What is a Battery?


I wanted to do an article on marine batteries, but with me not being anywhere of an expert on the subject and after finding this website, it appears to be so informative that I copied it here and am giving them credit.  In the past, I have just linked to good articles, only to down the road, find that the link has died for what ever reason, never to be found again.  The website copied here is  http://www.windsun.com/Batteries/Battery_FAQ.htm

Look very carefully at the chart below.  The older 6 volt car batteries when they got low, would still turn the starter over, just a lot slower.  But with the 12 volt, they need a FULL charge with anything below possibly 12.2 all you will get is maybe just the starter solenoid clicking.  Sure, it may run a radio but that takes a lot less power and is not near that voltage sensitive.

State of Charge

12 Volt battery
























One thing to remember is (probably twice a year) to remove the battery terminals from the battery, clean them, check the wiring for any corrosion, clean and coat the connections with a anti corrosion inhibitor.   Even do this on your towing vehicle so that you don't get in a situation where your vehicle may not start at an inappropriate time.


A battery, in concept, can be any device that stores energy for later use.   A rock, pushed to the top of a hill, can be considered a kind of battery, since the energy used to push it up the hill (chemical energy, from muscles or combustion engines) is converted and stored as potential kinetic energy at the top of the hill.   Later, that energy is released as kinetic and thermal energy when the rock rolls down the hill.   Not real practical for everyday use though.

Common use of the word, "battery" in electrical terms, is limited to an electrochemical device that converts chemical energy into electricity, by a galvanic cell.   A galvanic cell is a fairly simple device consisting of two electrodes of different metals or metal compounds (an anode and a cathode) and an electrolyte (usually acid, but some are alkaline) solution.   A "Battery" is two or more of those cells in series, although many types of single cells are usually referred to as batteries - such as flashlight batteries.

As noted above, a battery is an electrical storage device.   Batteries do not make electricity, they store it, just as a water tank stores water for future use.   As chemicals in the battery change, electrical energy is stored or released.   In rechargeable batteries this process can be repeated many times.   Batteries are not 100% efficient - some energy is lost as heat and chemical reactions when charging and discharging.   If you use 1000 watts from a battery, it might take 1050 or 1250 watts or more to fully recharge it.

Internal Resistance

Part - or most - of the loss in charging and discharging batteries is due to internal resistance.   This is converted to heat, which is why batteries get warm when being charged up.   The lower the internal resistance, the better.


Slower charging and discharging rates are more efficient.   A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate.   Much of this loss of efficiency is due to higher internal resistance at higher amperage rates - internal resistance is not a constant - kind of like "the more you push, the more it pushes back".


Typical efficiency in a lead-acid battery is 85-95%, in alkaline and NiCad battery it is about 65%. True deep cycle AGM's (such as Concorde and Deka) can approach 98%.


Practically all batteries used in PV and all but the smallest backup systems are Lead-Acid type batteries.   Even after over a century of use, they still offer the best price to power ratio.   A few systems use NiCad, but we do not recommend them except in cases where extremely cold temperatures (-50 F or less) are common.   They are expensive to buy, and very expensive to dispose of due the the hazardous nature of Cadmium.


We have had almost no direct experience with the NiFe (alkaline) batteries, but from what we have learned from others we do not not recommend them - one major disadvantage is that there is a large voltage difference between the fully charged and discharged state.   Another problem is that they are very inefficient - you lose from 30-40% in heat just in charging and discharging them.   Many inverters and charge controls have a hard time with them.   It appears that the only current source for new cells seems to be from Hungary.


An important fact is that ALL of the batteries commonly used in deep cycle applications are Lead-Acid. This includes the standard flooded (wet) batteries, gelled, and AGM.   They all use the same chemistry, although the actual construction of the plates etc varies.


NiCads, Nickel-Iron, and other types are found in a few systems, but are not common due to their expense, environmental hazards, and/or poor efficiency.

Major Battery Types

Batteries are divided in two ways, by application (what they are used for) and construction (how they are built).   The major applications are automotive, marine, and deep-cycle.   Deep-cycle includes solar electric (PV), backup power, and RV and boat "house" batteries. The major construction types are flooded (wet), gelled, and AGM (Absorbed Glass Mat).   AGM batteries are also sometimes called "starved electrolyte" or "dry", because the fiberglass mat is only 95% saturated with Sulfuric acid and there is no excess liquid.


Flooded may be standard, with removable caps, or the so-called "maintenance free" (that means they are designed to die one week after the warranty runs out).   All gelled are sealed and are "valve regulated", which means that a tiny valve keeps a slight positive pressure.    Nearly all AGM batteries are sealed valve regulated (commonly referred to as "VRLA" - Valve Regulated Lead-Acid).   Most valve regulated are under some pressure - 1 to 4 psi at sea level.

Lifespan of Batteries

The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors.   In extreme cases, it can vary to extremes - we have seen L-16's killed in less than a year by severe overcharging, and we have a large set of surplus telephone batteries that sees only occasional (5-10 times per year) heavy service that are now over 25 years old.   We have seen gelled cells destroyed in one day when overcharged with a large automotive charger.   We have seen golf cart batteries destroyed without ever being used in less than a year because they were left sitting in a hot garage without being charged.   Even the so-called "dry charged" (where you add acid when you need them) have a shelf life of 18 months at most.   They are not totally dry - they are actually filled with acid, the plates formed and charged, then the acid is dumped out.

These are some typical (minimum - maximum) typical expectations for batteries if used in deep cycle service.   There are so many variables, such as depth of discharge, maintenance, temperature, how often and how deep cycled, etc. that it is almost impossible to give a fixed number.

Starting, Marine, and Deep-Cycle Batteries

Using a Deep Cycle battery as a Starting Battery

There is generally no problem with this, providing that allowance is made for the lower cranking amps compared to a similar size starting battery.  As a general rule, if you are going to use a true deep cycle battery (such as the Concorde SunXtender) also as a starting battery, it should be oversized about 20% compared to the existing or recommended starting battery group size to get the same cranking amps. That is about the same as replacing a group 24 with a group 31. With modern engines with fuel injection and electronic ignition, it generally takes much less battery power to crank and start them, so raw cranking amps is less important than it used to be. On the other hand, many cars, boats, and RV's are more heavily loaded with power sucking "appliances", such as megawatt stereo systems etc. that are more suited for deep cycle batteries. We have used the Concorde SunXtender AGM batteries in some of our vehicles with no problems.

It will not hurt a deep cycle battery to be used as a starting battery, but for the same size battery they cannot supply as much cranking amps as a regular starting battery.

Battery Construction Materials

Nearly all large rechargeable batteries in common use are Lead-Acid type. (There are some NiCads in use, but for most purposes the very high initial expense, and the high expense of disposal, does not justify them). The acid is typically 30% Sulfuric acid and 70% water at full charge. NiFe (Nickel-Iron) batteries are also available - these have a very long life, but rather poor efficiency (60-70%) and the voltages are different, making it more difficult to match up with standard 12v/24/48v systems and inverters. The biggest problem with NiFe batteries is that you may have to put in 100 watts to get 70 watts of charge - they are much less efficient than Lead-Acid. What you save on batteries you will have to make up for by buying a larger solar panel system. NiCads are also inefficient - typically around 65% - and very expensive. However, NiCads can be frozen without damage, so are sometimes used in areas where the temperatures may fall below -50 degrees F. Most AGM batteries will also survive freezing with no problems, even though the output when frozen will be little or nothing.

Industrial Deep Cycle batteries

Sometimes called "fork lift", "traction" or "stationary" batteries, are used where power is needed over a longer period of time, and are designed to be "deep cycled", or discharged down as low as 20% of full charge (80% DOD, or Depth of Discharge). These are often called traction batteries because of their widespread use in forklifts, golf carts, and floor sweepers (from which we get the "GC" and "FS" series of battery sizes). Deep cycle batteries have much thicker plates than automotive batteries.

Plate Thickness

Plate thickness (of the Positive plate) matters because of a factor called "positive grid corrosion". This ranks among the top 3 reasons for battery failure. The positive (+) plate is what gets eaten away gradually over time, so eventually there is nothing left - it all falls to the bottom as sediment. Thicker plates are directly related to longer life, so other things being equal, the battery with the thickest plates will last the longest. The negative plate in batteries expands somewhat during discharge, which is why nearly all batteries have separators, such as glass mat or paper, that can be compressed.

Automotive batteries typically have plates about .040" (4/100") thick, while forklift batteries may have plates more than 1/4" (.265" for example in larger Rolls-Surrette) thick -  almost 7 times as thick as auto batteries. The typical golf cart will have plates that are around .07 to .11" thick. The Concorde AGM's are .115", The Rolls-Surrette L-16 type (CH460) is .150", and the US Battery and Trojan L-16 types are .090". The Crown L-16HC size has .22" thick plates. While plate thickness is not the only factor in how many deep cycles a battery can take before it dies, it is the most important one.

Most industrial deep-cycle batteries use Lead-Antimony plates rather than the Lead-Calcium used in AGM or gelled deep-cycle batteries. The Antimony increases plate life and strength, but increases gassing and water loss.  This is why most industrial batteries have to be checked often for water level if you do not have Hydrocaps. The
self discharge of batteries with Lead-Antimony plates can be high - as much as 1% per day on an older battery. A new AGM typically self-discharges at about 1-2% per month, while an old one may be as much as 2% per week.


Sealed batteries

Sealed batteries are made with vents that (usually) cannot be removed. The so-called Maintenance Free batteries are also sealed, but are not usually leak proof. Sealed batteries are not totally sealed, as they must allow gas to vent during charging. If overcharged too many times, some of these batteries can lose enough water that they will die before their time. Most smaller deep cycle batteries (including AGM) use Lead-Calcium plates for increased life, while most industrial and forklift batteries use Lead-Antimony for greater plate strength to withstand shock and vibration.

A few industrial batteries have special caps that convert the Hydrogen and Oxygen back into water, reducing water loss by up to 95%. The popular "HydroCaps" that we sell for flooded batteries do the same job for conventional ("wet"), golf cart, and fork-lift batteries. Lead-Antimony (such as forklift and floor scrubber) batteries have a much higher self-discharge rate (2-10% per week) than Lead or Lead-Calcium (1-5% per month), but the Antimony improves the mechanical strength of the plates, which is an important factor in electric vehicles. They are generally used where they are under constant or very frequent charge/discharge cycles, such as fork lifts and floor sweepers. The Antimony increases plate life at the expense of higher self discharge. If left for long periods unused, these should be trickle charged to avoid damage from sulfation - but this applies to ANY battery.

As in all things, there are trade offs. The Lead-Antimony types have a very long lifespan, but higher self discharge rates.

Battery Size Codes

Batteries come in all different sizes. Many have "group" sizes, which is based upon the physical size and terminal placement. It is NOT a measure of battery capacity. Typical BCI codes are group U1, 24, 27, and 31. Industrial batteries are usually designated by a part number such as "FS" for floor sweeper, or "GC" for golf cart. Many batteries follow no particular code, and are just manufacturers part numbers. Other standard size codes are 4D & 8D, large industrial batteries, commonly used in solar electric systems.

Some common battery size codes used are: (ratings are approximate)


34 to 40 Amp hours

12 volts

Group 24

70-85 Amp hours

12 volts

Group 27

85-105 Amp hours

12 volts

Group 31

95-125 Amp hours

12 volts


180-215 Amp hours

12 volts


225-255 Amp hours

12 volts

Golf Cart & T-105

180 to 225 Amp hours

6 volts

L-16, L16HC etc.

340 to 415 Amp hours

6 volts

Gelled electrolyte

Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries.

Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death. It is for this and other reasons that we no longer sell any of the gelled cells except for replacement use. The newer AGM (absorbed glass mat) batteries have all the advantages (and then some) of gelled, with none of the disadvantages.

AGM, or Absorbed Glass Mat Batteries

A newer type of sealed battery uses "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. We sell the Concorde (and Lifeline, made by Concorde) AGM batteries. These are also called "starved electrolyte", as the mat is about 95% saturated rather than fully soaked. That also means that they will not leak acid even if broken.

AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:

Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.

Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery.  These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls.  And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents.  The Concorde (and most AGM) batteries have no charge or discharge current limits.

AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries.  The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.

AGM's do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces.  The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.

Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. AGM's will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice.  AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes.  Not everyone needs these features.

Temperature Effects on Batteries

Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

Charge voltage vs Temperature for AGM and Flooded batteriesBattery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it does not work that well. Adding another complication is that large battery banks make up a large thermal mass.

Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature.

Click to see larger graph of Battery Capacity vs TemperatureEven though battery capacity at high temperatures is higher,  battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times. Click on the small graph to see a full size chart of temperature vs capacity.

One last note on temperatures - in some places that have extremely cold or hot conditions, batteries may be sold locally that are NOT standard electrolyte (acid) strengths. The electrolyte may be stronger (for cold) or weaker (for very hot) climates. In such cases, the specific gravity and the voltages may vary from what we show.

Cycles vs Life

A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how any cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

How depth of discharge affects cycle life on batteriesBattery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

Battery Voltages

All Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged. Batteries that are stored for long periods will eventually lose all their charge. This "leakage" or self discharge varies considerably with battery type, age, & temperature. It can range from about 1% to 15% per month. Generally, new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest. In systems that are continually connected to some type charging source, whether it is solar, wind, or an AC powered charger this is seldom a problem. However, one of the biggest killers of batteries is sitting stored in a partly discharged state for a few months. A "float" charge should be maintained on the batteries even if they are not used (or, especially if they are not used). Even most "dry charged" batteries (those sold without electrolyte so they can be shipped more easily, with acid added later) will deteriorate over time. Max storage life on those is about 2-3 years.

Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Lifespan is increased at the same rate if below 77 degrees, but capacity is reduced. This tends to even out in most systems - they will spend part of their life at higher temperatures, and part at lower.

Myth: The old myth about not storing batteries on concrete floors is just that - a myth. This story has been around for 100 years, and originated back when battery cases were made up of wood and asphalt. The acid would leak from them, and form a slow-discharging circuit through the now acid-soaked and conductive floor.

State of Charge

State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries.

"False" Capacity"

A battery can meet all the tests for being at full charge, yet be much lower than it's original capacity. If plates are damaged, sulfated, or partially gone from long use, the battery may give the appearance of being fully charged, but in reality acts like a battery of much smaller size. This same thing can occur in gelled cells if they are overcharged and gaps or bubbles occur in the gel. What is left of the plates may be fully functional, but with only 20% of the plates left... Batteries usually go bad for other reasons before reaching this point, but it is something to be aware of if your batteries seem to test OK but lack capacity and go dead very quickly under load.

On the table below, you have to be careful that you are not just measuring the surface charge. To properly check the voltages, the battery should sit at rest for a few hours, or you should put a small load on it, such as a small automotive bulb, for a few minutes. The voltages below apply to ALL Lead-Acid batteries, except gelled. For gel cells, subtract .2 volts. Note that the voltages when actually charging will be quite different, so do not use these numbers for a battery that is under charge.

Amp-Hour Capacity

All deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps x hours. If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) x .333 (hours), or 6.67 AH. The accepted AH rating time period for batteries used in solar electric and backup power systems (and for nearly all deep cycle batteries) is the "20 hour rate". This means that it is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours it supplies is measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications. The 6-hour rate is often used for industrial batteries, as that is a typical daily duty cycle. Sometimes the 100 hour rate is given just to make the battery look better than it really is, but it is also useful for figuring battery capacity for long-term backup amp-hour requirements.

Why amp-hours are specified at a particular rate:

Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some manufacturers and vendors have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are. Here are some typical battery capacities from the manufacturers data sheets:

Battery Type

100 hour rate

20 hour rate


Trojan T-105

250 AH

225 AH


US Battery 2200


225 AH

181 AH

Concorde PVX-6220

255 AH

221 AH

183 AH

Surrette S-460 (L-16)

429 AH

344 AH

282 AH

Trojan L-16

400 AH

360 AH


Surrette CS-25-PS

974 AH

779 AH

639 AH

State of Charge

Here are no-load typical voltages vs state of charge

(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.+

State of Charge

12 Volt battery

Volts per Cell


































Battery Charging

Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.

Bulk Charge - The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.

Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldom discharged, the float voltage should be around 13.02 to 13.20 volts.

Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave on for long periods. Among the regulated chargers, there are the voltage regulated ones, such as Iota Engineering and Todd, which keep a constant regulated voltage on the batteries. If these are set to the correct voltages for your batteries, they will keep the batteries charged without damage. These are sometimes called "taper charge" - as if that is a selling point. What taper charge really means is that as the battery gets charged up, the voltage goes up, so the amps out of the charger goes down. They charge OK, but a charger rated at 20 amps may only be supplying 5 amps when the batteries are 80% charged. To get around this, Statpower (and maybe others?) have come out with "smart", or multi-stage chargers. These use a variable voltage to keep the charging amps much more constant for faster charging.

We carry the Iota Engineering battery chargers and the Statpower Truecharge "smart" chargers.

Charge Controllers

A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types. Some of the various brands and models that we use and recommend are listed below. Note that a couple of them are listed as "power trackers" - for a full explanation of this, see our page on "Why 120 watts does not equal 120 watts".

Most of the modern controllers have automatic or manual equalization built in, and many have a LOAD output. There is no "best" controller for all applications - some systems may need the bells and whistles of the more expensive controls, others may not.

These are some of the charge controllers that we recommend, but almost any modern controller will work fine. Exact model will depend on application and system size and voltage.

Xantrex (All)
Morningstar (All)
Outback Power MX60 & 80
Blue Sky Energy (Solar Boost)

Using any of these will almost always give better battery life and charge than "on-off" or simple shunt type regulators.

Battery Charging Voltages and Currents:

Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. "C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps. Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. However, since very few battery cables can take that much current, we don't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.

Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Once the charging voltage reaches 2.583 volts per cell, charging should stop or be reduced to a trickle charge. Note that flooded batteries MUST bubble (gas) somewhat to insure a full charge, and to mix the electrolyte. Float voltage for Lead-Acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt battery. At higher temperatures (over 85 degrees F) this should be reduced to about 2.10 volts per cell.

Never add acid to a battery except to replace spilled liquid. Distilled or deionized water should be used to top off non-sealed batteries. Float and charging voltages for gelled batteries are usually about 2/10th volt less than for flooded to reduce water loss. Note that many shunt-type charge controllers sold for solar systems will NOT give you a full charge - check the specifications first. To get a full charge, you must continue to apply a current after the battery voltage reaches the cutoff point of most of these type of controllers. This is why we recommend the charge controls and battery chargers listed in the sections above. Not all shunt type controllers are 100% on or off, but most are.

Flooded battery life can be extended if an equalizing charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte. If the liquid in standard wet cells is not mixed, the electrolyte becomes "stratified". You can have very strong solution at the top, and very weak at the bottom of the cell. With stratification, you can test a battery with a hydrometer and get readings that are quite a ways off. If you cannot equalize for some reason, you should let the battery sit for at least 24 hours and then use the hydrometer. AGM and gelled should be equalized 2-4 times a year at most - check the manufacturers recommendations, especially on gelled.

Battery Aging

As batteries age, their maintenance requirements change. This means longer charging time and/or higher finish rate (higher amperage at the end of the charge). Usually older batteries need to be watered more often. And, their capacity decreases.

Mini Factoids

Nearly all batteries will not reach full capacity until cycled 10-30 times. A brand new battery will have a capacity of about 5-10% less than the rated capacity.

Batteries should be watered after charging unless the plates are exposed, then add just enough water to cover the plates. After a full charge, the water level should be even in all cells and usually 1/4" to 1/2" below the bottom of the fill well in the cell (depends on battery size and type).

In situations where multiple batteries are connected in series, parallel or series/parallel, replacement batteries should be the same size, type and manufacturer (if possible). Age and usage level should be the same as the companion batteries. Do not put a new battery in a pack which is more than 6 months old or has more than 75 cycles. Either replace with all new or use a good used battery. For long life batteries, such as the Surrette and Crown, you can have up to a one year age difference.

The vent caps on flooded batteries should remain on the battery while charging. This prevents a lot of the water loss and splashing that may occur when they are bubbling.

When you first buy a new set of flooded (wet) batteries, you should fully charge and equalize them, and then take a hydrometer reading for future reference. Since not all batteries have exactly the same acid strength, this will give you a baseline for future readings.

When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel - the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast.

Lead-Acid batteries do NOT have a memory, and the rumor that they should be fully discharged to avoid this "memory" is totally false and will lead to early battery failure.

Inactivity can be extremely harmful to a battery. It is a VERY poor idea to buy new batteries and "save" them for later. Either buy them when you need them, or keep them on a continual trickle charge. The best thing - if you buy them, use them.

Only clean water should be used for cleaning the outside of batteries. Solvents or spray cleaners should not be used.

Some Peukert Exponent values (not complete, just for info). We don't have a lot of data. Trojan T-105 = 1.25; Optima 750S = 1.109; US Battery 2200 = 1.20.

More information - Manufacturers Websites

US Battery Manufacturing Company - some good information and data.
Crown Battery - A major manufacturer of industrial and deep cycle batteries.
Trojan Battery - not a lot of real technical info here, but has all the specifications.
Exide - not much here but marketing stuff, but you can buy Exide T-shirts. We don't sell Exide.
Surrette - Specs and data on the Surrette deep cycle and marine batteries
Concorde - specs and data on all the Concorde batteries, including Lifeline.

Please feel free to email us with any comments about this page or batteries in general.



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For a look at marine battery usage concerning dual batteries & or dual motors  here is a link that has wiring diagrams of different versions with the pros & cons of each.  The section below was taken from the following website message board



Dual Battery Wiring

This article describes several techniques for interconnecting outboard motors and dual batteries.  There are two distinct applications: single engine installations and dual engine installations.

Single Engine/Dual Battery

The use of a single engine and dual batteries is one of the most common installations found in outboard powered boats. Interconnection is straightforward. The diagram below shows the typical wiring.


Schematic Diagram
Dual Battery/Single Engine Typical Schematic
NOTE: The battery positive leads are shown in red. The negative leads are shown in green for clarity; code suggests the use of yellow wire. Wiring gauge is determined by current demands and length of the run. Typically 2-AWG is used to connect to the batteries; 10-AWG is used to connect to the distribution panel. The single switch is an OFF-1-BOTH-2 switch.


The use of a switch and wiring like this is recommend with dual battery installations. A brief explanation of the operation of the switch in this circuit follows.

In the OFF position, the two batteries are disconnected from all loads. The OFF position is used when the boat is being stored or otherwise not in use. It prevents any drain from the batteries. This can be useful if a circuit has been accidently left on, say a cabin lighting circuit or similar drain. Such a load can completely discharge a battery in a day or two, leading to an unanticipated dead battery situation when you return to your boat. On some switches a key lock is provided, allowing the switch to locked in the off position. The provides another level of security in preventing the boat from being used when left in storage or unattended.

In the 1 position, all loads and charging currents are connected to the No. 1 battery (the PORT battery in the illustration). Starting current for the engine starter motor is supplied by the PORT battery. When the engine is running, surplus current developed by its charging circuit will flowing into the PORT battery. Current to lighting and other loads will flow from the PORT battery. The STDB battery is completely isolated and has no load current, nor does it receive any charging current.

In the 2 position, all loads and charging currents are connected to the No. 2 battery (the STBD battery in the illustration). Now it supplies current required by starting and running, and it receives all the charging current from the engine. The PORT battery is totally isolated.

In the BOTH position, the two batteries are connected in parallel. This has a number of implications. Unless the batteries have exactly the same state of charge, the combined voltage to the two batteries in parallel will sag to a voltage somewhat lower than the highest battery's terminal voltage. Current from the higher voltage battery will flow into the lower voltage battery and begin charging it. As long as the state of charge in one battery is higher than the other, the lower battery is more of a load than a source of power. Eventually, the batteries will reach an equilibrium, and they will both have the same terminal voltage. At that point they will both tend to supply current to loads that are attached to them, and they will both receive charging current furnished by the engine.

It would seem like operating in the BOTH position would be beneficial, but that is not always the case. Even thought the batteries will eventually rise or fall to the same terminal voltage when connected together, they will not necessarily become exactly the same. A battery (or any source of electrical engery) can be though of as having an internal resistance. The lower this internal resistance the greater the current it can supply. The internal resistance will also affect how the battery absorbs charging current. Even though they are connected in parallel, it is possible that they will supply unequal currents to the loads, and it is also possible that they will accept unequal currents from an the engine charging source.

If the batteries are significantly different in their age, their type of construction, and their state of charge, this unequal distribution of current can be more significant. To describe the situation in the simplest of terms, when two batteries are connected in parallel, they will probably tend to behave more like the weakest battery of the two than the strongest.

Paralleling the batteries can come in handy in some situations. For example, both batteries may be discharged to a point where neither alone can provide enough current to crank the starter motor, but combined in parallel they can turn the engine over.

If one battery is fully charged and the other is totally discharged, connecting them in parallel (by using the BOTH position) can cause very high currents to flow between the batteries. Extreme heat can be generated by the sudden charging of the discharged battery. Use caution in this situation. It is better to recondition a discharged battery by slowly re-charging it with an AC-operated battery charger.

The arrangement of the contacts of the typical OFF-1-2-BOTH permits the operation of the switch in the range of 1-2-BOTH without ever disconnecting the batteries from the load or the outboard charging circuit. This is important, as it is possible to cause damage to the charging circuit if the battery is disconnected while the engine is running. By choosing the path of rotation of the switch, it is possible to change from 1 to 2 without moving through the OFF position.

Dual Engine/Dual Battery

Dual engines and dual batteries require careful interconnection to prevent damage to the engine charging circuits. Three approaches are shown, one as believed to be used by Boston Whaler, a second alternative approach (that I have since discovered is described by West Marine in their catalogue), and a third configuration which I am currently using in my Boston Whaler.

The Problem

The use of dual batteries in boats is quite common, but most often the dual batteries are associated with just one engine or charging source. The wiring and interconnection of two batteries and one engine alternator is quite well covered in the boating literature. There is a less common and less written about situation that arises when there are two batteries and two engines or charging sources.

The problem with having two charging sources is that care must be taken to not connect the two chargers together in a way that damages one or both of them. Experience suggests that if two engines' chargers are wired in simple parallel and charge a common battery or bank of batteries, it is not unusual for one of the chargers to suffer a burned out stator coil. The cause of this is most easily explained if a fundamental rule of charging circuits is understood: the charger should never be operating without a load (a battery) connected.

In circuits with parallel connection of two engine charging circuits, it is likely that the voltage produced by them will not be precisely equal. If one unit has an output approximately one volt greater than the other, the effect of this will be to electrically disconnect the load from the lower voltage output engine. This may result in damage to the stator coil of the engine producing the lower voltage output.

Boston Whaler Factory Installation

Several models of Boston Whaler boats come rigged from the factory with dual outboards and dual engines. Below is the wiring which is believed to be currently used by Boston Whaler in these installations.


Schematic Diagram
Dual Battery/Dual Engine Schematic
NOTE: The starboard battery positive is shown in dark red for clarity; use red wire. The negative leads are shown in green for clarity; use yellow wire. All wiring is AWG-2 unless indicated. Switchs are OFF-1-2-BOTH style.
Adapted from a sketch supplied by Boston Whaler by JWH.


Normal Operating Procedures

The Whaler Factory Installation operating procedure is believed to be:

  1. Normal operating position of PORT switch is Position 1
  2. Normal operating position of STBD Switch is Position 2
  3. To parallel batteries for starting, turn both switches to ALL Position. Return each switch to its normal operating position after starting.
  4. PORT engine can be started from STBD battery by turning PORT Switch to Position 2. Return to Position 1 after starting.
  5. STBD engine can be started from PORT battery by turning STBD Switch to Position 1. Return to Position 2 after starting.

If the suggested procedure is followed, the charging circuits of the two engines will not be connected together in normal operation. One engine can charge two batteries, but two engines should not charge a common battery. Prolonged running of both engines operating on same battery is to be avoided; prolonged running of both engines with both batteries paralleled is to be avoided.

Alternate Dual Battery Configuration

An alternative approach that is not as flexible but is simpler to install and operate is shown below.


Schematic Diagram
Dual Battery/Dual Engine Alternative Schematic
NOTE: The starboard battery positive is shown in dark red for clarity; use red wire. The negative leads are shown in green for clarity; use yellow wire. All wiring is AWG-2 unless indicated. All switches are simple On-Off switches.


Normal Operating Procedures

For the alternative dual battery wiring, the operating procedure is:

  1. For normal operation PORT switch is in On position.
  2. For normal operation STBD switch is in On position.
  3. For normal operation TIE switch is in Off position.
  4. To parallel batteries for starting, turn TIE switch to On position. Return TIE switch to Off as soon as starting finished.

In some cases, the TIE Switch can be omitted and replaced by a temporary connection between batteries using a jumper cable. It is probably a good idea to carry jumper cables in any case. Connecting the two battery negative terminals is also probably a good idea, even it the TIE switch is not used.

If the suggested procedure is followed, the charging circuits of the two engines will not be connected together in normal operation. One engine can charge two batteries, but two engines should not charge a common battery.

"New" Dual Battery Configuration

A new approach that I just implemented this spring on my Whaler is shown below. Again, this arrangement is not as flexible as the "factory" wiring, but it is the most simple and easiest to install.


Schematic Diagram
Dual Battery/Dual Engine "New" Schematic
NOTE: The battery positive leads are shown in red. The negative leads are shown in green for clarity; code suggests the use of yellow wire. All wiring is AWG-2 unless indicated. The single switch is an OFF-1-BOTH-2 switch.


Normal Operating Procedures

In the "new" dual battery wiring, the two batteries and the two engine charging circuits are entirely isolated so long as the OFF-1-BOTH-2 switch is NOT in the BOTH position. The switch serves a dual function. In the OFF position it disconnects the house load from the batteries. In the 1 or 2 position, the house load is powered from a single battery as selected. In the BOTH position, the house load is powered from both batteries, and the two batteries are connected in parallel. The BOTH position should only be used for special cases, such as attempting to start an engine and needing additional battery power.

To prevent the paralleling of the engine charging circuits, when operating in the BOTH position it is advisable to only run one engine at a time. The only time the BOTH position may be needed is in starting an engine whose normal battery is too weak to crank it over. In that case, the selector can be moved to the BOTH position, temporarily paralleling the batteries and allowing the engine (whose battery is weak) to be started.

Once the engine is running, the switch can be moved out of the BOTH position, and the second engine started from its battery (which should have enough charge remaining to crank it).

Thus the normal operating procedure is:

  1. For normal operation, select OFF, 1, or 2 as appropriate to attach the house load to a selected battery .
  2. To parallel batteries for starting, turn the switch to the BOTH position. Return the switch to OFF as soon as first engine starting finished. Start second engine. If house load is needed connect to strongest battery, either 1 or 2

If the suggested procedure is followed, the charging circuits of the two engines will not be connected together in normal operation. One engine can charge two batteries, but two engines should not charge a common battery.

Comments About the Three Designs

Whaler Factory

The more complex Whaler factory design allows for more flexibility, but at the same time allows for the possibility that a wrong setting of the switches could be made. The result could be damage to the engine charging circuitry. This arrangement is probably most appropriate on larger, more complex vessels


The principal advantage of the "alternate" wiring shown is its simplicity. If the TIE switch is not thrown, there is no chance to connect two engine charging circuits together. Sometimes in situations of high stress, like a hard starting engine in a rough sea, simplicity has very desirable advantages.

Notice that the alternate arrangement connects the circuit breaker panel to only the Starboard battery. This prevents the Port battery from being discharged by other loads (like lamps left on or a stuck bilge pump switch), thus preserving it for starting duty only. This can be an important advantage.


The principal advantage of the "new" wiring is its extreme simplicity. Only one switch is used and it serves a dual purpose. The batteries and charging systems are kept isolated except when the BOTH position is selected for special or emergency starting conditions.

The disadvantage of this arrangement is there is no electrical disconnect of the batteries from the engines. However, there are millions of outboard powered boats that operate without such a disconnect, save for the case of opening the battery box and removing the leads from the terminals. The only need for such a disconnect would be in the unusual case of the engine electrical circuit malfunctioning, as could occur with trim relay, starter motor contactor, or other electrical load remaining stuck in an "ON" condition.

Other Wiring Arrangements

A great deal of other possible wiring and connection of batteries, loads, switches, starters, chargers, and engines is possible. In many cases, additional equipment like isolators and separate chargers are added. Some engine alternators have specialized regulator circuits which permit separate sensing leads to be connected to the battery to compensate for other voltage drops in the circuit. In the main, none of this is applicable to outboard powered boats for two very simple reasons. First, the starter motor and alternator are usually connected together under the cowling of the outboard motor and are not easily separated without significant electrical modification of the outboard. Second, even if one were to re-wire their outboard motor, it would be necessary to add a great deal of external wiring coming from the motor to the various external components (like the isolators, the switches, the chargers, etc.). All this wiring would have to be made flexible and be capable of withstanding the turning and tilting inherent in the operation of an outboard motor.

In many years of boating, I have never heard of or seen anyone having undertaken the modification of an outboard engine electrical under-cowling wiring to interconnect isolators, switches, and other external circuit components. If I did encounter such an outboard, I would be extremely wary of it. I doubt that typical owners, do-it-yourself-er, or factory trained mechanics would be comfortable working on such a modified engine. In short, I do not endorse making that type of modification.

Caution Advised

Wiring of low-voltage/high-current circuits can be hazardous because of the extremely high short circuit current capacity that exists when working with large batteries. All the information provided is believed to be accurate and representative of techniques and practices in common use in the marine applications described. Examine any wiring carefully for proper configuration before applying power. Low-voltage/high-current short circuits can produce extremely rapid heating and can be dangerous. Never wear a ring or metal jewelry when working with high-current sources like storage batteries.

Comments or Questions

If you have a comment or question about this article, please post it in the Whaler Forum.

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This article first appeared February 10, 2001. A revised version appeared February 14, 2001



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Updated 04-14-2013   ***