Category Archives: Batteries – Questions and Answers

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!