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 system” reliability 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:
- boats built with a single, central battery bank that “does everything.”
- 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
- 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;
- separate “start” banks for multiple engines
- “house bank” split into two parts, Port and STBD; fore and aft,
- 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
- “starting” the propulsion engine,
- “house” DC loads, and
- 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:
|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|
|Bilge Pump||1||Automatic; must work|
|“High Bilge” Alarm||1||Indicates potential emergency|
|Sump Pump||1||Automatic – doubles as bilge pump|
|Potable Water Pressure Pump||1||Manual pump not installed|
|Anchor/Deck Washdown Pump||1||Doubles as a fire-fighting option|
|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)|
|Chart Plotter 1 (Autopilot)||1||Flybridge|
|Chart Plotter 2 (RADAR)||1||Flybridge|
|Chart Plotter 3||3||Salon; redundant iPad, iPhone apps|
|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.
|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|
|8||BMS HTCO||>135℉ charging1;
>160 ℉ discharging
|9||BMS LTCO||<25℉ charging; <-4℉ dischg|
|10||Energy Density||Lowest per unit weight & volume;
|Slightly better than wet cells;
|Slightly better than AGM||3X >PBSO4 by weight & volume;
|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
Ref: IEEE 450-2010
|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:
- 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
- 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.
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:
- lead-acid batteries are “dumb batteries” that require “smart chargers,” and/or
- 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.
- lead-acid batteries are “best charged” by a multi-stage (“smart”) charger;
- 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;
- a modern lead-acid “smart charger” with both lead-acid and LFP charging programs will charge both battery chemistries just fine.
- 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.
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.
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:
- 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?
- 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.
- 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.
- 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.
- 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.
- 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?
- 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.
- 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?
- 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:
- how the owner uses the boat, in cruising and in layup,
- how much longer the owner plans to own the boat,
- analysis of the existing electrical system in place aboard the boat,
- prioritization of the safety importance of DC circuits aboard the boat
- analysis of owner electrical skills and conversion costs,
- characteristics, strengths, weaknesses and maturity of current LFP solutions, and
- 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.
- DIYers possessed of “advanced” or “expert” electrical skills,
- living-aboard boats that remain in continuous service year-’round,
- fit with large solar systems (800W or greater), and
- 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.”