bypass” position.

When in “Bypass,” the switch’s external AC “power in” (red lines) comes from the hot and neutral lines that also feed external AC to the inverter.  Note that the “hot” feed for the bypass switch is upstream of the inverter’s power switch on the “Shore 1” AC panel.  This arrangement allows for bypassing the inverter while at the same time enabling a service technician to apply AC power to the inverter for diagnostic testing and repair verification.

When planning for the installation of an inverter, two pre-purchase considerations are, 1) what branch circuits will be powered from the inverter, and 2) what does the capacity of the inverter need to be in order to support the load of those circuits?  Aboard Sanctuary, we determined that we wanted to have AC power in the galley and at other utility outlets while underway.  That allows us to use our coffee maker, microwave, toaster and crockpot (not all at the same time), keep our DVR and AC lighting active, and occasionally charge utility batteries for my power tools.  We selected a 2kW inverter/charger to do that, which provides a maximum continuous AC output of 15A, shared by our four utility branch circuits.  That has served us well for 12 years.

Following is a “cut ‘n paste” from my “project plan” for the installation of our UL-458 compliant inverter/inverter-charger into Sanctuary’s DC and AC electrical systems, and timeframes based on my personal DIY-install timeline.  My need to “reconfigure” the B+ and B- DC busses on Sanctuary was because I consolidated the batteries from two separate banks (“house” and “start”) into a single bank at the same time, and updated the battery monitor from a stand-alone Xantrex monitor to a Magnum BMK. Combining banks greatly simplified battery charging from both the inverter/charger and the engine alternator.  Those steps are not specifically necessary for the inverter installation, but I like the consolidated battery bank.  Click to see my article describing that change.

Harmonic Distortion of AC Power

Initial post: 6/7/202
Minor edits: 6/8/2020

I’m posting this here because it came up on a boating club Forum that I follow.  As I have said often, my “target audience” is people in boating that do not have much prior background in matters of electricity.  This topic is a bit arcane, and does tend to be an advanced topic.  But at the same time, it does show up as a symptom that affects some boaters in some situations, so I offer it here for awareness.

Here is the question that started the discussion:

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“I would like to elicit opinions from the electrically minded of us regarding the following.  When running my NL 9Kw gen at anchor my Dometic/Cruisair heat pumps (240V, 16000 btu) work fine with just one of my Magnum Energy MS2812 (2800W, 125A charger) active to charge the batteries. But, when the 2nd charger is activated (now balanced loads on the gen legs), the heat pump compressors stop active function (no heating/cooling), fan drops to minimum level, but, amp load is unchanged. The above occurs whether 1 or all 3 Dometic units are running (this is not about trying to start one of the compressor motors with the gen loaded).  I have not noted this interference when the battery charging load is minimal.  The gen amp output at 100% is 37.5/240V.  Max charger demand is 17A both legs.  All 3 heat pumps together draw 13-14A. There is no problem if the water heater is run (240V/10A) with the heat pumps on and just one charger (brief test – 40A on one leg).

“It seems as though there must be some type of electrical interference that is occurring when the 2nd charger is added to the circuit affecting the heat pump compressor motor function. Any ideas as to what this might be and how it can be tested for? Emails were sent to NL and Dometic with no response. Thanks!”

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Here is my response to this question, edited for completeness, which I offer to others who may be experiencing similar intermittent, “weird” symptoms:

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What you are describing sounds like a somewhat out-of-the-ordinary (but not “extraordinary”) problem called Harmonic Distortion.  Here’s the electrical theory of HD in four sentences: A pure resistance – water heater heating element, light bulb, running motor – draws current in linear proportion to its impedance (according to Ohm’s Law).  Electronic devices do not follow Ohm’s Law;  they can and do draw current in short bursts within the AC sine wave voltage cycle.  These electronic devices are called “non-linear” loads.  Since in non-linear loads, current does not follow Ohm’s Law against voltage, the apparent internal impedance of the source can cause the waveshape of the AC voltage to distort (dip, flatten at the top and bottom), rather than be or remain a pure sine wave, clean as the driven snow.

So in your situation, the inverters are AC loads being used for battery charging, but the battery charger’s internal DC circuits are non-linear, “switch-mode” devices.  That creates non-linear current demand on the input AC waveform that is reflected back into the source.  The system doesn’t fail on shore power because the apparent impedance of the shore power source is many, many, many times less than the apparent impedance of the genset.  That doesn’t mean the phenomena isn’t there on shore power.  It just means the source is big enough to overcome the magnitude of the non-linear load component.  On shore power, the ratio of load impedance to source impedance is sort of analogous to David-on-Goliath.   But with the much smaller capacity of the genset, the aggregate effect of the switch-mode current demand can affect the shape of the genset’s output voltage sine wave.  Here, the ratio of load impedance to source impedance is definitely David-on-David.  What tends to happen with Harmonic Distortion is that the positive and negative peaks of the AC sine wave flatten, although more complex distortion is possible in extreme cases, even to the point of approaching a square wave with a flat top and very low peak voltage.

You mentioned in your post that you have a 9kW NL genset.  Nine kilowatts is somewhat under-sized for a 250V, 50A boat.  The power that can be absorbed by a 240V, 50A load is 12000 Watts, or 12 kW.  What you have is NOT “bad” from the perspective of genset loading or the perspective that you rarely need the entire capacity of the generator anyway.  But, if what you have is a symptom related to Harmonic Distortion, the smaller genset will have a higher apparent impedance than a larger genset would have.  The higher the apparent impedance of the source, the more likely it is that Harmonic Distortion would present itself as a noticeable and annoying symptom.

My conjecture that this is Harmonic Distortion is easily confirmed with an oscilloscope.  In the old days, that was the only way to see it.  But today, you can confirm it easily it if you have a means to read TRUE RMS voltage and a means to measure the TRUE PEAK voltage.  The peak of a 60Hz sine wave should be 1.414 times the RMS value.  I use an Ideal SureTest 61-164 or 61-165 circuit tester for this task.

So let’s assume you have a stable 60Hz voltage at 118V when running on the genset.  And we must also assume you have a stable 60hZ frequency, ±2 hZ, when running on the generator.  Multiply the 118 x 1.414, and the peak of the voltage waveform should be 167V.  If you then measure the actual peak, and it’s – let’s say – 156V, then you know you have Harmonic Distortion taking place, and the wave form isn’t a pure sine wave.

Now, the tolerance of the inverter/charger(s), the SMX Controller electronics and the blower drive electronics of the heat pump to AC voltage waveform shape, for which they, themselves, are responsible for distorting in the first place, may not be favorable.  That is a vicious circle.  It’s creating something that it, itself, can’t live with.  Since the genset is also feeding the Dometic SMX heat pump control unit and the blower and compressor control electronics of the heat pumps, those circuit boards can also be impacted by distortion of the voltage waveform.  Symptoms across the onboard system can be unpredictable, and can vary from attachment to attachment.  Pure resistance loads will not be affected, but electronic devices can be to varying extents.

Harmonic Distortion and Power Factor are two of the most challenging problems power utility companies have to manage.  A distorted AC voltage sine wave waveform is called “dirty power,” and it costs utilities a lot of money to manage.  Buildings with banks of computers and servers cause huge HD problems on the power grid, often affecting their neighbors and neighborhood.  Virtually all electronic devices cause Harmonic Distortion, right down to the family flat screen TV and stereo.  Power quality is a huge problem at the level of commercial power utilities serving residential neighborhoods.

And by the way, from the perspective of the 9kW NL generator itself, the higher apparent impedance and distorted wave shape will cause additional heat in the windings of the genset.  That heat is not related to useful work done by the generated power.  It amounts to excessive waste heat of which the genset’s cooling system has to dispose.  This can be worse than having unbalanced 120V loads on each side of the genset.

The fix?  You’d need a bigger capacity generator; i.e., one with lesser internal impedance.  With a lower reflected impedance, the genset would maintain the shape of the waveform for equivalent non-linear loads.  Or, your can just choose to live with it…

I have not written about Harmonic Distortion or Power Factor for my website because it’s definitely not a beginner’s/layman’s topic.  (Well, I have now, haven’t I?)  And even if you have HD, there’s little that can be practically done.  But if you want to read more about HD, click here for a fairly readable and reasonably good explanation from Pacific Gas & Electric; and click here for a better explanation of non-linear loads.  Start on page 3, at the heading called “ELECTRICAL HARMONICS.”  Skip the math; you don’t need it to understand the concepts.

Hope this helps.  And of course, this is only a guess on my part…   Cough, cough, choke, choke…

I wish I could recommend something practical that would make this better, but in the current system configuration, I think it’s a permanent restriction.

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Understanding Harmonic Distortion is complex and it’s definitely an advanced problem in an electrical distribution system.  What I’ve written above is just the very tip of the the technical iceberg.  But, although relatively rare, HD can produce observable symptoms related to the performance of boat AC electrical attachments.  It can affect the quality of sound from an entertainment system or produce what looks like interference (snow, lines) on a TV.  And, it can affect the operation of other types of equipment, like network routers, DVRs and printers.  If you have these symptoms and all else has been ruled out, consider Harmonic Distortion as a possible cause.  If you have these symptoms, it will be necessary to call in a skilled professional electrical technician to troubleshoot and confirm the diagnosis.  The tools that are necessary are expensive, and the skills to appreciate and understand the causes are advanced.  This is not a job for a residential electrician.