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How Lead Acid Batteries Work

Here is a short run-through of how lead-acid batteries work.  I'll start with some basics and work my way up - hence the absence of an alphabetical order.  Depending on your familiarity with the subject, you may want to scroll down more or less.


Voltage is an electrical measure which describes the potential to do work.  The higher the voltage the greater its risk to you and your health.  Systems that use voltages below 50V are considered low-voltage and are not governed by as strict (some might say arcane) a set of rules as high-voltage systems.


Current is a measure of how many electrons are flowing through a conductor.  Current is usually measured in amperes (A).  Current flow over time is defined as ampere-hours (a.k.a. amp-hours or Ah), a product of the average current and the amount of time it flowed.


Power is the product of voltage and current and is measured in Watts.  Power over time is usually defined in Watt-hours (Wh), the product of the average number of watts and time.  Your energy utility usually bills you per kiloWatt-hour (kWh), which is 1,000 watt-hours.

What is a Lead-Acid Battery?

A lead-acid battery is a electrical storage device that uses a reversible chemical reaction to store energy.  It uses a combination of lead plates or grids and an electrolyte consisting of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again.  The electrolyte of lead-acid batteries is hazardous to your health and may produce burns and other permanent damage if you come into contact with it.  Thus, when dealing with electrolyte protect yourself appropriately!

Deep Cycle vs. Starter Batteries

Batteries are typically built for specific purposes and they differ in construction accordingly.  Broadly speaking, there are two applications that manufacturers build their batteries for: Starting and Deep Cycle.

 As the name implies, Starter Batteries are meant to get combustion engines going.  They have many thin lead plates which allow them to discharge a lot of energy very quickly for a short amount of time.  However, they do not tolerate being discharged deeply, as the thin lead plates needed for starter currents degrade quickly under deep discharge and re-charging cycles.  Most starter batteries will only tolerate being completely discharged a few times before being irreversibly damaged.

 Deep Cycle batteries have thicker lead plates that make them tolerate deep discharges better.  They cannot dispense charge as quickly as a starter battery but can also be used to start combustion engines.  You would simply need a bigger deep-cycle battery than if you had used a dedicated starter type battery instead.  The thicker the lead plates, the longer the life span, all things being equal.  Battery weight is a simple indicator for the thickness of the lead plates used in a battery.  The heavier a battery for a given group size, the thicker the plates, and the better the battery will tolerate deep discharges.

 Some "Marine" batteries are sold as dual-purpose batteries for starter and deep cycle applications.  However, the thin plates required for starting purposes inherently compromise deep-cycle performance.  Thus, such batteries should not be cycled deeply and should be avoided for deep-cycle applications unless space/weight constraints dictate otherwise.

Regular versus Valve-Regulated Lead Acid (VRLA) Batteries

Battery Containers come in several different configurations.  Flooded Batteries can be either the sealed or open variety.

Sealed Flooded Cells are frequently found as starter batteries in cars.  Their electrolyte cannot be replenished.  When enough electrolyte has evaporated due to charging, age, or just ambient heat, the battery has to be replaced.

Deep-Cycle Flooded cells usually have removable caps that allow you to replace any electrolyte that has evaporated over time.  Take care not to contaminate the electrolyte - wipe the exterior container while rinsing the towel frequently.

VRLA batteries remain under constant pressure of 1-4 psi.  This pressure helps the recombination process under which 99+% of the Hydrogen and Oxygen generated during charging are turned back into water.  The two most common VRLA batteries used today are the Gel and Absorbed Glass Mat (AGM) variety.

Gel batteries feature an electrolyte that has been immobilized using a gelling agent like fumed silica.

AGM batteries feature a thin fiberglass felt that holds the electrolyte in place like a sponge.

Neither AGM or Gel cells will leak if inverted, pierced, etc. and will continue to operate even under water.

Battery Cells

Battery Cells are the most basic individual component of a battery.  They consist of a container in which the electrolyte and the lead plates can interact.  Each lead-acid cell fluctuates in voltage from about 2.12 Volts when full to about 1.75 volts when empty.  Note the small voltage difference between a full and an empty cell (another advantage of lead-acid batteries over rival chemistries).

Battery Voltage

The nominal voltage of a lead-acid battery depends on the number of cells that have been wired in series.  As mentioned above, each battery cell contributes a nominal voltage of 2 Volts, so a 12 Volt battery usually consists of 6 cells wired in series.

State of Charge

The State of Charge describes how full a battery is.  The exact voltage to battery charge correlation is dependent on the temperature of the battery.  Cold batteries will show a lower voltage when full than hot batteries.  This is one of the reasons why quality alternator regulators or high-powered charging systems use temperature probes on batteries.

Depth of Discharge (DOD)

The Depth of Discharge (DOD) is a measure of how deeply a battery is discharged.  When a battery is 100% full, then the DOD is 0%.  Conversely, when a battery is 100% empty, the DOD is 100%.  The deeper batteries are discharged on average, the shorter their so-called cycle life.

For example, starter batteries are not designed to be discharged deeply (no more than 20% DOD).  Indeed, if used as designed, they hardly discharge at all: Engine starts are very energy-intensive but the duration is very short.  Most battery manufacturers advocate not discharging their batteries more than 50% before re-charging them.

Battery Storage Capacity

The Amp-hour (Ah) Capacity of a battery tries to quantify the amount of usable energy it can store at a nominal voltage.  All things equal, the greater the physical volume of a battery, the larger its total storage capacity.  Storage capacity is additive when batteries are wired in parallel but not if they are wired in series.

Most marine, automotive, and RV applications use 12V DC.  You have the choice to either buy a 12V battery or to create a 12V system by wiring several lower-voltage batteries/cells in Series.


When two 6V, 100Ah batteries are wired in Series, the voltage is doubled but the amp-hour capacity remains 100Ah (Total Power = 1200 Watt-hours).

You may decide to wire batteries in series because a single 12V battery with the right storage capacity is simply too heavy, unwieldy, or awkward to lift into place.  Batteries consisting of fewer cells (and hence lower voltage) in series can provide the same storage capacity yet be portable.  It is not unusual to see solar power installations where the battery bank consists of a sea of 12V batteries that have been wired in series.

parallel battery diagram

Two 6V, 100Ah batteries wired in Parallel will have a total storage capacity of 200Ah at 6V (or 1200 Watt-hours).

Battery banks consisting of 12V batteries wired in parallel are often seen on OEM installations in boats and RVs alike.  Such banks are simple to wire up and require a minimum of cabling.  However, the wiring must have the capacity to deal with a full battery bank.

You should fuse each battery individually in such a bank to ensure that a battery gone bad will not affect the rest of the bank.

Series-Parallel Battery Diagram

Battery banks wired in Series-Parallel are even more complicated.  Here, four 6V cells are wired in two "strings" of 12VDC that were then wired in parallel.  Using 6V, 100Ah batteries, this system will have a storage capacity of 200Ah at 12V or 2,400Wh.

Since such a system has more wiring, it is very important to group "strings" logically and to label everything.  Furthermore, it is a very good idea to fuse every "string" of series-wired batteries to ensure that a problem in one part of the battery bank does not take the whole bank down.

We use Group GPL4C batteries exclusively on our boat.  Since these batteries have a nominal voltage of 6V, we have wired them in series for the starter bank (2 batteries) and series-parallel for the house bank (4 batteries).

Despite advances in instrumentation, the battery industry mostly still advertises amp-hours as a capacity measure instead of watt-hours.  Hopefully, the battery and marine power instrumentation industry will make a transition to Watt-hours (Wh) in the future.

Available Capacity versus Total Capacity

Since batteries depend on a chemical reaction to produce electricity, their Available Capacity depends in part on how quickly you attempt to charge or discharge them relative to their Total Capacity.  The Total Capacity is frequently abbreviated to C and is a measure of how much energy the battery can store.  Available Capacity is always less than Total Capacity.

Typically, the amp-hour capacity of a battery is measured at a rate of discharge that will leave it empty in 20 hours (a.k.a. the C/20 rate).  If you attempt to discharge a battery faster than the C/20 rate, you will have less available capacity and vice-versa.  The more extreme the deviation from the C/20 rate, the greater the available (as opposed to total) capacity difference.

However, as you will discover in the next section, this effect is non-linear.  The available capacity at the C/100 rate (i.e. 100 hours to discharge) is typically only 10% more than at the C/20 rate.  Conversely, a 10% reduction in available capacity is achieved just by going to a C/8 rate (on average).  Thus, you are most likely to notice this effect with engine starts and other high-current applications like inverters, windlasses, desalination, or air conditioning systems.

For example, the starter in an engine will typically quickly outstrip the capacity of the battery to keep cranking it for any length of time.  Hence the tip from mechanics to wait some time between engine start attempts.  Not only does it allow the engine starter to cool down, it also allows the chemistry in the battery to "catch-up".  As the battery comes to a new equilibrium, its available capacity increases.  A very elegant equation developed in 1897 by a scientist called Peukert describes the charging and discharging behavior of batteries.

The Peukert Effect

As you can see below, the Peukert equation consists of several factors.

Peukerts Equation: I n x T = C


  • I is the current (usually measured in amperes)
  • T is time (usually measured in hours)
  • n is the Peukert number / exponent
  • C is the theoretical storage capacity of the battery (usually measured in amp-hours).  Use the C/100 capacity or add 10% to the storage capacity at the C/20 rate.

  • As you can see, the available current is dependent on the rate of discharge and the Peukert exponent for the battery.  The closer the exponent is to 1 (one), the less the available capacity of a battery will be affected by fast discharges.  Peukerts numbers are derived empirically and are usually available from manufacturers.  They range from about 2 for some flooded batteries down to 1.05 for some AGM cells.  The average peukerts exponent is 1.2 though the exact number depends on the battery construction and chemistry.

    The following image shows the dramatic impact of the Peukerts exponent on the available capacity of a 120Ah battery, depending on the ampere draw.  As you can see, the lower the Peukerts Exponent, the lesser the effect on available capacity.  Note the dramatic difference in Available Capacity between the average flooded cell (n = 1.20) and a deep cycle AGM (n = 1.08) with high-current applications.


    In the above picture, note how the low exponent battery (topmost curve) has more than four times the available capacity over a high-exponent battery (lowest curve).  This chart uses a linear scale.

    When the time comes to charge a battery, the Peukerts effect also comes into play.  The capacity of a battery to absorb a charge during the bulk phase is also dependent on it's Peukerts number.  This is one of the reasons why AGM cells can be bulk charged at much higher rates than either Gel or Flooded cells.

    Reserve Minutes

    Reserve Minutes are a measure of how long your battery can sustain a load before it's available capacity has been completely used up.  This measure is especially useful for folks who want to run inverters, fridges, and other large loads.  The following chart has a logarithmic time scale (minutes) - hence, the non-linear nature of the Peukert effect is smoothed out quite a bit.


    Note how batteries that have a high Peukerts Exponent will quickly run out of capacity with high loads.  Here, the low-exponent battery will last over 100 minutes with a 50 ampere load, while the high-exponent battery will last about 20 minutes.  Thus, anytime you deal with large loads relative to the battery capacity available, chose a low-exponent battery.  This is why many wheel-chairs and other electrically motorized vehicles use AGMs.

    This chart answers why starter batteries are built to have a low Peukerts exponent.  Otherwise, they'd simply not be able to crank an engine for more than a few seconds.  However, the thin plates that allow flooded cells to work as starter batteries also make them too fragile for deep-cycle use.

    Conversion Efficiency

    The conversion efficiency denotes how well a battery converts an electrical charge into chemical energy and back again.  The higher this factor, the less energy is converted into heat and the faster a battery can be charged without overheating (all other things being equal).  The lower the internal resistance of a battery, the better its conversion efficiency.

    One of the main reasons why lead-acid batteries dominate the energy storage markets is that the conversion efficiency of lead-acid cells at 85%-95% is much higher than Nickel-Cadmium (a.k.a. NiCad) at 65%, Alkaline (a.k.a. NiFe) at 60%, or other inexpensive battery technologies.

    Battery Life

    Battery manufacturers define the end-of-life of a battery when it can no longer hold a proper charge (for example, a cell has shorted) or when the available battery capacity is 80% or less than what the battery was rated for.  The life of Lead Acid batteries is usually limited by several factors:

  • Cycle Life is a measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out.  The greater the average depth-of-discharge, the shorter the cycle life.

  • Age also affects batteries as the chemistry inside them attacks the lead plates.  The healthier the "living conditions" of the batteries, the longer they will serve you.  Lead-Acid batteries like to be kept at a full charge in a cool place.  Only buy recently manufactured batteries, so learn to decipher the date code stamped on every battery... (inquire w/manufacturer).  The longer the battery has sat in a store, the less time it will serve you!  Since lead-acid batteries will not freeze if fully charged, you can store them in the cold during winter to maximize their life.

  • Sulphation is a constant threat to batteries that are not fully re-charged.  A layer of lead sulphate can form in these cells and inhibit the electro-chemical reaction that allows you to charge/discharge batteries.  Many batteries can be saved from the recycling heap if they are Equalized

  • Equalization

    Sulphation layers form barrier coats on the lead plates in batteries that inhibit their ability to store and dispense energy.  The equalization step is a last resort to break up the Sulphate layers using a controlled overcharge.  The process will cause the battery electrolyte to boil and gas, so it should be only done under strict supervision and with the proper precautions.

    It is much more tricky to equalize a VRLA battery than a flooded battery with removable caps.  However it apparently can be done as described at the Ample Power web site.  Since I do not have the space here to describe the Equalization process in detail, I'd consult some of the links on the index page instead.


    Batteries start to gas when you attempt to charge them faster than they can absorb the energy.  The excess energy is turned into heat, which then causes the electrolyte to boil and evaporate.  The evaporated electrolyte can be replenished in batteries with removable caps such as most flooded deep-cycle batteries.  Many car batteries are sealed and thus need to be replaced when their electrolyte evaporates over time.

    Since AGM and Gel cells are always sealed, it is very important to guarantee they are not overcharged.  The only way to ensure this is to use a temperature-compensated charging system.  Such chargers use a temperature probe on the battery to ensure that the battery does not get too hot.  As the battery heats up, the charging current is reduced to prevent thermal runaway, a very dangerous condition.

    Thermal Runaway

    This is a very dangerous condition that can occur if batteries are charged too fast.  The byproducts of Gassing are Oxygen and Hydrogen.  As the battery heats up, the gassing rate increases as well and it becomes increasingly likely that the Hydrogen around it will explode.  The danger posed by high Hydrogen concentrations is one of the reasons that the American Boat and Yachting Council (ABYC) requires that batteries be installed in separate, well-ventilated areas.


    The self-discharge rate is a measure of how much batteries discharge on their own.  The Self-Discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside.

    For instance, flooded cells typically use lead alloyed with Antimony to increase their mechanical strength.  However, the Antimony also increases the self-discharge rate to 8-40% per month.  This is why flooded lead-acid batteries should be in use often or left on a trickle-charger.

    The lead found in Gel and AGM batteries does not require a lot of mechanical strength since it is immobilized by the gel or fiberglass.  Thus, it is typically alloyed with Calcium to reduce Gassing and Self-Discharge.  The self-discharge of Gel and AGM batteries is only 2-10% per month and thus these batteries need less maintenance to keep them happy.

    Battery Group Size

    To further complicate matters, manufacturers for marine batteries make them in all sorts of sizes and voltages.  Battery case sizes are typically denoted by a "Group Size" which has nothing to do with the actual size of the battery.  For example, Group 8D batteries are much larger than Group 31 batteries. Here are some examples:

    Battery Group Units
    21 24 27 30H 31 4D 8D T105 GPL4C L16
    Voltage 12 12 12 12 12 12 12 6 6 6 Volts
    Length 8.19 10.25 12.06 13.5 13 20.75 20.62 10.38 10.35 11.62 inches
    Width 6.81 6.81 6.81 6.81 6.72 8.75 10.95 7.13 7.06 7 inches
    Height 8.75 8.87 8.75 9.25 9.44 9.88 10.17 11.2 11.57 17.56 inches

    Note: Dimensions are approximate and vary by manufacturer. Consult manufacturer data sheets for exact dimension of container, location and type of terminals, etc.

    Table of Battery Group Sizes, Voltages, and Approximate Exterior Dimensions:

    The group size will merely indicate the approximate exterior dimensions (including terminals) and voltage of the battery in question.  However, the exact dimensions can only be directly obtained from each manufacturer.

    Nickel-Cadmium Cells

    Several people have inquired about NiCad cells for Marine environments.  I am not a great fan of them due to their toxicity and their low power conversion efficiency. See my NiCad info page for more information about the pros and cons of NiCad technology for marine applications.