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

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. 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.

ABYC Electrical Standard Mapped to Sanctuary’s AC System

4/20/2020: Significant editorial updates to content.
5/27/2020: Added borders to images via HTML edits.


All boaters at one time or another get involved in discussions about what boats “are required by standards and codes to have or to do.” This comes up every time the owner is faced with getting a boat survey. A boat survey report usually makes copious references to “ABYC Standards” and to “industry best practice.” But the vast majority of boat owners do not work in a world of industrial codes and standards and are not familiar with what they are, what they are intended to do, and how they are used throughout the marine and commercial business world (especially, the insurance risk world).

This article is in the form of a stand-up classroom presentation. Slides are presented along with text (“speaker notes”) that describes the slide’s content. This is a mix of “engineering” and “safety.” My hope is that this material will make sense in this format. What I do in this article is look at the “electrical system” of our own boat, and compare that to the requirements of the principle ABYC electrical standard, E11, “AC and DC Electrical Systems for Boats.”

Our trawler, Sanctuary, is a Monk36 Trawler fit with two 120V, 30A shore power service cords. In our case, the shore power cords are configured so that one feeds the house AC loads and the other feeds our heat pump AC loads. Many boats are configured in the same way, but other configurations are possible. Our house loads include a battery charger for our genset start battery, fridge, hot water heater, inverter/charger and several utility outlets. The heat pump loads include one 5kBTU self-contained unit and one 16kBTU self-contained unit and a raw water circulator pump.

While configurations of individual boat electrical systems may be different, the ABYC Electrical Standard E11, “AC and DC Electrical Systems on Boats,” applies equally to all electrical system configurations on all boats of all designs and hull forms. Boats that adhere to the ABYC electrical standard are highly likely to be safe and compatible with 2020 shore-side infrastructure (marinas, boatyards, community, condo, municipal and residential docks). These standards are intended to maximize the safety of the boat; safety from shock hazards, freedom from ground faults, freedom from accidental fire hazards and much worse. I strongly encourage boaters to bring their boats into compliance if that is not already done!


Figure 1 shows an “energy flow diagram” of the total electrical system of a typical cruising boat, comprised of three separate divisions. The central electrical system is the vessel’s DC division (shown in red). This is the division that starts the engine and powers navigation lights, pumps, windlass and miscellaneous navigation equipment. All engine-powered boats have DC systems, but AC divisions are optional. Sanctuary’s platform also has an AC division (shown in green) which allows captain and crew to enjoy the comforts of a shore-side residence. Interfacing between the DC and AC divisions is a means to charge the batteries, and optionally, also use the batteries to power all or part of the AC division.


Note: in this topology view, solar battery charging systems would be part of the DC Division.

Note: out-of-scope for this article is the Bonding System Division of the electrical system. Those interested are referred to my article “Bonding System Design and Evaluation.”

Figure 2 shows the interfacing division with an inverter/charger instead of a battery charger. The red highlighted lines show the Inverter/charger in “Invert” mode. For the inverter to be in “Invert” mode, no other AC power source is available to the vessel; ie, no shore power, and no onboard generator running. Absent a source of AC power, the inverter draws DC power from the batteries, converts it to AC, and provides AC power to a subset of AC circuits on the boat. This operating mode would be the typical operating mode for boats at anchor, or boats underway on a travel day. While at anchor or underway, power is available for an AC coffee maker, a microwave, a crockpot, AC space lighting and entertainment systems, and an AC charging source for computers, onboard routers, smart phones and tablet computers. At least, that’s what we do aboard Sanctuary.


In Figure 3, the red highlighted lines show the flow of AC power when the boat is connected to shore power via a dock-side pedestal. AC Power enters the boat at the SHORE POWER INLET, passes through a MAIN DISCONNECT BREAKER to, and through, the GENERATOR TRANSFER SWITCH and on to a DISTRIBUTION PANEL which supplies HOUSE LOADS. AC Shore Power passively “Passes Through” the INVERTER/CHARGER to power a subset of AC loads, and the inverter/charger device acts as a DC BATTERY CHARGER.

Note: in this topology view, the inverter/charger is fully integrated into the boat’s electrical system, and automatically switches between “Standby/Pass Through” mode and “Invert” mode as AC power from another source comes and goes. If a boater in a neighboring slip accidentally turns off Sanctuary’s pedestal breaker(s), our inverter/charger automatically transfers to “Invert” mode to maintain AC power to it’s attached loads. This configuration is the ONLY use case that ABYC supports for inverters or inverter/chargers installed aboard boats.

Figure 4 shows the above AC Electrical System components mapped to the actual wiring diagram detail of Sanctuary’s installed AC electrical system. The remainder of this article focuses on ABYC requirements of the E11 standard related to the AC Division of the boat platform.


Note: Sanctuary is not fit with an Isolation/Polarization transformer (shore power transformer). Shore power transformers have a number of unique ABYC requirements and considerations. Consult the E11 standard for the treatment of these devices.

Note: I occasionally hear that an isolation transformer has been recommended as a means of avoiding the need to “spend unnecessary money” in order to fix/correct conditions aboard a boat that cause dock-side ground fault sensors to trip AC shore power “off.” I strongly discourage that thinking. The conditions that cause ground fault sensors to trip are often serious, potentially dangerous electrical safety or fire hazards. Transformers do mask safety problems which can be a threat to the boat and its occupants, but they DO NOT CORRECT THE UNDERLYING ELECTRICAL FAULT-CAUSING CONDITIONS.

Figure 5 is a clear view of the wiring detail of our AC electrical system. Notice that the neutral buss for house circuits has been divided so that the circuits fed from the Inverter/charger are separated from the house circuits that are not. Further, except as necessary for explanation, AC safety ground wiring is not shown on this diagram; that is a conscious choice made in the interest of simplifying the diagram.



The ABYC electrical standard is quite extensive and complex. This presentation only covers the major highlights that apply to the AC system division. Similar requirements apply to the DC division. Get these basics right and the boat will be well on its way to being safe. This presentation does not include a discussion of the requirements of onboard 120V load circuits; it focuses on the power distribution components of the AC division, to which we normally give little specific consideration.


By far the most common 120V, 30A shore power connectors are National Electrical Manufacturers Association (NEMA) L5-30R and L5-30P pairs. These are found on the familiar 120V, 30A commercial cordsets. I do not like them because I feel they are not nearly robust enough for the repetitive removal and replacement to which shore power cords are subjected in normal use. NEMA L5-30 connectors were designed 80 years ago for light industrial applications where outlets were sometimes ceiling mounted and machinery cords hung from ceiling receptacles. They were plugged in and given a twist, and they were rarely touched again. They are not intended to be roughly handled by boat owners and dock assistants, dropped on docks, stepped-on, rained-on, snowed-in and otherwise abused in routine service.

Which brings up an important point about all ABYC standards. The “requirements” stated in ABYC E11 are MINIMUM PERFORMANCE REQUIREMENTS. They do not require a particular piece of equipment or a particular manufacturer’s product. They simply specify minimum compliance requirements. So, NEMA L5-30P/R connectors ARE NOT “required” by the standard. What is required is a “grounding plug that locks into place” so it can’t “fall apart.” Also realize, ABYC standards apply to boat manufacturers, marine equipment manufacturers, and service technicians. Only indirectly do they apply to boat owners. The standards DO NOT contemplate that DIY electrical work will be done by owners, but they do contemplate that all work done by anyone will comply with the requirements.

0DA35F75-5CF5-4F08-B340-40F94896936A_1_201_aI have personally chosen to replace the OEM NEMA L5-30P shore power inlet receptacles with those made by SmartPlug, LLC ( (no personal financial interest; just a very happy customer). I personally feel SmartPlugs are much safer and more robust than L5-30 twistlocks, and they meet all NEC (UL, cUL, eTL) and ABYC requirements. That said, the SmartPlugs EXCEED the minimum performance requirements of the E11 standard.

The following slide shows requirements for the shore power CORDS and the shore power INLETS of the boat. The E11 standard refers to the “Type” of the wire. The cord’s “Type” descriptor is part of the information printed on (or molded into) the cord’s insulation, and should be easily readable on all marine-complaint cordsets. Don’t worry about the “Type” descriptor on Shore Power cables unless for some reason (I discourage this) doing a DIY shore power cord fabrication project. Simply buy products made by marine manufacturers and certified for marine use. The cordset manufacturers will have covered all that’s necessary for ABYC standards compliance.


The following slide illustrates a very important concept for shore power systems which all boaters should know; most especially, those who do DIY electrical projects!  At the head of the dock, in the facility’s electrical service infrastructure, the safety ground conductor is bonded (connected) to the neutral conductor. This is an NEC code requirement for all sources of AC power throughout North America, and results in a system referred to as a “Grounded Neutral System.” In a “Grounded Neutral System,” the neutral is intended to carry all of the current returning from the boat to the shore-side source. By design, the ground conductor IS NOT intended to carry current except to trip a circuit breaker in a fault situation. Thus, the neutral-to-ground bond is located in the facility’s infrastructure for both 120V and 240V systems.


The following slide emphasizes the boat-side of the shore power connection. The E11 standard requires that there be no neutral-to-ground bond(s) on the boat. At this point, for clarity, that firm statement can be modified to read, “there must be no neutral-to-ground bond(s) on the boat when operating on shore power.” The reason for this distinction now will become clear later, but for shore power, if there is a neutral-to-ground bond on the boat, that wrongly-placed bond creates a connection between the neutral conductor and the ground conductor that electrically parallels the two conductors all the way back to the dock-side infrastructure’s correct ground bond. Since the ground conductor on the boat is in direct contact with the sea water in which the boat is floating, this also parallels-in a ground path through the sea water. When all of these paths are in parallel, current that should flow only on the neutral will divide and flow in equal amounts on both conductors, and in some amount, through the water itself. By definition, this is a “ground fault,” and it will trip power “off” if there are ground fault sensors on the dock-side pedestal, but it can also kill people, pets and wildlife in the water. Incorrect neutral-to-ground bonds on boats are a primary cause for AC power leaking into the water, and can lead to incidents of ELECTRIC SHOCK DROWNING. For further information, readers are referred to my article on “Electric Shock Drowning.”


The following slide shows correct and incorrect wiring examples. In my article entitled “AC Electricity Fundamentals – Part 1,” I explain that a boat connected to a pedestal is intended to be wired like a sub-panel in a residential installation. Many residential electricians and DIY boat owners do not understand that technical detail, and so often connect neutrals and grounds together as they would in the main panel of a residence. On boats, as explained above, this is WRONG and DANGEROUS. Those who DIY must understand this natty technical detail.


The following slide shows the next major component in the flow of AC power into the boat: the Shore Power MAIN DISCONNECT BREAKER. This device is mainly for overload (and since 2012, ground fault) protection. Note that for 120V, 30A circuits, both the hot conductor and the neutral conductor must be switched, so this disconnect must be a 30A, “double-pole” circuit breaker with either a single operator handle or operator handles that are mechanically interconnected so if one side trips, the other side is also opened.


Boats built before 2012 will not have OEM ELCI (Equipment Leakage Circuit Interrupter) circuit breakers installed. That is OK. Although required since 2012 on new construction boats, ABYC states that boats that complied with the version of E11 that was in effect at the time the boat was built by the OEM manufacturer are “grandfathered” for compliance. Note that MANY MARINE SURVEYORS do not choose to adhere to/acknowledge the ABYC “grandfathering” policy. That can result in an inappropriate non-compliance finding in a boat survey.

The following slide shows the MAIN DISCONNECT SWITCH on a boat fit with 240V, 50A service.  The significant difference is that here, only the two hot conductors (L1 and L2) are switched. The neutral is not switched. Thus, a double-pole breaker rated at 50A is appropriate here. As before, this breaker must have either a single operator handle or operator handles that are mechanically interconnected so if one side trips, the other also opens.


Note that the neutral-to-ground bond is only correctly located in the shore power infrastructure, which is one of the National Electric Code (NEC) “rules” for residential and light commercial 120V/208V/240V electric services.

The following slide illustrates another very important wiring detail. Recall, Sanctuary is served by two 120V, 30A circuits. Earlier, we saw that neutrals and grounds MUST NOT be connected together aboard the boat. This is a similar case, and for the same reason. Here, it’s essential that the neutrals from Shore Power Circuit 1 and the neutrals from Shore Power Circuit 2 be SEPARATED aboard the boat. The reason is, both of the neutrals run back into the marina pedestal, or may run all the way back to the marina main service panel. If they are connected together on the boat, they become electrically paralleled all the way back to wherever they are ultimately joined together (pedestal junction, panel neutral buss, etc). All current returning from the boat will divide and flow equally on both neutrals. By definition, that is a “ground fault” at the pedestal circuit breakers, which will trip both breakers and interrupt power to the boat. But even more importantly, if one of the shore power cord neutral conductors were to fail open (due to, for example, a burned blade on a NEMA L5-30P twistlock plug), the other neutral circuit would become overloaded and could easily become a fire hazard aboard the boat. Preventing that fire hazard is why understanding and complying to these standards is important.




And by the way, the “wrong way” is a common way to find neutral wiring done on older boats.

Check your boat.

The following slide highlights the need for Equipment Leakage Circuit Interrupter (ELCI) devices for protecting against ground faults on the boat.


Those interested can read more about ELCI circuit breakers in my article entitled “ELCI Primer.”

The ELCI requirement was added to ABYC E11 in 2012 for new boats. ELCI devices are intended to both protect from overloads and detect ground faults. Ground faults on boats can result in dangerous levels of AC power being dumped into the water, which is a hazard that can lead to Electric Shock Drowning (ESD), as discussed previously.

An ELCI device on the boat is the same thing as a “ground fault sensor” on the dock-side pedestal (ground fault sensors on docks have many acronyms, including “EPD,” “GFD,” “GPD,” and “RCD;” don’t worry about what they’re called. By any name, they do the same thing.) ELCI devices also do the same thing as pedestal sensors, but the ELCI is physically installed aboard the boat. The value of having an ELCI on the boat is twofold. First, the simple act of installing an ELCI will flush out any silent, hidden wiring problems that currently exist on the boat. Second, ELCI will trip instantly upon the spontaneous emergence of a ground fault issue on the boat at some later date, so the boat owner will become aware of it, and be able to initiate repairs, as soon as it surfaces as a safety issue.

The following slide introduces the concept of a GALVANIC ISOLATOR. Galvanic Isolators are very important to controlling corrosion of underwater metals on any boat.


Galvanic Isolators are installed IN SERIES WITH the safety ground conductor AT THE POINT WHERE THE GROUND CONDUCTOR ENTERS/EXITS THE BOAT. Nothing – NOTHING – should be connected to the side of the isolator that leads to the shore power inlet connection except the actual safety ground conductor, itself.


The E11 standard considers Galvanic Isolators to be “optional” equipment, but if they are installed, the standard provides installation requirements.

If a Galvanic Isolator is NOT installed, the rest of the GROUNDING CONNECTIONS are still mandatory.


Earlier, above, the ABYC requirement that “there must be no neutral-to-ground bond on the boat when connected to shore power;” was mentioned with the proviso that it would “become clear later.” Now is the time to clarify as we look at the topic of POWER-SOURCE SWITCHING. The following slide shows the three possible sources of AC power on Sanctuary: 1) shore power, 2) genset, and 3) Inverter. The North American design standard for ALL AC power sources is, ALL power source neutrals are grounded at the source. Since shore power sources are grounded on land in the facility infrastructure and NOT aboard the boat, and since both the generator and the inverter are located aboard the boat, then how is it possible for them to be “grounded at the source” if neutral-to-ground connections are not allowed on the boat? Well, compliance is accomplished through appropriate source transfer switching.


Note the construction of the GENERATOR TRANSFER SWITCH shown on this slide.  That Generator Transfer Switch on Sanctuary is a three position rotary switch: “Shore,” “Off,” “Generator.” When the switch is in the “Shore” position, the generator’s neutral-to-ground bond is switched out of the circuit, thus meeting the shore power separation requirement. When the switch is in the “Generator” position, the shore power circuit is switched out of the boat’s electrical platform, thus permitting the onboard neutral-to-ground bond at the generator. The same type of logical switching is accomplished for the inverter by a relay located within the inverter.

Note: ABYC A31 requires that Inverters installed on boats be certified to UL458 (Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts) to ensure this grounding management relay is present. ABYC E11 includes ABYC A31, amongst other boat electrical standards. BOAT OWNERS SHOULD ENSURE THAT ANY INVERTER INSTALLED ON A BOAT IS COMPLIANT WITH UL458. Especially, be aware that inverters from Harbor Freight and other discount sources will not be compliant to UL458 and are not suitable for use on mobile platforms like boats and RVs.

Following is a close-up of Sanctuary’s Generator Transfer Switch. This is a three-position rotary switch. There are other switching styles that use lockout slide mechanisms to accomplish the same thing. Here, the breaking of the neutral conductors is highlighted by the red ellipses.


The following slide is just a reminder of what we looked at earlier WITH RESPECT TO SHORE POWER SOURCES. For Shore Power, the neutral-to-ground bond is in the shore power infrastructure and NEVER on the boat.


And this slide shows the generator neutral-to-ground bond that is switched into the boat’s onboard AC system when the genset is running…


And this slide shows requirements specific to inverters…


The following slide moves to another very important safety issue. Rarely, it is possible to encounter a 120V dock power pedestal source in which the black (hot) and white (neutral) wires (or red and white) are physically reversed inside the pedestal or other location in the dock-side infrastructure. No, it should not happen. Yes, it should be found by the installing electrician before the circuit is put in service. But folks, it does happen (rarely, thankfully). I have seen it three times in 16 years of cruising.


What’s particularly bad about a “reverse polarity” situation is that it can be present and also be entirely symptomless on the boat. Electrical equipment aboard the boat will work normally. But, touch potential shock hazards are likely. Because this condition is largely symptomless, it’s important to detect it and warn the boat operator of the potential life-safety issue. The “RP” warning lights (and/or audible alarms) are connected between the Safety Ground (green) and the Neutral (white) conductors on the boat. There should never normally be more than a volt or two between those conductors. Anyone who sees a ”Reverse Polarity” warning light(s) illuminated on their boat should immediately DISCONNECT (physically unplug) the shore power cord from the pedestal and report the condition to facility management. This can be a potentially lethal condition in the right (wrong!) circumstances.


This slide shows “Reverse Polarity” warning lights wired between the safety ground and neutral conductors aboard Sanctuary.

Actually, Sanctuary has some duplication here. Our Generator Transfer Switch has Reverse Polarity indicators, as do both shore power distribution panels.
ABYC specifies the minimum impedance of RP detection devices must be ≥ 25kΩ. Since these devices are connected between the neutral and the safety ground, they are a possible path for small “ground fault” currents, and properly installed sensors on some boats can cause false trips of a dock-side or ELCI ground fault sensing device. This would be caused by either multiple sensors in combination or older incandescent sensors having too low an impedance, thus allowing too high a “ground fault leakage current.”

The most current revision of the ABYC E11 Standard (July, 2018) as of this writing (April, 2020) is 67 pages of “shall” and “shall not” requirements, technical tables and example electrical drawings. Far more than I have covered here. Furthermore, there are several other ABYC Standards that apply to electrical subjects, such as A27, “Alternating Current Generators,” A28, “Galvanic Isolators,” A31, “Battery Chargers and Inverters,” E2, “Cathodic Protection” and E10, “Storage Batteries.”

These standards – and all ABYC Standards – make us all safer. They save property damage losses and they save lives. A marina fire is one of the most terrifying things any boater can ever experience, and there have been several this year alone (winter, 2019-20). When we aboard Sanctuary arrive at a marina, we must assume all of the boats that will be our new dock neighbors are safe. All of those boaters must also assume that we are safe. These standards are the reason we can all have some confidence in those forced assumptions. If there are condition(s) aboard your boat that you know need to come into compliance, please do so. The family you save may be your own!

For the record, I’m not much of a fan of covered slips, either. Those roofs help with UV damage and weather, but in a fire, heat arising from the fire’s origin is contained by the cover and spreads linearly along the dock until the cover finally burns through. This greatly foreshortens escape time; and, not a good thing for survivability of boats that were otherwise uninvolved in the first place. Always think fire safety and escape routes…

Nomad Portable AIS Transponder

In the Spring of 2019, I had the most fortuitous experience of being gifted a portable AIS transponder.  The unit is a Nomad,™ manufactured by Digital Yachts, Ltd.® in the UK.  This small, portable unit is a Class “B” transponder.  The manufacturer states the target market for the Nomad is “charter and delivery captains, pilots, tenders and back up for main systems.”

The Nomad comes with a short, non-removable power cord terminated in a USB connector, and a removable rubber-ducky VHF antenna on a 20′ length of coax terminated with a BNC connector.  The unit has a wi-fi interface, and a removable wi-fi antenna is included in the box.  The unit is designed to be portable, hence power is provided via a USB A/B-style connector.  Necessary product support software is downloadable from the Digital Yacht website.  Users will need this software to get the most from the unit.

The Nomad has an internal wi-fi access point supporting connection of up to 7 wi-fi client devices.  The wi-fi Access Point gateway IP address is not configurable, and the device cannot, itself, be used as a client on a local LAN.  The wi-fi interface supports access to two different types of data: internal performance data and AIS target data.  The AIS target data consists of NMEA0183 AIS sentences (!Axxx) destined to running on smart devices.  Any app on any operating system platform that can interpret NMEA0183 sentences can display the data (Aqua Maps®, Navionics®, SEAiq®, Coastal Explorer®, OpenCPN®, MacENC®, etc).

An application software package called “ProAIS2“ is available for Windows and Mac operating environments.  Via ProAIS2, the user performs initial configuration of the vessel name and MMSI number associated with the vessel, and re-programs the vessel identity information when the unit is relocated to a different boat.  Not counting download and installation time, initial programming of the Nomad is very straight-forward and takes less than 10 minutes.

An Android-only utility app called “AISConfig,” downloadable from the Google Play Store, allows the user to connect an Android® device to the Nomad via the wi-fi Access Point.  This utility displays key internal operating parameters. including internal operating voltage, VHF antenna Standing Wave Ratio (SWR), transmit and receive message counts, and status of the internal GPS receiver.  The app is useful in optimizing the location of the rubber-ducky VHF antenna on the host boat.

For convenience, click here for the Digital Yacht America download site.

(Note: in preparing this review, I found “AISConfig” in the Google Play Store with an update date of October, 2019.  The description suggests the app may now include the capabilities of ProAIS2, but that was not my experience in May, 2019.)

During the 2019 cruising season, Sanctuary and crew cruised round-trip from Charlotte Harbor, SW Florida, to Fairport, NY, on the Western Erie Canal.  That round trip gave me 3500 statute miles of experience observing Nomad performance on the Okeechobee Waterway, the A-ICW through the Port of Charleston, the Elizabeth River and the Port of Norfolk, the Chesapeake and Delaware Bays, the Port of New York, the Hudson River and the Erie Canal.  Northbound, we were accompanied by my brother, who has an Android tablet on which we installed the DY “AISConfig” utility app.  Southbound, I did not have an Android device, so monitoring the internal operation of the transponder was not possible.   South-bound, we cruised with a companion boat from Baltimore, MD, through Myrtle Beach, SC.  Our companion’s boat was fit with a permanently-mounted competitor’s AIS transponder.   On this 3500 StM cruise, I feel we utilized our Nomad in much the way a charter or delivery captain might use it.

In my use, the Nomad portable AIS transponder performed quite well.  Although I did encounter certain limitations, the Nomad was completely adequate for providing Sanctuary’s visibility and separation safety in busy commercial marine traffic areas.  Southbound, I followed both Sanctuary and our companion’s boat on the iOS version of the MarineTraffic® app.  The Nomad VHF radio performed almost as well as the permanently-mounted unit on our companion’s boat.  The slight performance differences seem to be due to antenna gain and placement associated with our companion’s permanently-mounted AIS installation vs. the VHF rubber-ducky antenna we were using.

Figure 1, above, is a screenshot of the MarineTraffic app as Sanctuary transited Northbound from Isle of Hope, SC, in the lower left, across the Savannah River, through Calibogue Sound past Hilton Head Island, across Port Royal Sound and into Beaufort, SC, at the upper right.  I had limited previous experience with the MarineTraffic app.  From an understanding of the technology and modest prior experience, I knew the app isn’t reliable on “rural” waterways like the St. Lawrence River, the Great Lakes and portions of U. S. Inland Rivers.  I was quite surprised at the number of coverage voids along the A-ICW.

Looking at the MarineTraffic screenshot, there are many obvious voids in our track.  Large bulk cargo ships and CNG tankers regularly use the Savannah River.  I expected better coverage from AIS land stations around this area, and in the vicinity of Parris Island, SC.

Figure 2, following, is a MarineTraffic screenshot as Sanctuary transited into and through the C&D Canal and down Delaware Bay.  In this screenshot, land station coverage in the region seems significantly better than in Figure 1, although there are still some void coverage areas.  Note that this screenshot shows the detail of our overnight stop in Delaware City.

Figure 3, following, shows two side-by-side screenshots in the area of the Neuse River and Adams Creek on the A-ICW in North Carolina.  The left hand view shows Sanctuary’s track as reported by our Nomad.  The right hand view shows the track reported via the permanently-mounted unit on our companion’s boat.  The permanently-mounted VHF antenna did somewhat better hitting land stations than we did with our rubber-ducky.  That said, it’s clear that the Nomad with the rubber-ducky antenna is perfectly adequate for purposes of safe on-water vessel separation.

Figure 4, following, shows two side-by-side screenshots of tracks transiting the A-ICW Southbound from Morehead City, NC, through Bogue Sound and Camp Lejeune, to an area south of New River Inlet.  The left hand view show’s Sanctuary’s track as reported by our Nomad.  The right hand view shows the track reported by the permanently-mounted AIS on our companion’s boat.  Our two AIS tracks appear on MarineTraffic as nearly identical.

Years before we installed our Nomad, Sanctuary had been fit with an Icom® MXA-5000™ AIS receiver.  The receiver is integrated into an NMEA0183 network aboard the boat.  Aboard Sanctuary, I have a multiplexer installed that allows my iGadget apps to see all NMEA0183 and N2K data aboard.  This includes AIS data from in-range AIS targets, HDG, COG, SOG, BTW, DTW, XTE, DPT, DBT and much more.  Since I had this solution installed and working long prior to installing the Nomad, I did not use the Nomad’s limited built-in wi-fi data feed to display AIS targets on my iPad™.  Instead, my Nomad appears to apps on our iPad to be just another nearby AIS target.

This arrangement actually worked for me as an alternative to having the AISConfig utility to monitor our Nomad.  We have been using our iPad as our primary navigation tool for several years.  First with SEAiq, and in the recent two years using Aqua Maps® U.S. & Canada™ with the Aqua Map “Master” extensions.  When my Nomad transmits a location datapoint, Aqua Maps running on my iPad shows me as a target 7 feet away.  As I continue to move, prior to the next position transmission, I see separation distance increase.  At the next Nomad position transmission, the target distance closes again.  More importantly, I know when Sanctuary’s name disappears from the display that the Nomad had stopped transmitting.  At that point, I can verify the LEDs, confirm an issue, and take corrective action.

The Nomad is a functional, cost-effective and easily transported tool appropriate to any charter or delivery captain’s portable toolkit, and certainly is an alternative for permanent installations.  However, all Class “B” AIS units share significant AIS Data Architecture protocol limitations, so If purchasing a new transponder for permanent installation aboard a pleasure craft, I recommend either a Class “A” or a Class “B+” transponder, depending on the buyer’s cost tolerance.  Class “A” is best, and Class “B+” is significantly better than Class “B.”

The Nomad product and the Nomad unit both had some usability limitations.  These were manageable inconveniences.

  1. A GPS receiver is built-into the Nomad.  The GPS antenna is located inside the unit, and the manufacturer’s instructions are to have that end facing the sky.  On large sounds and bays and other open waters – areas with a clear view of the sky – the unit functioned well when located tucked away inside our flybridge’s fiberglass console cabinet.  However on narrow waterways like the Hobucken Cut in NC (A-ICW MM155-MM158), the Rock Pile in SC (A-ICW MM353 – MM356), the Waccamaw River (A-ICW MM 367–385) and along much of the Erie Canal, the crown of the adjacent forest could and did block the weak GPS satellite radio signals.  This resulted in GPS dropout even when the Nomad was located out in the open atop our flybridge instrument console.  When GPS dropout does occur, there are no position transmissions.
  2. From the perspective of this iPad owner, a monitoring utility for the Nomad that runs on an Apple® iOS platform is missed.  I had start-up problems with my unit which required that we work with Digital Yacht Tech Support.  The problems I encountered were intermittent, and took several hours of runtime to expose themselves.  I do not have the space on my flybridge to mount and use a PC in a manner that provides physical security for my PC.  My Tech Support experience was excellent and responsive.  However without the coincidental availability of my brother’s Android tablet and the DY AISConfig app, it would have been more difficult to obtain the necessary diagnostic data.  The ProAIS2 configuration utility can collect the data, but in my use case, was not a practical alternative.  I am certainly not the only cruising boater who has only Apple® products, so I see this as a support gap which I hope Digital Yacht will address.
  3. The electronics of the Nomad’s internal VHF radio monitors the SWR of the VHF rubber-ducky.  When the SWR gets “too high,” the VHF radio quits transmitting.  Performance here was unpredictable and erratic.  Northbound, I could monitor SWR status via the Android AISConfig utility on my brother’s Android tablet.  Sanctuary’s flybridge is fit with a full enclosure supported by a Stainless Steel frame.  That frame seems to interact with the Nomad VHF antenna.  At times, a given antenna location on the flybridge showed a 1.2 SWR, which is quite good.  Other times, the same location showed a 2.0 SWR, which is quite bad.  Sometimes the unit would work fine with an SWR of 1.8, and sometimes it would not transmit with an SWR of 1.4.  By changing the location of the antenna, I could “get it to work,” but it took more attention to the device than I thought was appropriate, particularly on narrower sections of waterway.
  4. There are four colored LEDs on the Nomad related to AIS operation, and two LEDs related to wi-fi network operation.  The four AIS status LEDs are on one end of the unit and the two wi-fi network activity LEDs are on the other end.  Three of the operational status LEDs indicate fault conditions, and one – “Power” – indicates the unit is happy (and presumably transponding.)  The LEDs are in a place that can be hard to see.
  5. The internal power supply in the Nomad contains a buck-boost regulator that converts the 5v USB input voltage to >19v inside the Nomad, so that it has enough power to make its transmissions.   Some PC computer USB ports can provide sufficient power (Amps) to the Nomad, but some cannot.  The manufacturer recommends using a USB3 source rated at 2.4A.  Using my Macbook Pro with current-generation USB-C connectors to power the Nomad was not an option for us.  I tried multiple options to power my Nomad, including a 12v cigarette lighter adapter, with varying degrees of “success.”  The one that worked best for me was a 120V-to-USB “power brick” that comes with the newest version of the Apple iPad Pro (18 Watt).  That brick was able to provide sufficient power (Amps) at 5V for the Nomad unit to operate reliably.  I also verified that an Anker® 20100mAhr external LiON battery could reliably power the Nomad, but battery life limitations made that unsatisfactory as an all-day solution.  I found that if input power was marginal or inadequate, the unit experienced random GPS position errors and/or failed to transmit.  Charter and delivery captains need to plan carefully to provide an adequate 5V power source.

Finally, there are some legal considerations for Nomad users in the United States.  FCC regulation in the U.S. prohibits end users from editing the vessel identity information in DSC radios and AIS transponders.  U.S. Federal Agency (FCC) regulations have the force of law, so it’s “illegal” for U.S. users to program the vessel identity data in an AIS transponder.  Most of us never have a need to do that, but obviously charter and delivery captains were not taken into consideration when the regulations were developed.  Note: If the Nomad is not programmed with vessel identity information, it operates as an AIS receiver-only, not a transponder.

There is a disclaimer in the Digital Yacht ProAIS2 transponder configuration software reminding U.S. users of the FCC prohibition.  Users must “accept” the disclaimer to proceed.  A charter or delivery captain with a need to periodically re-program the unit will also need a “special code” from Digital Yacht to reset the unit once it is initially programmed.  The good news is, Digital Yacht does make the reset code available upon request.  The user accepts responsibility for their use in accordance with the laws of their nation.  My personal attitude is, as long as vessel identity data that legally corresponds with the host vessel is programmed into the unit, users are “in compliance” with the spirit and intent of the regulatory requirements.  That doesn’t make it “legal” to program the unit, but for some captains in some cases, it may make it risk-worthy for the potential safety advantage that AIS provides.

Figure 5, right, is a screenshot of the ProAIS2 utility running on Windows 10.  It shows the operational status of our Nomad  after being correctly configured with Vessel Name and MMSI Number.  The three red Xs indicate problems with the GPS receiver, the AIS transponder and the VHF antenna.  Note, this screenshot also shows the internal chipset voltage is low, at only 14.8v.  After correcting the low voltage, the red Xs were cleared.


Figure 6, left, shows a screenshot of the “AISConfig,” Android-only, utility app showing realtime Nomad internal performance and status data on an Android Tablet.  There is no way to obtain this data on an Apple iOS platform at this time.  The data can be extracted using the ProAIS2 utility on a Mac OSX or MS-Windows operating system platform.

The four status indicators shown on the app correspond to the physical operational LEDs on the Nomad device.  As above, when the unit is transponding normally, the “Power” LED is the only LED illuminated.  Observe also in this screenshot, the SWR being reported by the AISConfig utility is quite high at 2.0:1, yet the device appeared to be working normally at that time.  I cannot explain how this is engineered to work, but as reported above, the realtime behavior I experienced over 3500 StM was erratic in regard to SWR.


A personal disclaimer: I am generally not a fan of AIS transponder use on pleasure boats.  A great many pleasure boat “captains” do not understand the tool or the limitations of the underlying technology.  Many abuse the tool by leaving it “on” all the time.  I believe the tool creates a false sense of safety and security in/for many users.  That is especially true for Class “B” AIS transponders.  It remains my opinion that there are only 5 situations where AIS Transponders are appropriate for continuous use on pleasure boats (at least in the U.S., where AIS carriage on recreational vessels is NOT mandatory):

  1. any operations on the great inland rivers of the U.S.
  2. operations in conditions of reduced visibility (<1 NM in fog, t’storms, snow)
  3. offshore passage operations
  4. night operations
  5. vessel-not-under-command situations

Aboard a slow-moving trawler/cruiser/sailboat on the open sounds and bays of the US East Coast, other than the above five cruising situations, there is just no compelling safety need for an AIS transponder.  As required by USCG Navigation Rules, pilots at the helm of recreational craft should just keep a proper helm watch by looking out the windows.  In most of the U.S. Southeast, and in many densely populated pleasure boating areas everywhere, AIS “clutter” caused by owners leaving their units “on” and transmitting while the boat itself is safely secured at the dock completely obscures the chart plotter screens of those boats that are in transit in the area.  I recently heard it described as “looking like an active beehive.”  This makes it impossible to rely on a distance proximity alarm, and creates a huge distraction for a pilot at the helm of a transiting boat.  It’s often impossible to differentiate moving vessels from stationary vessels that should have AIS “off” anyway.

For those who choose to have AIS on their boats, I implore:  please, do not abuse AIS; turn it “off” when operating in clear conditions of visibility.  Turn it off when secured in a slip or on a mooring.

For navigation safety, keep a constant and competent visual watch.  In combination with the fact that the vast majority of recreational boats do not carry AIS at all, especially in congested areas, do not let the “glass helm” distract from what’s happening on the water around you.

Additional information on the technology of AIS can be found in an article on this website.

Electrical Behavior of a 208V/240V Boat

This article discusses the electrical behavior of the two 120V AC circuits on a boat that is natively wired for 125V/250V, 50A shore power service.  Topics include current flow (Amps) in the different appliance loads, power limitations when connected through a “Smart Splitter,” and the constraints and limitations encountered with the use of certain shore power transformers when powered from 208V dock utility voltages.

Use Case 1: a boat wired with a 125V/250V, 50A shore power cord, but not fit with 240V appliance loads.

Figure 1 is a generic wiring diagram illustrating this use case.  The system includes a genset and a Galvanic Isolator.  In Figure 1, the dock power source is on the far left and the boat’s appliance loads are on the far right.  Dockside 50A circuit breakers are omitted for simplicity.  The 50A shore power cord is highlighted in the red oval.  One 120V load (the heat pump) is highlighted in red.  Other 120V loads (house loads) are shown in black.  This boat DOES NOT have 240V loads.  This use case is a very common “50A” boat configuration.

Use Case 2: a boat wired with a 125V/250V, 50A shore power cord adapted to two 120V, 30A pedestal outlets to obtain limited 208V/240V power.

Figure 2 is a generic wiring diagram illustrating this use case.  Most commonly, a “Smart Wye” splitter adapter is used (ref: Appendix 1).  A “Smart Wye” splitter has two 30A twistlock plugs (NEMA L5-30P) and one 50A receptacle (NEMA SS2).  The two 30A receptacles (NEMA L5-30R) are on the dock pedestal.  The splitter and the 3-pole, 4-wire, 50A power cord are shown in the red ovals.  The rest of this system is identical to Figure 1.

Figure 3 applies to both Use Case 1 and Use Case 2 configurations.  Figure 3 shows logical blocks instead of actual circuit detail in order to make it easier to visualize the electrical behavior in this AC system.  In Figure 3, incoming power is shown as being derived from “any suitable 240V source.”  Electrically, we really don’t care how we get shore power as long as it’s “3-pole, 4-wire” of the right voltages.  In Figures 1 and 2, the loads were shown as they are wired, but Figure 3 shows them as they are logically arranged in the overall electrical circuit.  As the drawing shows, the red-highlighted 125V, L2 heat pump load is connected in series with the black-highlighted 125V, L1 appliance loads.  These two load groups share a common “Neutral” conductor.  The Neutral conductor anchors and maintains the midpoint voltage of the series connection under varying demand conditions.

Visualizing this electrical configuration in the mind’s eye as two 120V loads connected in series across a 240V source is the first key concept in this article.

Having identified the electrical arrangement of the two 120V appliance load groups of this 240V system, further analysis is on a) the voltages present, b) current flows, and c) power available to do work.

Figure 4 shows the two series load components of this boat’s 240V boat system, each with 120V across them.  The L2 load group is comprised of the boat’s heat pump(s) and raw water circulator.  The L1 load group is comprised of the hot water heater, fridge, battery charger(s) and multiple utility outlets.  Measuring across the L2 load between points A and B, there are 120V.  Measuring across the L1 load between points B and C, there are 120V.  The series pair receive the 240V mains supply voltage measured between points A and C.

Next, consider the electrical currents (measured in Amps) flowing through the two series load groups in a variety of specific but different load circumstances.  Understand that in the following analyses, different specific devices are “on” and others are “off” at any specific point in time.  Assume the following scenario: the boat’s owners have been away from their boat for a mid-summer week.  Upon late day arrival at the boat, outside air temperatures are in the mid-to-high 80s with 85% relative humidity.  Our boat owners will turn on some space lighting, and will immediately turn on the heat pump for air conditioning.  They will turn on the hot water heater and battery charger, stow fresh veggies, ice cream and adult beverages into the fridge, and perhaps turn on the DVR/TV.

Electrically, assume the heat pump draws 20A.  Also assume that house loads (hot water heater, battery charger, fridge, space lighting, computers and DVR/TV) add up to drawing 20A.

In Figure 5, the heavy red line represents this 20A flow of current (Amps).  This example is a special case called a “balanced-load” condition; that is, both of the 120V loads just happen to draw the same amount of current (20A).  The Amps flow from the dock pedestal into the loads on one of the energized line legs (L1), and flow back to the pedestal on the other energized line leg (L2).  In this balanced-load condition, no current flows in the neutral conductor (N).

Very importantly, notice that no more than 20A is flowing anywhere in this system. A double-pole 30A circuit breaker that serves the boat via a Smart Splitter at the dock pedestal sees 20A on both legs, L1 and L2.  Since there is no place in the system carrying more than 20A, the 30A pedestal circuit breaker is perfectly happy.  The second extremely key concept to take from this article is that the 20A flowing to power the heat pump circuit is the same 20A that flows through the House circuits to power the water heater, battery charger, fridge and utility outlets.

The word “power” is highlighted above to make the point that the same 20A flowing in the two 120V loads does useful work in both 120V load groups.  The basic formula for “Power” is P = Volts x Amps.  So in the heat pump load group, we have 120V * 20A = 2400 Watts.  In the house appliance load group, we also have 120V * 20A = 2400 Watts of power doing useful work.  In total, we have 4800 Watts of work being done at this time, in this system.

Up to 30A is available from a 30A shore power pedestal without exceeding the capacity of the circuit breakers.  The maximum power possible for each load is 120 * 30 = 3600 Watts.  Because the two load groups are in series, the maximum work that can be done by 30A, in total, is 7200 Watts.  If the boat had access to its design maximum of 125V/240V, 50A shore power, there would be the potential for 240 * 50 = 12000 Watts, total.  It quickly becomes clear why careful load management is necessary when running with two 30A cords feeding a 50A boat through a 30A Smart Splitter.

Following from our earlier scenario, after an hour or so, the hot water heater has done its water heating work, the fridge has done its cooling/freezing work, and the batteries are fully charged.  But, the heat pumps are still running to cool the boat.  Now, although we have 20A flowing in the heat pump load, current on the house side has dropped to 4A for the DVR/TV and space lighting.  Figure 6 shows what happens electrically.

The heavy red line represents the 20A needed by the heat pump.  But this time, there are only 4A needed by the house, represented by the thin red line continuing through the House circuit.  There is no longer a balanced-load.  The arithmetic difference between the heat pump demand and the house demand is 16A.  That 16A returns to the pedestal in the system’s neutral (N) conductor.  In this example, as before, there are 120 * 20 = 2400 Watts of work being done in the heat pump load group, and 120 * 4 = 480 Watts of work being done in the House load group.  There are never more than 20A flowing in any part of this system.  Neither the shore power pedestal breakers nor the Neutral conductor are overloaded.  All is safe and well within specifications.

At the end of the evening, when our sample boaters retire to bed, assume they turn off all of the house loads.  The hot water heater is satisfied, the battery charger is satisfied, the fridge is satisfied, the TV is “off,” the laptop and iGadget batteries are charged (and the screens have gone “dark”), and the space lighting is “off.”  Now, there is no current at all flowing in the House loads.  Ah, yes, but the air conditioning is still needed.

Figure 7 represents the electrical status in this case.  Since the heat pumps are still running, there are 20A flowing in the heat pump circuit.  Since there is nothing “on” in the House load group, the arithmetic difference is 20 amps, which returns on the neutral (N) conductor.  Again, no part of the circuit carries more than a total of 20A.




Use Case 3: a boat wired with a 125V/250V, 50A shore power cord, but fit with 240V appliance loads aboard.

Figure 8 shows the addition of pure 240V loads at the far right of the drawing.  Boats with 125V/250V, 50A shore power service which have both 120V and 240V appliance loads (hot water heater, cooktop, electric dryer, heat pump compressor) are electrically very similar to those without 240V appliances.  Very few “240V appliances” are “pure” 240V devices.  The only ones that come to mind are 2-pole, 240V deep well pumps and 2-pole, 240V hot water heaters.  Appliances like heat pumps, cook tops, ovens, clothes dryers and watermakers, are usually “hybrid devices;” ie, they need both 120V and 240V to operate.  The control circuits in hybrid appliances are generally 120V circuits.  In a dryer, for example, the heating elements are 240V but the motor that turns the drum and the clock timer circuit both require 120V.  Hot water heaters can be pure 240V-only loads which do not need or have a neutral conductor.

In Figure 8, the pure 240V appliance loads are electrically in parallel with the two 120V series loads, and the 240V loads add to the amps drawn in the 120V supply mains, L1 and L2.  So, if we had the 20A L2 load running a 120V heat pump, as has been the example throughout this article, and in addition, a 240V hot water heater simultaneously calling for 12A, the result would be a 32A total Amps in L2.  Attached to a 50A pedestal, all would be OK, but attached to a 30A splitter, the result would be a tripped 30A pedestal circuit breaker.  So again for emphasis, it is up to the boat owner/operator to understand load management and ensure that pedestal breaker capacity is not exceeded.

Potential Power Issues with Certain Shore Power Transformers

The utility power on docks can originate from two kinds of public utility sources.  “Single phase” sources will appear as conventional 120V/240V.  “Three phase” sources will appear as 120V/208V.  Because this electrical fact is a well-understood, and very common in boating, UL Marine certified electrical appliances are designed to accommodate the difference between 240V and 208V.  Residential appliances MAY NOT have have that same flexibility.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for  125V/250V, 50A applications   are manufactured in three “flavors:”

  1. Basic, single input, single output, 240V transformer; least expensive flavor.
  2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input to achieve a constant 240V output.
  3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.   While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.   Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”   The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.    The output of this transformer (the secondary) provides single phase, 3-pole, 4-wire power to the boat. In English, that means there is a conventional black, red, white and green output.    If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.   But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances.   With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a low utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances.  Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.  Microwaves will run but will take slightly longer to cook.   Coffee pots will perk, but will take slightly longer to perk.    Electric blankets will keep sleepers warm and cozy.   Water Heaters will heat water, but take slightly longer to reach target temperature.   Stovetop burners will heat, but not get as hot at the same setting.  Heat pump compressors and fans should all run, but some motors may overheat and cut out to protect themselves from damage.  Marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.   One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V should be designed to detect the condition, put up a power warning fault light, and self-disconnect.    Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer marine heat pump designs do that.

WARNING:  if there is a 240V shore power supply voltage applied to a manual transformer set to a 208V input voltage, then the AC voltages aboard can get high enough to “damage” appliances.

Article Summary:

  1. When operating a 125V/250V, 50A boat which does not have 240V loads, total loads of up to 2 * 3600 Watts can be supported with two conventional 30A pedestal outlets.  In this case, neither the energized (hot) conductors nor the Neutral conductor are ever overloaded.  No individual circuit conductor ever conducts more than 30A.
  2. When operating a boat with pure 240V loads, the Amps required by the 240V loads add to the Amps needed in the 120V loads.  The owner/operator must monitor total amps drawn/power used to keep total power consumed below 3600 Watts per side.
  3. Some shore power circuit breakers are housed in inaccessible, locked locations ashore.  If a boater accidentally trips a shore power circuit breaker, particularly after hours, it may not be possible to gain access to it in order to reset it
  4. It is necessary for boat owner’s to closely monitor power usage and limit the amount of  current used to prevent tripping shore power circuit breakers.  Care must be exercised to not run high amp draw appliances (coffee pots, microwave ovens, inductive cookware, hair dryers, clothes washer/dryers and similar devices) at the same time.  Boats with multiple heat pumps will probably be unable to run all of them at the same time on 30A services.
  5. The examples in this article assume that the heat pump circuit is on one 30A load leg and house loads are on the other leg.  Obviously, some boats are wired differently. Systems with heat pumps and house loads distributed across both incoming energized 120V legs will have to monitor loads and current draws in the same manner, but the electrical principles discussed above remain the same.
  6. The specific balance of currents in the load one group and the load two group changes constantly.  L1, L2 and Neutral current (Amps) never exceeds 30A.

Appendix 1:

To the right is the electrical diagram of a typical “Smart Wye” splitter.  This Figure represents the electrical circuit detail of the splitter shown in Figure 2 in the earlier text.  Note that the splitter contains a relay – labeled “K” in the drawing.  The relay requires 208V or 240V to close.  Without at least 208V, the relay will not close and the splitter will not pass any power through to the boat.

Following is a link to my article describing Smart Splitters, and the receptacles required for their successful operation.