Category Archives: Battery Topics

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.

Boat Batteries – Charging and Care


For all boat owners, an understanding of lead-acid batteries is important to optimizing the reliability and performance of the boat’s DC electrical system.  This article introduces some important concepts in order to enable further reading in boating publications, magazines, websites and Internet posts.  This article has no math, and only conceptual references to battery chemistry and physics.

It is essential to understand the rate at which different types of lead-acid batteries can be charged.  For boaters, the subject has a direct bearing on battery and battery charger choices.  Owners must also understand the importance of battery care, including routine monitoring of state-of-charge (SOC), battery electrolyte management, equalization charging, and related battery maintenance subjects.  Good battery maintenance will avoid premature battery failure and maximize battery service life.

Batteries (what they are/what they do):

Lead-acid batteries contain chemical energy in the materials of which they are made.  Batteries release that stored energy in the form of DC electric current.   The rate at which that energy can be delivered by a battery into an electrical load is stated in Cranking Amps (CA), Cold Cranking Amps (CCA), Marine Cranking Amps (MCA) and Reserve Capacity (RC).  Deep-cycle (traction) batteries are typically rated in Ampere Hours (aHr) returned over 20 hours (US measurement system; 10 hours is typical in Europe).  Start service batteries are normally rated in CA/CCA/MCA and RC.  The total amount of energy a lead-acid battery can release is determined by the physical properties of the battery materials, its construction, and the length of time over which the energy is drawn off.

Charge Acceptance Rate:

The technical details related to Charge Acceptance Rate (CAR) are quite complex, but there are some simple “rules-of-thumb” that illustrate and explain basic battery charging concepts.  CAR is determined by battery materials and construction technology.  The ill effects of incorrect charging (over charging and under charging) are extremely difficult to measure and may never be fully appreciated by retail battery buyers/owners.  Boaters need to know that poorly designed charging systems can result in premature battery failure and shortened battery service life.

Assume that the lead-acid house bank has been discharged to a point where between 50% and 60% of its total capacity is still remaining.  In other words, 40-plus percent of the battery capacity has been “used,” and the batteries are presently at 50% to 60% State-Of-Charge (SOC).  This condition is an appropriate and reasonable time to begin recharging the batteries.

The CAR “rule-of-thumb” for flooded wet cells at 50% SOC is that they will accept 25%, stated in Amperes, of that cell’s aHr capacity.  In some articles, the capacity number is represented by the letter “C.”  So, CAR for flooded wet cells is 0.25C.  For a 100 aHr battery at 50% SOC, CAR would be 25A.   That is, that 100 aHr flooded wet cell will accept a charge current of 25A, but no more.  That 25A of charging current can come from a single battery charger or from several charging sources working cooperatively together.

Similarly, the CAR “rule-of-thumb” for AGM and Gel cells at 50% SOC is that they will accept 40%, stated in Amperes, of the cell’s aHr capacity.   So, CAR for AGM and Gel cells is 0.4C.   For a 100 aHr AGM or Gel cell at 50% SOC, CAR would be 40A.  Indeed, some specialized AGM cells (Thin Plate Pure Lead, Carbon Foam) can accept larger charging currents, but the batteries found on most boats tend to be of types that cannot.   An owner who has batteries that can take higher CARs would know that.  Those batteries are expensive, and owners would not install those batteries without knowing what they can do and why they would want/need them.

Just because a battery has a theoretical CAR does not make charging at that rate the right thing to do.

Rules-of-thumb are useful for learning and understanding concepts.  Battery owners should verify the manufacturer’s ratings for their particular batteries.  Different battery lines do have different specific details.   Lifeline, for example, specifies lesser than “rule-of-thumb” maximum CAR, and for good reason.

For lead-acid batteries, the positive and negative lead plates are composed of two different forms of lead: pure lead and lead dioxide (lead dioxide is also called “sponge lead” by some authors).  Lead-acid batteries release electric current in a chemical reaction that converts lead and lead dioxide into lead sulfate and simultaneously, sulfuric acid into water.  In that chemical reaction, free electrons are released.  Charging a lead-acid battery is the process of recombining the lead sulfate and water back into lead/lead dioxide and sulfuric acid.  Lead is a crystal lattice metal.  In the recombination, electrons are restored to the structure of the crystal lattice of the metal.

Most boaters are aware that multi-stage battery charging is the optimum technical solution to maximize battery service life and ensure the reliability of their DC electrical system.  A simplified view of the nominal charge-cycle that multi-stage chargers should produce is as follows:

Lead-acid battery charging stages

Lead-acid battery charging stages

In the “bulk” stage, the charging source maximizes the charge current to the level the battery charger can supply up to the level that the batteries/bank can accept.  During bulk, battery terminal voltage starts out low, and slowly increases to it’s pre-determined setpoint as SOC increases.  In the “absorb” stage, terminal voltage is held at a pre-determined level, and  charging current decreases as SOC continues to increase.  In the “float” stage, terminal voltage is held slightly above the battery’s resting voltage and a very small current trickles through the battery.  The “equalize” stage is a short duration, occasional, over-charge performed on a fully-charged battery.

I recommend that charging systems be designed to result in charge rates that are slightly less than battery manufacturer specifications for CAR.  The reason is, with a partially discharged battery, the crystal lattice of the lead plates is depleted of electrons.  Lead has been converted to lead sulfate and the plate surfaces are “etched” of material.  During charging, it takes time for the conversion of lead sulfate back into lead and lead dioxide, and it takes time for electrons to equalize throughout the interior deep regions of the lattice of the lead plate material.   If the rate of electron migration is driven to try to proceed above the maximum CAR, the process can result in a phenomena called “surface charge.”   The surfaces of the lead plates of the battery can become electron-saturated even while the deep structure of the lead lattice remains electron deficient.  Generally, electron migration rate-control is one of the key purposes of the “absorb” stage of the battery charging curve.

Modern multi-stage AC battery chargers and multi-stage engine alternator voltage regulators (like the Balmar MC614 and ARS-5, and the Xantrex XAR) use

  1.  battery terminal voltage,
  2.  elapsed time,
  3.  battery temperature, and
  4.  the amount of charge current flow

to determine when to switch from stage-to-stage.   If a battery is charged too fast by a charging source capable of “force-feeding” more current than the battery can naturally accept, surface charge “over-saturation” will occur.  Under this condition, electrolyte gassing will also occur.

Worse than gassing (discussed later in this article), battery terminal voltage will appear to have reached a charger stage-transition setpoint, and a multi-stage charger can think that it’s time to switch into the next later stage of charging.  For example, the “bulk” charging target voltage can be reached early, and the charger will then switch to “absorb.”   This particular condition doesn’t have serious consequences, but it does prolong the overall time it takes to achieve full charge.

However, the “absorb” charging current threshold can appear to be met prematurely.  In that case, “absorb” can terminate early, and the charger can enter “float” prematurely.   If absorb ends prematurely, the lattice structure of the plates will be in an only partially reconstructed (electron-depleted) state.  Thereafter with the continuing passage of time, the excess surface charge that caused the premature stage switch will bleed into the deep lattice.   Once electron equilibration completes, the result is that the entire lead lattice remains electron deficient.   That is, in English, the battery is undercharged (not fully re-charged).   That means some lead sulfate remains behind.

Complicating the above, some experts now feel that the duration of the “absorb” phase in modern multistage chargers is not sufficiently long enough to fully recharge the battery, even when everything else is optimum.  Because the volume of available lead sulfate decreases as the battery charges, the speed at which it’s conversion back into lead and lead dioxide necessarily slows.  Some experts say that the final 15% of charge, from 85% to 100%, requires 8 hours.  Shore power chargers that drop into float for long times can accomplish the same thing, but if running on a genset at anchor, not likely.

All of this also means the energy restored into the battery is only a large fraction of what should be there; let’s say, instead of 100 aHr for a 100 aHr rated battery, maybe only 95 aHr or 96 aHr.  Following is a marketing (conceptual) chart from a major manufacturer of solar charging systems and PWM and MPPT controllers showing the effect of charging on battery life:

Effects of charging on battery service life.

Effects of charging on battery service life.

An example: assume a 1000 aHr battery bank made up of flooded wet cells that is around 50% SOC; its theoretical CAR is 250A (25% of aHr capacity).   A charging source controlled to drive only a “bulk” charge of 150A or 200A would avoid the build-up of excess surface charge, but a charger (or multiple charging sources working cooperatively) that drove to 400A would indeed create a massive surface charge that could result in a battery bank that is chronically undercharged while also needing a lot of electrolyte replacement.

Realize also that in all lead-acid batteries, as SOC increases, CAR decreases.  That is, the “internal resistance” of the battery’s “equivalent circuit” goes up.  That also increases the possibility of excessive surface charge.  The net is, charger selection and capability are extremely important.  The charger and the charging program need to be able to sense battery behavior and compensate for changing CAR throughout the charging process.  All battery banks of all sizes can exit the “absorb” stage early, wind up in “float” prematurely, and be chronically undercharged.   If that happens on a regular basis, battery service life will be lessened.


Battery charging is indeed a trade-off between what is actually best for the physics and chemistry of the battery and what the owner views as “best” or “necessary” for them in their daily lives.   What’s best for the battery is to be fully recharged, and to be fully charged somewhat slowly compared to its theoretical maximum capability.   Of course, that means increased generator run time for anchored boats.    Many cruisers charge to the end of bulk and quit; that point is, as a “rule-of-thumb,” the “gassing” point, or about the 85% SOC point.   Most cruising boaters have heard of re-charging in the range between 50% SOC and 85% SOC.   That is not a strategy to maximize battery service life or return total lifetime amp hours.   To achieve those goals, owners must do what is best for the batteries. Most battery manufacturers recommend fully charging batteries at least every 10 days to two weeks.

A side benefit of solar charging systems is that they have lower charging capacities and do their work over longer periods of time; slowly.

Battery Monitoring:

I recommend the use of a coulomb counter to monitor battery State-Of-Charge (SOC).   These devices use a shunt to precisely measure the current (amperes) leaving the battery/bank, and average that load over time to calculate the amount of energy withdrawn, measured in aHr.  One such legacy stand-alone coulomb counting device is the Xantrex Link10/Link 20, but it has some limitations.  There are several aftermarket coulomb counters available today, such as the Bluesea Systems M2 DC (p/n 1830), Victron BMV-770 or BMV702, Xantrex LinkPro and Mastervolt BTM-III.

I feel the best choice for boaters is to install a coulomb counting battery monitor from the manufacturer of the charger or inverter/charger.   Aboard Sanctuary, a Magnum MS-Series Inverter/Charger is installed.   I replaced my former Xantrex Link 20 with a Magnum Battery Monitor, ME-BMK.   Our monitor calculates Peukert effect and Charge Efficiency Factor (CEF) in real time.  This is especially important for those who run AC appliances such as coffee makers, food processors, chopper/grinders, microwave ovens, vacuum cleaners, washer/dryers and such on inverters.   These types of loads on the 120VAC side are very demanding on the 12VDC battery side.   Heavy DC loads (thrusters, windlasses, winches and DC motors [watermaker “Clark Pump”]) also distort (lessen) the 20-hour rating of energy capacity that batteries can return.

Many boaters rely on conventional DC voltmeters to track state-of-charge.   Battery terminal voltage is a late indication of SOC.   Particularly when large DC loads are involved, voltmeters do not provide a reliable indication of SOC.  Conductance testers are great at measuring CA/CCA/MCA, but not so good at aHr.   In my opinion, coulomb counters remain the best available practical alternative for boaters.

Electrolyte Level:

Recalling  the purposes of the “bulk” and “absorb” stages of charging: “bulk” is a constant-current stage of large current flow (full-fielded alternator) that is targeted to take advantage of the Charge Acceptance Rate physics of the lead plates and battery electrolyte technology.  “Absorb” is a constant-voltage stage that is intended to restore the electron equilibrium of the deep crystal lattice of the lead plates.  However, electron replacement in the plates is only part of what’s going on inside the battery.   The chemistry of the liquid electrolyte is also dynamic and changing throughout the recharge and discharge cycles.

When flooded lead-acid batteries are fully charged, the “normal” mixture for the liquid electrolyte is approximately 35% sulfuric acid and 65% pure water, so the Specific Gravity (SpGr) of the electrolyte mixture is 1.265 at 80ºF.   With flooded wet cells, SpGr can be measured with a hydrometer; it’s not practical to measure SpGr with sealed AGMs or Gels. The process of discharging a battery removes sulphur from the sulfuric acid to form a sulfate of lead. Timely recharging of the battery reverses that reaction.   With the formation of lead sulfate during discharge, the percentage of sulphuric acid in the electrolyte mixture decreases and the percentage of water in the electrolyte mixture increases.  The SpGr of the electrolyte mixture goes down.  (Note: measurement of SpGr is the single most reliable measure of battery state-of-charge; perhaps not practical, but by far the most accurate).

As a lead-acid battery begins to charge, the electrolyte is restored toward the 35/65% “normal” equilibrium mixture concentrations of sulphuric acid and water.   During charging, there comes a point when electrons cannot move as fast into the lead lattice as the chemical process is capable of proceeding. That point is called the “gassing” voltage; or the point at which significant bubbling begins to appear in the liquid electrolyte; the electrolyte appears to be “boiling.”   “Gassing” is the point at which hydrogen gas is given off to the atmosphere.  At the gassing voltage, CAR is overcome by the build-up of surface charge; that’s when “gassing” begins.

One result of this evolution of electro-chemistry is a drop in the liquid level of the electrolyte in the battery cell.   Some people say “water has boiled off.”   Not exactly, but that description is OK relative to the externally observable symptom.   The liquid electrolyte level does go down, and if it goes far enough, the plate tops will get exposed.   Another consequence of the gassing is that some lead sulfate remains un-reconstituted, so over time, battery capacity is diminished.

The “gassing” voltage (or ideally, a voltage immediately below the gassing voltage) is the point where lead-acid battery chargers should  switch from “bulk” to “absorb.”   At that point, the battery is around 85% state-of-charge.   One major system design reason for a charger to switch from constant current (bulk) to constant voltage (absorb) at that 85% level is to reduce gassing to a minimum.   Implication: if there is an ongoing need to replace distilled water in the battery electrolyte mixture, the ONLY reason is that the charging source(s) is not switching from bulk to absorb at the most optimal point in the charging cycle FOR THOSE BATTERIES AT THEIR OPERATING TEMPERATURE.   Other than a physical damage to the battery case, there’s no other reason to lose electrolyte.

Electrolyte Stratification:

I believe flooded wet cells offer the best overall return-on-investment for boats that get a lot of use.  AGMs or Gels are a better choice for boats that get relatively little use.

Stratification is a phenomena that happens to flooded wet cells over a relatively long period of time (weeks) of disuse.  As discussed above, the liquid electrolyte in lead-acid batteries is a mixture of sulphuric acid and water.  Over time, the sulphuric acid and water components will settle in layers in the battery cell chambers (stratify), water “floating” on top of acid.  Charge/discharge cycles set up convective circulation in the electrolyte mixture, so stratification does not occur when batteries are used.  Boats also agitate the electrolyte mixture as jostle about in a seaway.  Equalization charging also creates convection current which mixes electrolyte liquid.  Boats at risk for electrolyte stratification are boats with flooded wet cells that get infrequent use, have high DC electrical demands, and have small charger capacity and/or are not recharged for sufficient time to achieve a full charge.

Stratification in flooded wet cells is a problem for several reasons.  Acid concentration at the bottom of the plates causes open circuit terminal voltage to appear higher than it actually is.  Uneven distribution of the acid throughout the cell reduces CA/CCA/MCA and causes uneven plate etching.  Sulfates on plates are not evenly reconstituted.  Over long periods of time, these issues cause premature loss of capacity and lead to early battery failure.

Digital Battery Charging Programs:

Charger manufacturers are different entities from battery manufacturers.  Chargers from any one manufacturer will charge batteries from many battery manufacturers.  Default charging programs make assumptions (via “rules-of-thumb”) based on averages for battery materials, construction and operating temperatures.   Occasionally in real life, factory defaults must be tweaked.   Not all flooded wet cells are the same; not all AGMs are the same.  Balmar voltage regulators allow the setpoints for charging-stage voltage, stage duration and battery temperature to be adjusted.   Most newer shore power chargers allow for some parameters to be adjusted.  Many older technology chargers do not.  Shore power chargers should be equipped with temperature probes for the batteries they charge.  If the charger’s battery-type setting (flooded, AGM, Gel) is correct for the battery being charged, and batteries nevertheless continue to lose electrolyte, reduce the bulk setpoint voltage in steps of 0.1 volt and monitor the results.   Electrolyte loss should decrease.  Do that incrementally until full charging occurs with minimal electrolyte loss.   The effect will be that it may take a few minutes longer to achieve a fully charged battery (total time from start-of-bulk through end-of-absorb), but there will be less electrolyte loss, less hydrogen outgassing, and more complete re-constitution of lead sulfate.

These same processes occur inside AGM and Gel batteries.   The difference is, because AGM and Gel batteries are sealed, it can’t be seen or measured.   These batteries have pressure operated valves (Valve Regulated Lead Acid) that retain the outgassed hydrogen under pressure, and allow it to resorb into it’s electrolyte over time. That does work, to a point.  However, it’s way better to avoid overcharging sealed batteries.  For maximum return on service life, set charging parameters so that sealed batteries charge slowly.  Especially so with Gels, because hydrogen gas escaping in gel cells can actually create gaps (channels) in the Gel, which can result in permanent loss of capacity in the cell.


Equalization (called “conditioning” by some manufacturers) is the process of applying an intentional over-voltage charge to a battery.  Because the battery to be equalized starts out fully charged, DC currents forced through the battery during equalization are relatively low.  Equalizing is very hard on the mechanical structure of the plates and the plate support frames.  It also causes aggressive release of hydrogen gas (gassing).   These negatives are why very few AGMs, and no Gels, can be equalized.  Battery and charger manufacturers recommend equalizing flooded wet cells only when the Specific Gravity (SpGr) in resting cells differ from cell-to-cell by from 0.15 to 0.20.

Measuring SpGr is not hard, but it is tedious.  Achieving a “resting” state is impractical on most boats while batteries are in use.  Lead-acid flooded wet cells are 2.2VDC each, so there are 6 cells in each battery of nominally 12V.   A bank of three 12V batteries will have 36 individual cells.   Only the most dedicated owners will measure the SpGr of 36 cells to determine if equalization is actually appropriate.

Then there is the matter of, “how long to equalize?”   Manufacturers recommend monitoring of cell SpGr hourly, with the equalization overcharging voltage being discontinued when the SpGr is consistently equal across all of the cells of the battery/bank.  When the SpGr is equal across all cells in the battery, that is THE DEFINITION of “equalized.”   That is the condition where the internal resistance and internal losses of each cell are identical, and balanced, so each cell carries a fully equivalent part of the total load.   This state is reached after a variable duration of time, depending on the starting condition of the battery when the equalization process was begun.

The net is, while there is real science and manufacturer’s recommendation on how and when to equalize, and when to quit equalizing, the tedious and inconvenient nature of “doing this right” makes this task ripe for shortcuts and dockside lore.   It takes a lot of time to do it right; I assume, more time than most people are willing to invest.  Can it be a good thing to do?  Absolutely.   Can it be a bad thing if it’s over done?   Absolutely.

Batteries – Questions And Answers

Following are questions frequently asked about the lead/acid batteries found on boats:

Question 1: Is it OK to combine batteries of different lead/acid construction types?  That is, is it OK to combine wet cells with AGMs, or mix ‘n match wet cells, AGM and/or Gel cells?
Answer 1: For short periods, like starting an engine in an emergency: yes.  Regularly, long-term, “permanently:” no.  NO!
Discussion 1: All electrical devices, including batteries, possess the electrical property of “resistance.”  The physics of resistance in batteries is more complex than the physics of resistance in a length of copper wire.  Referred to as “internal resistance,” the resistance of batteries is a function of the chemistry, construction design, current state-of-charge (SOC), temperature and the cumulative damage done to the battery by its average and maximum discharge usage history, age measured in accumulated charge/discharge cycles, rate-of-discharge, and history of operating temperatures.

Newly manufactured batteries of different construction (wet cells vs AGMs vs Gels) have different “natural” internal resistance characteristics.  The internal resistance of older batteries is higher than that of new batteries.

Combining batteries of different types guarantees they will not be electrically equivalent when combined.  The imbalance will result in circulating currents caused by the differences in internal resistance and battery construction.  These circulating currents will hasten self-discharge, internal sulfation and premature loss of capacity.  Batteries of different construction charge and discharge at different rates.

Question 2: Is it OK to combine batteries of different service class?  I have a pair of flooded Group 31 starter batteries to start my engine and two flooded 8D deep cycle batteries to support my  inverter bank.  Can I keep my battery selector switch (Off-1-2-Both) set to “Both” all the time?
Answer 2.  For short periods, like starting an engine in an emergency: yes.  Regularly, long-term, “permanently:” no.
Discussion 2: Batteries used in “start” service should be labeled by their manufacturer as “start” batteries.  Batteries used in “deep cycle,” or “house” or “inverter” service should be labeled by their manufacturer for “deep cycle” service.   Start and deep cycle batteries have different internal electrical properties, and are not compatible as long-term peers in a combined battery bank.

The difference between start and deep cycle batteries is not in battery chemistry, but rather in construction materials and technique.  “Start service” batteries have a large number of thin plates that give up and restore their energy very quickly, while “house service” batteries have fewer but much heavier plates that give up and restore their energy more slowly.

Start service batteries are not designed for, and should never be, deeply discharged.  Because they are built with thin plates and lightweight separator frames, start service batteries will be permanently damaged with even a very few deep discharges.  House service batteries are designed to handle the mechanical stresses that deep discharges apply to lead plates and plate frame construction.

Question 3: Is it OK to combine batteries of the same service category and construction type, even if they are of different capacities?  I have two flooded 8D start batteries and eight flooded 6V golf cart batteries for my house/inverter bank.
Answer 3: In parallel, not recommended, but may be OK; in series, no; never.
Discussion 3:  Resting circulating currents in this situation are minimized because the battery technology is the same for both start and house bank.  However, high currents – particularly large load and large charging currents – are not balanced in the bank due to voltage drop in the inter-connecting cables and differences in battery capacity.  In parallel configurations, the lesser capacity batteries in the cluster will be “supported” by their greater capacity peers, and will become charged and discharged at approximately the same rate in proportion to their capacity.

Question 4: Is it really necessary to replace all of the batteries in a bank at the same time?
Answer 4: With only one exception, yes, that is what I recommend.
Discussion 4:  Batteries of the same type and capacity, connected in a bank cluster while performing similar service, age at approximately the same rate.

I discuss this in a post on my website, here:  No matter how carefully battery banks are managed, battery aging will occur.  Over time, un-reconstructed sulfation will reduce plate surface area.  Shedding of plate surface material will reduce the lead volume of plates, reducing overall energy storage capacity.  Good management will only affect the rate at which the aging processes progress.

Ignoring for now how the determination is made, let’s assume that one battery in a bank of three 12V, 8D batteries in “house” service (including a 3kW inverter) has been identified as having failed.  There are two sets of questions that arise.  First,  questions around placing a new battery with low internal resistance into a bank of aged peers with much higher internal resistances.  Second, questions of economics and convenience; i.e., when will another aging member of the bank cohort fail (week/month/quarter)?  Where will the boat be when that happens?  Will replacement batteries of the required capacity and type be available in that locale, or will they have to be shipped in?  What will be the impact in boat operations if the bank fails?

The answers to the economic and convenience questions will be unique to each boat and owner, but the fact is, the remaining still-serviceable batteries have aged and will fail at some point, relatively sooner than later.

Question 5: How do I decide how much battery capacity I need on my boat?
Answer 5: The rule-of-thumb: lead/acid batteries should not be discharged more than 50% of their total capacity per charge/discharge cycle.
Discussion 5:  There are well established approaches to calculate the average daily load a boat will have.  Each boat and owner is unique.  Different boaters have different lifestyles aboard.  Some live on the boat as if they were “camping out;” others want “all of the comforts of home.”  Some boats are used infrequently; only a few hours per year in local day or weekend outings.  Others are used hundreds of hours per year in long distance cruising.  Some boats rarely leave their home mooring and shore power; others are rarely connected to shore power.  But for all battery systems, the rule-of-thumb applies.

Deep-cycle batteries should be used in “house/inverter” applications, and should not be discharged more than 50%.  Lesser average depth-of-discharge (DOD) is better.  One study of Lifeline batteries suggests the best lifetime return (ROI) of amp hours occurs at the 40% average DOD level.

Each boat owner will need to confirm the battery capacity necessary to maintain that average discharge.  I would note here that rare – RARE – deep discharge will not hurt service life, provided batteries are immediately and fully recharged.  Even though alternating between 30% DOD and 70% DOD averages to 50%, frequent discharges below 50% DOD will shorten battery service life.

Question 6: How do I keep track of state-of-charge of my batteries?
Answer 6:  Install a made-for-purpose battery monitor.
Discussion 6:  “Coulomb counters” are devices that have large shunts in the bank’s DC return line.  They measure in real-time the amount and net direction of energy flow into and out of the battery bank.  Coulomb counters must be pre-programmed to “know” the battery construction type and total amp hour capacity of the bank being monitored.  The boat owner must then monitor battery use and recharge the batteries when that “rule-of-thumb 50% DOD” is reached.

A new class of devices called “Capacitance monitors” report state-of-charge directly.  They do not use shunts or install shunts in large diameter battery wiring.  They pass a high frequency AC signal through the battery/bank from which they calculate both total amp hour capacity and internal resistance.  Based on battery construction type, they read out SOC as a percentage, and require no complex user-setup programming.

Most charger manufacturers offer coulomb counters as options.  Balmar, Blue Sea Systems, Newmar and others also offer independent options.

Question 7: Do I really need a battery monitor to monitor my battery’s SOC?  Can’t I just use a DC voltmeter?
Answer 7: A battery monitor is by far the preferred choice!
Discussion 7:  In general, the terminal voltage of a battery bank is a lagging indicator of SOC.  This can often result in repeated but unintentional over-discharge or incomplete recharge of the batteries.  Voltages that reliably reflect battery SOC are measured open circuit, batteries disconnected from the host circuit, after a period of an hour or more of “resting” to allow electrons to diffuse through the crystal matrix of the lead plates.  That approach works in a laboratory environment, but is not practical on any boat that is actively in use.  A made-for-purpose battery monitor is much more accurate than a DC voltmeter in tracking battery capacity.

Question 8: Do I really need a multi-stage battery charger?
Answer 8: Yes!
Discussion 8:  Single-voltage battery chargers are best used occasionally, and then primarily on start service batteries.  They are typically regulated to produce 14.6V – 14.8V.  Those voltages are way too high for all but the bulk phase of deep cycle battery charging.  At that, they are only suited to be the target voltage for the bulk phase of charging, not a steady voltage.  High charging voltage will permanently damage Gel cells, and can damage AGMs.  During periods of over-voltage, excessive current flow in wet cells causes the batteries to gas and causes oxides on the plates of the battery to “shed” and fall to the bottom of the cell.  Prolonged high charging voltage will result in damage to all batteries, and premature failure.

Remember also, there are at least two separate and independent battery charging systems on most trawlers.  One operates from shore or generator-supplied AC.  The second is the alternator on the propulsion engine.  BOTH OF THESE SYSTEMS SHOULD BE REGULATED, MULTI-STAGE CHARGING SOURCES.  Few OEM engine alternators come with built-in multi-stage voltage regulation.

All lead/acid batteries have a “natural Charge Acceptance” curve.  This curve is an “electrical property” of a battery, determined largely by battery construction type.  All lead/acid batteries discharged to 50% SOC can accept charge fairly rapidly up to about the 80% state-of-charge level.  The last 20% of charge can take more time to achieve than that first 80% step, but full charging is critically important to maximizing lead/acid battery service life.  Full charging is recommended by most battery manufacturers at least bi-weekly (fortnightly).

Multi-stage chargers are designed to take advantage of the natural charge acceptance characteristics of lead/acid batteries.  The initial period of rapid charge acceptance is called the “bulk” stage, followed by a period of slowing charge acceptance called the “absorption” stage.  When a battery is almost fully charged, a multi-stage charger will go into its “float” stage.  To the human observer, it’s fairly easy to see that point between bulk and absorb, because that’s when flooded wet cells begin to visibly, actively and aggressively produce hydrogen gas.  Outgassing is always considered ‘bad” for lead/acid batteries, and multi-stage battery chargers are designed to switch to float in order to minimize the conditions that cause gassing.  This is particularly important for AGM and Gel batteries, because replacing liquid electrolyte in these batteries is not possible, and gassing will permanently damage these batteries.

Question 9: How do I decide how big my battery charger needs to be?
Answer 9: Balance between battery bank capacity (in amp hours) and charger cost.
Discussion 9:  One of the many electrical characteristics of batteries is the rate at which the battery can accept charge.  This is called Charge Acceptance Rate (CAR).  Boaters will frequently read in advertising and on the Internet that flooded wet cells have a CAR of 25%.  What that means is flooded wet cells will accept a charge measured in AMPS, that is numerically equal to 25% of the capacity of the battery/bank, stated in AMP HOURS.  To illustrate the concept, assume the capacity of a flooded wet cell battery bank is 650 amp hours (a bank of 6 golf cart batteries).  The CAR for that wet cell bank is equal to 0.25 x 650 aHr, or 162.5 amps.  For AGM and Gel batteries, the “nominal” CAR is around 40%.  So, an AGM or Gel battery bank of 490 amp hours capacity (two 8Ds) would have a CAR of 0.40 x 490 aHr, or 196 amps.  For the average 40′ – 45′ trawler, a charger capable of between 100 and 150 amps is both functionally adequate and economically feasible for battery charging.

It is EXTREMELY IMPORTANT to realize that these “rule-of-thumb” CAR numbers (25% and 40%, respectively) are the rate at which a significantly discharged battery can accept charge.  As SOC increases, the Charge Acceptance Rate decreases.  A battery at 80% SOC will have a CAR well below the nominal 25% or 40% numbers.

Furthermore, it’s actually better for lead/acid batteries to charge at a rate that is slightly less than maximum CAR.  The reason is technical.  As charging voltages are applied to a battery, charge builds up on the surface of the lead plates.  It takes time for electrons to diffuse into the interior structure of the crystal matrix of the lead plates.  To the extent electrons accumulate on the surface of a battery’s plates, the battery appears to be at a higher SOC than it actually is.  If the battery charger has too large a charging capacity, it can switch charge stage prematurely.  If that happens, batteries can be chronically undercharged.  This allows sulfation to accumulate and results in shortened battery service life.  So, for trawlers and other power boats that run their engines continuously, it’s better to charge at a slightly slower rate than the maximum theoretical rate.

Question 10: Do different boats have different battery charging needs?
Answer 10:  Definitely, yes!
Discussion 10:  Many trawler owners have previously enjoyed sailing.  Sailboats motor out of the home mooring to the sailing field, sails are deployed, and engines are shut off.  At the end of the sail, the engine runs for only a short time while returning to port.  Thus,  sailboat propulsion engines have very short duty cycles.  During the time sailboats are under sail, their navigation instruments and lighting are supported by their batteries.

With trawlers, propulsion engines run continuously (well, we hope they do).  Under way, trawler navigation equipment and lighting – and the batteries themselves – are supported by the output of the engine alternator.  Rarely are batteries actually called on to provide power.  Sailboats cruising the ICW probably have engine duty cycles more similar to that of trawlers.  Anyway, sailboats that actually sail have very different battery charging requirements from trawlers and other cruising power boats, and the requirements of one does not generalize well to the other.  For sailboats that sail, when the engine is running, alternator output must be absolutely maximized, and typically operates in bulk mode.  For trawlers, battery charging is much more complex, and must automatically adjust to the changing requirements of the batteries during the overall charging cycle.

Question 11: Is there such a thing as “the best choice of batteries?”
Answer 11: Yes/No/Maybe/Maybe not…
Discussion 11:  There are many considerations in this question.  There is no “one-size-fits-all” answer.  My net is, for those who use their boats a lot, and prefer anchoring to staying in marinas, flooded wet cells provide the best ROI if they are charged and maintained properly.  That is a big if, because the statistics do not suggest that many owners are disciplined about maintaining their batteries.  Live-aboards and frequent use boaters are most likely to maintain their batteries properly.  For those who cruise from marina to marina, rarely or never anchor out, use their boats infrequently, or use them as floating winter condos in a marina in Florida, AGMs may be a good investment.  In that use, batteries would rarely if ever be deeply discharged, and being maintenance free is an advantage to their owners.

I wrote a post on my website, located here, that discusses these compromises in more detail:

Question 12: Can lead/acid batteries freeze?
Answer 12: Yes, but…
Discussion 12: If a lead/acid battery is partially discharged, the electrolyte may freeze.  At a 40% SOC, the electrolyte solution will freeze at around -16ºF.  When a battery is fully charged, the electrolyte will not freeze until the temperature drops to around -92ºF.   While few boats will encounter either of these temperatures, it is always wise to keep batteries fully charged before storing the boat for the winter!

Battery Replacement

It is the industry-best-practice, standard advice of battery retailers to recommend changing all the batteries in a bank at the same time and to have all of the batteries that make up a bank of the same brand, type, capacity and age.  Despite the up-front cost of replacing a large bank all at the same time, that advice is based on sound underlying technical considerations.   All batteries have life spans.  Batteries of the same type that make up a bank are likely to have similar life spans based on the physical and chemical make up of the batteries and the profile of the service the bank has been called upon to perform.  The useful life span of any particular battery depends on a lot on factors: operating temperature, vibration, suitability of charging and discharging profiles, number of charge/discharge cycles, average depth-of-discharge and the extent to which they have been fully re-charged and equalized over their service lifetimes.

Individual cells in batteries all age at slightly different rates. Plate surfaces become etched at slightly different rates. Plates sulfate at slightly different rates. The Specific Gravity of electrolyte changes at slightly different rates.  Specific gravity is the most reliable indicator of battery state-of-health, but the construction of AGM and Gel batteries makes it impossible to measure SpGr.  In the “equivalent circuit” of a battery, age-related variation in the Specific Gravity means the “internal resistance” of individual battery cells increases; some cells increase at higher rates than others.

Batteries wired into banks are functional sub-components of a “power distribution network.” Individual batteries are affected by whatever happens in the rest of the network. Regardless of age or state-of-health, a miscreant neighbor in the bank will affect the entire bank, almost always negatively.  Plate and electrolyte variations between individual cells in batteries in a parallel network leads to circulating currents that hasten the self-discharge process and lead to unbalances in both load-sharing and charging currents.  Within that battery bank network, the internal resistance of the individual battery cells and variations in the external resistance of the terminals, wiring crimps and conductors that interconnect the batteries affect the way large charging and load currents will be distributed among and between the members of the bank.  Circulating currents that do flow within the bank are distributed unevenly within a bank.  Even a bank that is physically disconnected from outside circuits will have internal circulating currents where more highly charged cells will try to “equalize” with cells that are less charged.   All this leads to cell deterioration over time.  A battery is considered “failed” when any cell in its construction  is failed.

The size of a bank also affects the above phenomena.  In a bank of two batteries in parallel, there are few interconnections.  In a bank of 6 or 8 paralleled batteries, there are many interconnections to affect the distribution of circulating currents.  In very large banks, many more interconnections again.  In series/parallel banks – two 6V batteries in series to create a 12V battery, or two 12V batteries in series to create a 24V battery – are extremely dependent on the length of the jumpers that make up the series-connection.  Very slight differences affect the resistance of the series-connection, which in turn affects the distribution of load and charging currents.

When any one battery in a bank reaches end-of-life, by definition, that means the other batteries in that bank *are not* new.  Internal sulfation has already occurred to some extent in all of the batteries in that bank.  Some amount of lead plate deterioration has happened in all of the batteries in that bank.  When one battery in a bank fails, there is no way to know the extent of internal, microscopic deterioration that has been accumulated and incurred on the not-yet-failed neighboring kinsman.  There is also no way to know if the others have one day of remaining service life or one year of remaining service life.  There is no way to know if the others will fail in a graceful, peaceful manner or go out in a blaze of glory!

If a new battery is placed in an bank network of older batteries, the resulting bank may “work” for some period of time; or perhaps, not.  A new battery has lower internal resistance than aged, older batteries.  Adding a new battery – even of the same brand, construction and technical specifications – will cause unbalanced currents flow in the bank.  That new battery will carry a disproportionally large share of the total load.  So, it may actually hasten the failure of another, older peer in the bank.

If a remaining, unknown state, older battery in the bank suffers an internal short, it may take other batteries with it, including a new one added only recently and wasting that investment.  Catastrophic battery failure can create heat, smoke and worse.  If that same remaining, unknown state, older battery just dies a quiet and uneventful death at anchor one night, then maybe you’ll be able to start the engine, and maybe you’ll be able to get the boat to safe harbor, and maybe a local chandlery will be able to provide a replacement battery, and maybe that replacement will be at a cost you’ll feel OK about.  Or, maybe, none of the above.

The only time I’d consider replacing just a single battery in a bank would be if I had very recently replaced the entire bank and subsequently had an “early life” failure – not more than 6 months – with one of the “new” batteries.  If that failure was peaceful, and not glorious, I’d replace just that one battery in just that one case.  In that one case, there would also be a warranty claim to ease the financial pain and cruising inconvenience.

Of course, this entire choice also depends on how the boat is used and the level of reliability the owner feels is necessary.  A boat that is always less than 50 miles from home, always in sight of land, and also carries towing insurance, has one set of reliability considerations.  A boat that cruises to remote regions, where banjo music and wolves baying at the moon in the distance can be heard in the isolated, remote anchorage, has – perhaps – a different set of reliability considerations.

My personal choice: to maximize reliability and peace-of-mind, I bite the bullet and replace all the batteries in a bank at the same time.

Separate or Combined House and Start Battery Bank?

The question of having a separate “house” battery and “start” battery or having a combined, dual purpose battery bank is largely a matter of personal preference.  Each arrangement has pros and cons, and there is nothing inherently bad about either arrangement.

Modest sized cruising trawlers have limited space for everything, including batteries.  Sanctuary had never been used by her previous owners as a cruising boat.   Her previous owners may have anchored occasionally, but their preference had been to use marinas.  When they did anchor, they were clearly in “camping out” mode, not in “living normally” mode.  So when we bought the boat, she was in her OEM configuration, fit with two 8D batteries aboard.  One 8D was dedicated to starting the Cummins 4BT-3.9 propulsion engine.  The other 8D was used to power house DC loads.  In that configuration, the “start” battery was seriously under-utilized; and, the house battery was seriously over-utilized.  There was a manual “1-Both-2” switch to combine the batteries, but using it effectively required human awareness and diligent attention.   As I “wind down” toward bed time in the evening, I often fail most gloriously in the areas of awareness and diligence.  So, Sanctuary’s configuration was a very poor arrangement in terms of commodity utilization and human usability.

Starting the engine requires many amps for a very short period of time; perhaps 500A for 5 seconds or less.  The battery needs to supply cranking amps (CA, CCA, MCA, Reserve Capacity) but not amp hour (aHr) capacity.  High, short term demand is not the design profile of loading of a deep cycle battery.  At the same time, a single 8D was not adequate to handle house DC requirements in cruising service, and we needed more energy capacity for house use (refrigeration, water pumps, space lighting, computer, TV/DVR).  However, we did not have the space to install a third 8D battery.  It was obvious that combining the two existing 8D batteries into a dual purpose bank was the alternative that returned the best utility for our situation.  We ran with the two 8Ds combined into a single bank configuration for several years after that with no problem.  Then the day came when that bank went casters-up.

In the summer of 2012, both of the 8D AGM batteries failed, a week apart.  We were cruising in New England at that time, not near our home port, familiar marinas or regular marine chandleries.  Fortunately, a cruising friend offered us a mooring ball at his yacht club, and the use of his pick-up.  I am no longer able to lift and move an 8D battery by myself, so we decided to permanently replace the 8Ds with 6, 6V Golf Cart batteries, wired in a series-parallel configuration, to support our 12V DC system.   The yacht club where we were moored allowed me to pull up to their dock (they were closed on Monday), where I loaded the batteries and spent the afternoon fabricating battery cables.  I made up 8″ intermediate cables to make the series connections, and buss cables to add what was effectively a third 12V battery into the bank.  The resulting bank occupied the same floor footprint as the 2 8Ds had needed.

Following is a diagram of the resulting DC Distribution System aboard Sanctuary”


Anyone who wishes can download an Adobe .pdf copy of this diagram by clicking this link: 20161022_dc_electrical_distribution_system.

A combined battery bank also solved several related problems, including the proper charging and load balancing on the two batteries, while giving us the additional house capacity we needed.  So that’s how we have run for the last several years.  We think it has been a net gain in several respects.  We think there is nothing inherently bad about having a single bank that feeds both the house loads and starts the engine.  For a combined battery bank, however, there are two clear cautions: 1) the bank must be big enough so that when it is discharged to the 50% point, it can still easily start the propulsion engine, and 2) there must be a reliable and convenient means to track the state-of-charge of the battery bank to avoid deep discharging to more than 50%.  Fortunately, several battery monitoring products are available that enable reliable, accurate battery monitoring.  Meeting these two requirements is therefore quite easy and relatively inexpensive.

There are, of course, only two backup options to a severely discharged/dead battery bank.  One is to run a genset to re-charge the bank, and the other is good towing insurance.  If the genset option fails, the final option is to call for a tow.  Our genset does have its own stand-alone start battery.  The genset charges its own start battery when it is running.  We also have a small, stand-alone external AC charger for the genset start battery.  We use it periodically – not continuously – when we are docked for extended periods of time.  That keeps the genset start battery conditioned and compensates for self-discharge.

Which Batteries are Best?

On boats, there are two very different sorts of DC electrical loads.  Electronic navigation equipment (VHF Radio, RADAR, Chart Plotter, GPS, AIS), navigation lights and space lighting, water pump(s), refrigeration, computer equipment and entertainment systems require relatively modest amounts of electrical energy over rather a prolonged period of time.  Propulsion and generator engine starter motors, thruster motors and windlass motors require rather large amounts of electrical energy over very short-period bursts.

Lead-acid battery options are manufactured to support the needs of both of these electrical load profiles.  “Traction” batteries are designed to operate in a physically harsh environment and be relatively deeply-discharged many times.  These batteries are intended to supply small-to-moderate amounts of power for long periods of time.  Automotive and light truck “start” batteries are designed to provide very large amounts of energy (amperes) over short periods of time.  They are not designed to be deeply discharged, and can be permanently damaged by deep discharging.  A compromise in manufacturing technique is the “commercial” battery.  These are often found in RV and marine applications to power house DC loads.  They are not a true start battery, but they are also not a true traction battery: they are a compromise between the two.

Battery manufacturers rate traction batteries in Ampere-Hours (aHr), which is a function of lead mass.  Start batteries are rated in terms of Cranking Amps (CA), Cold Cranking Amps (CCA), Marine Cranking Amps (MCA) and Reserve Capacity, a function of lead surface area.  These technical ratings lead to the very different profiles of electrical load the two very different battery-types are intended to support.  These ratings also reflect the very different construction and materials used in these different purpose batteries.

In the US, traction batteries get their aHr rating based on carrying a load from the fully-charged state to the fully-discharged state in a period of 20 hours.  All lead-acid battery manufacturers recommend against discharging traction batteries to more then 50% of the battery’s rated amp-hour capacity.  That means a 200 aHr overnight DC electrical requirement would have to have a minimum of 400 aHr of installed battery capacity.  Batteries that experience a lesser average level-of-discharge will generally return a longer installed service life.  Traction batteries that are discharged at a rate greater than their 20-hour rate are not able to deliver the full 20-hour-rated aHr label capacity.

Traction batteries are built with relatively few, but relatively thick, lead plates.  Start batteries have a relatively greater number of lead plates than traction batteries, but the plates are also relatively thinner.  This design difference gives start batteries a much greater surface area of lead from which chemical energy can be quickly converted into electrical energy.  Traction batteries can be used in starting service, but generally need to be of larger capacity (and therefore, physically larger and heavier) than a made-for-purpose start service battery.

There are two grades of traction batteries.  The 4D and 8D batteries commonly used on boats and RVs are “commercial grade” batteries.  They are used in commercial services to start engines on large trucks and in many types of construction equipment.  They tolerate moderate amounts of mechanical vibration and offer large CCA capacities.  They support a reasonable mix of starting and deep-cycle applications.  Electric car/golf cart batteries are true deep-cycle batteries, again with thicker lead plates and electrolyte reservoirs that are deeper below the plates than their commercial cousins.  Because of plate thickness, they release their energy relatively more slowly, but for much longer periods.  The thickness of the lead plates minimizes sulfation, and improves plate remodeling during the charging process.  Particularly in hard-service applications, these design elements extend their service life.

In RV and boat installations where there are separate battery banks for “start” and “house” applications, start batteries would be most appropriate to power boat engine starter motors, thrusters and windlasses, and traction batteries would be most appropriate to power navigation, computer and entertainment system electronics, refrigeration, water pumps and DC lighting needs.  Battery charging and combining issues often incent boat owners to use traction batteries for all DC loads on their boats.  That can be inefficient in terms of electrical usage, but can also be quite workable.  Aboard Sanctuary, we have 6, 6V traction batteries (deep-cycle golf cart batteries) arranged to comprise a single 12VDC bank that powers all of our DC loads, including engine starter motors and our windlass.  Our genset has it’s own, dedicated 12V “dual-purpose” battery.  That combination gives us greatly more Reserve Capacity and CCA than necessary for engine starting (inefficient), but very adequate aHr reserves for overnight anchoring in seasons with short hours-of-daylight.  This minimizes our need to run the genset for battery charging.  (Our thruster system is hydraulic, not electric.)

In consideration of the above, the question, “which batteries are best for a boat,” remains a bit of a “religious” discussion, like which anchor is best or what’s the best micron size of fuel filters.  To a great extent, the best answer is, “it depends on how you plan to use the batteries and the boat.”

First, I’d suggest avoiding all 8D form factor traction batteries, not because they’re inherently bad, but because they are very heavy.  At 160# or so per battery, more for some, I can not handle 8Ds by myself without risking personal injury.   When they fail – and they do fail – you need to be able to handle them yourself, because it won’t happen when you’re at your home slip with lots of help available!  Two 6-volt GC2 form factor Electric Car batteries will easily fit in the same foot print as a single 8D, and will have very nearly the same energy capacity (measured in Amp Hours).  Three GC2 batteries will “just fit” in most 8D battery boxes, so that would allow six GC2s in the same space as two 8Ds.  If so, that will give you significant additional aHr capacity in the same space two 8Ds require.  For example, most 8D batteries range around 225 – 245 aHr; a pair of GC2 batteries in series will result in a 12V battery in the range of 215 – 230 aHr.

The technology choices for deep cycle “Traction” batteries (“marine”/”commercial”) boils down to the selection of “flooded wet cell,” “AGM” or “Gel” batteries.  All three are lead-acid technology.  Optima spiral-wound batteries are a special – and expensive – case of lead-acid AGM technology.  Firefly Carbon-Foam AGM batteries are another lead-acid technology.  Each of these technologies has pros and cons associated with them.  Some of the pros and cons will carry personal value for some boaters, but not for others.

Flooded lead acid batteries require regular maintenance.  That means periodically checking and adding distilled water, and periodically equalizing them.  Flooded lead acid Electric Car (Golf Cart) batteries are “commodities,” available everywhere in the world; even in the third world.  Flooded batteries have relatively high self-discharge rates.  AGM and Gel batteries may not be either available or affordable in many places, even in the US.  AGM and Gel technologies are “maintenance-free,” meaning electrolyte isn’t lost in normal operation, but also can’t be added.  AGM and Gel batteries generally can’t be equalized. (Some AGMs can, but that is manufacturer-specific, and the general rule is: not.)  AGM and Gel can be mounted in any physical orientation, including standing-on-side and standing-on-end.  AGM and Gel batteries are adversely affected by ambient temperature, Gel more than AGM, so mounting them in the engine room can result in shortened service life in some installations.

Charging traction (deep cycle) lead-acid batteries – regardless of flooded, AGM or Gel – should be done with a modern 3-stage charger set to the correct charging program for the battery technology.  There are lots of technical issues around battery charging which I’m not discussing here (see my separate article, here).  Suffice it to say that a charger with a 100A – 125A DC output for a 675 aHr battery bank is a perfectly acceptable solution; not perfect, but a highly acceptable compromise from a lifetime ROI perspective.

Flooded lead acid batteries are the least expensive traction batteries to buy (lowest capital cost).   The commodity cost of lead has driven battery cost way up in the last few years.  With the Chinese buying the entire world-wide reserves of lead ore, that commodity cost will probably continue to rise.  My summer, 2012, experience was that I bought six, 6V EGC-2 batteries rated at 230 aHr in June/July, 2012, for $92 apiece, from Sam’s Club.  At that time, a single Deka AGM 8D was $600.  So I was able to buy 690 aHr from Sam’s Club for what I would have paid for one 245 aHr AGM commercial battery at Hamilton Marine in Portland, ME.

Now, here following is the “religious” part of the battery topic.  The real issue is, “what’s the best ROI on the battery technology selection you buy?”  And the answer is, again, “it depends.”  From the reading I’ve done and the experience I’ve had aboard Sanctuary since 2004, running 60K miles and 7500 hours engine hours, and anchoring out 1/2 to 2/3rds of the time while traveling,  I am persuaded that if you are an active cruiser –  if you actually use your boat, put many hours on it each year, and anchor out in preference to using marinas – flooded wet cells provide the best ROI.  They are the least expensive to buy, and they return good charge/discharge cycle life.  The Sam’s Club batteries with Duracell branding are manufactured by East Penn in the United States.  They are the same batteries that you’d buy retail, for much more money, over the counter at NAPA or West Marine.  Yes; literally the same batteries, save for the house labels, made by the same people on the same production line in Pennsylvania.

My own personal experience with AGM batteries (Deka, the East Penn brand label) did not meet my desires or expectations.  I have had two sets of Deka AGM batteries fail after only 3 – 4 years in service (less than 300 charge/discharge cycles).  They were not excessively discharged in service.  State-of-charge was monitored with a Xantrex Link20 battery monitor, later upgraded to a Magnum ME-BMK battery monitor.  Both of my battery chargers (Magnum MS2012 inverter/charger on shore power, Balmar 712110 alternator with Balmar MC-614 (earlier, ARS-5) voltage regulator on the main engine) are multistage chargers.  The chargers use the correct AGM charging profiles.  And when these batteries failed, they failed overnight, not slowly; no advance warning. They did not return the charge/discharge cycle life they should have been able to return, and at $600 apiece in 2012, I no longer feel they’re the best choice for our cruising profile for lifetime ROI.

That said, for people who don’t use their boats much, for those who use marinas rather than anchoring out, etc., the AGM and Gel maintenance free batteries may be a good choice.  These owners will get several years of service from the batteries.  In these applications, it may not matter that they haven’t returned their rated cycle life.


One final “religious” argument comes up with comparing Sam’s Club batteries to high-end Trojans, Rolls Surette, Lifeline, Optima, or other “high-end” battery brands.  Again, “it depends.”  There is no denying those brands are excellent, well made batteries.  They are also expensive to buy.  If two boats have equivalent aHr battery banks, equivalent loads, and equivalent usage profiles, it is likely that these premium brands will, indeed, outlast my Sam’s Club batteries.  However, the real question is, will they outlast them by the ratio of their purchase price to mine?  If mine last 5 years, will theirs last 15 years?  Unlikely, I think.  So, my Sam’s Club batteries may well offer better ROI and lesser total-cost-of-ownership than those elegant premium brands.  My trade-off opinion then is, for coastal cruising in the US, the Bahamas and Canada, Sam’s Club batteries are just fine.  If you’re a blue water cruiser who’s going to circumnavigate and travel to places like Figi or the Marquesas, or cruise south of 60º S latitude in the Channel Islands or Straights-of-Magellan of Chili, well, maybe in that case the premiums offer more comfort and re-assurance to their owners.  But I’m a mid-size trawler cruiser.  Most single engine trawler owners are not going to circumnavigate, cross the Atlantic, or cruise the Windwards.  So for me, Sam’s Club batteries are just fine for coastal and near-coastal cruising in the US, the Bahamas and Canada.

Optima batteries are a variation of AGM technology called “thin plate pure lead” (TPPL).  They return lots of aHrs per unit of weight and space, and are relatively more efficient (lower Peukert’s exponent) to charge than other AGM batteries.  They are expensive to purchase, per aHr.  The technology has great potential, but not for me at current consumer price levels. In automotive start service batteries, Sears Diehards are of this “spiral wound” TPPL design.

LiON batteries are an entirely new and “emerging” battery technology.  They deliver several times the amount of energy at 1/4 the weight compared to their lead-acid counterparts.  LiON batteries (the lithium, iron, phosphate variant) are stable and safe in operation.  Some early adopters have installed LiON battery systems and are getting very encouraging results.  That said, in 2013/2014, I consider LiON systems as an emerging technology.  The technology requires very different charging and battery monitoring equipment which is not compatible or interchangeable with lead-acid charging equipment or systems.  LiON charging and monitoring equipment is currently (2014, 2015) only available from hobbyist and custom-developer sources.  It is not yet available as a commercial product from a reliable equipment manufacturer with stated length-of-life expectations, reliability and quality standards and consumer product warranties.  LiON batteries are currently quite expensive.  Supplier sources and product availability are highly limited.  My net is, for the average boat owner/cruiser, from the perspective of fit-up cost, lifetime ROI and in-service maintenance, I consider conversion from lead-acid to LiON technologies to be impractical at this time.  Yes, it works.  Yes, it is highly efficient.  Yes, piece parts are available from specialty sources.  But, significant technical knowledge is required to install and manage these systems.  If an outage occurs, the necessary service knowledge is not yet generally available in the marine service industry.   This technology undoubtedly represents the future, but not yet the present.

For those interested, following is the chemistry of a flooded wet cell during discharge.  When the lead/acid galvanic cell discharges into an electrical load, the following reactions occur:

Anode half-cell reaction:
Pb(s) + HSO4(aq) + H2O(l) → 2e + PbSO4(s) + H+(g) + H2O(aq)

Cathode half-cell reaction:
PbO2(s)+ HSO4(aq) + 3H3O+(aq) + 2e  → PbSO4(s) +5H2

Add the two half-cell reactions together, the full-cell discharge reaction is:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(aq)

During discharge of a lead/acid cell:

  • Notations: (s)=solid, (l)=liquid, (g)=gas, (aq)=aqueous solution
  • PbSO4 (lead sulfate) precipitates out and deposits on BOTH the anode and the cathode.
  • Free hydrogen (H+) from the sulfuric acid electrolyte (H2SO4(aq)) produces water (H2O(aq)) at the cathode.
  • The concentration of free hydrogen (H+) decreases over time.
  • The concentration of sulfuric acid (H2SO(aq)) decreases over time.
  • The pH of the electrolyte (H2SO4(aq)) increases over time.
  • Two electrons are transferred in the overall reaction.
  • Both half-cell reactions go from left to right when load is applied to the battery.
  • The half-cell reactions are different processes.
  • For each mole of lead sulfate produced, two moles of electrons travel through the external circuit.
  • During discharge, the “-” plate is the “anode” (since the “-” plate material is being oxidized),
  • During discharge, the “+” plate is the “cathode” (since the “+” plate material is being reduced).