Category Archives: Batteries – Charging and Care

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, 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 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.  So, 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 for a 100 aHr AGM or Gel cell at 50% SOC, CAR would be 40A.  Indeed, some specialized AGM cells (Thin Plate Pure Lead) 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.  The batteries are expensive, and owners would not install those batteries without knowing what they can do and why they’d 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.  Batteries do have different specifics.   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 batteries/bank can accept.  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.  1) battery terminal voltage,
  2.  2) elapsed time,
  3.  3) battery temperature, and
  4.  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 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 in 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 running on a genset at anchor, not so much.

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 flooded wet cell battery bank that is around 50% SOC; it’s 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).   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 these days is to get a coulomb counting monitor from the manufacturer of the charger or inverter/charger.   Aboard Sanctuary, a Magnum MS-Series Inverter/Charger is installed.   I replaced my 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 alternative today.

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.  “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 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 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.