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Glossary of Battery Terms

An extensive dictionary of battery terms and usage. Also see our glossary on solar terms and usage.


Select the first letter of the word from the list above to jump to appropriate section of the glossary.

- A -

Ampere, or Amp
The unit of measurement of current flow. One volt placed across a one ohm resistance will cause a current of one Amp to flow. One amp for one hour is called an "amp-hour" or AH. The fundamental unit of electric current, named for the French physicist Andr-Marie Ampre (1775-1836), one of the pioneers in studying electricity. The official definition of the ampere goes like this: suppose we have two parallel conductors, infinitely long and having negligible cross section. Place these conductors one meter apart in a perfect vacuum. One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length. (You're welcome to object that no one can make an infinitely long conductor, nor a perfect vacuum. But scientists can use the idealized definition to construct appropriate real-world equipment in their laboratories.) The other electrical units are all defined in terms of the ampere. For example, one ampere represents a current flow of one coulomb of charge per second. One ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential.
Ampere-hour or AH
The unit of electrical capacity - this tells you how much power the battery will store. Current multiplied by time in hours equals ampere-hours. A current of one amp for one hour would be one amp-hour; a current of 3 amps for 5 hours would be 15 AH. Similar to the "gallons per day" measure of water. Amp-hour ratings will vary with temperature, and with the rate of discharge. For example, a battery rated at 100 AH at the 6-hour rate would be rated at about 135 AH at the 48-hour rate. Ampere-hours (AH) designates the storage capacity of the battery.  SLI batteries are not rated in AH, but in "CCA", or cold-cranking amps (marine batteries are often rated in "marine cranking amps").. Terms such as "6 hour rate" or "20 hour rate" indicate that the battery is discharged steadily over 6 or 20 hours, and the Amp-hour capacity is measured by how much it puts out before reaching 100% DOD, or 1.75 volts per cell. For examples of this, see the Concorde AGM battery listing.
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- B -

An electro-chemical device that stores energy. Consists of one or more cells.
British thermal unit (Btu)
a unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In America the British thermal unit is sometimes called the heat unit. In defining the Btu, it is necessary to specify the temperature of the water; thus there have been several definitions over the years. However, one Btu is equal to about 251.996 (small) calories, or 0.251 996 of the (kilo-)calories counted by dieters. Using the current definition of the calorie (the IT calorie), one Btu equals approximately 778.169 foot pounds, 1.055 056 kilojoules or 0.293 071 watt hour.
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- C -

CA or Cranking Amps
A rather optimistic market driven rating, especially for "economy" or "value priced" batteries. The same as CCA, but at at 32 degrees F (0 C) temperature. The standard BCI rating is CCA, at 0 degrees F (about -18 C). The MCA, or Marine Cranking Amps is basically the same as CA. CCA is about 20% less than CA or MCA.
CCA or Cold Cranking Amps
The maximum amperes that can be continuously removed from a battery for 30 seconds at zero degrees F before it's voltage drops too low to use (1.2 volts per cell, or 7.2 volts). This term is used only for engine starting batteries, and has little to do with the amp-hour capacity or deep cycle batteries. This rating will also appear on many deep cycle marine batteries. See also CA and MCA.
Charge Controller
An electronic regulator that controls the amount of voltage and/or current going from the PV array into the batteries. Regulators come in three general types: on-off, single stage, and 3-stage. Regulators may control the current with a relay or transistor (usually a FET). The simplest type, the on-off, applies full array power to the battery up to a certain voltage, and then cuts off. On-off regulators are not recommended, although there are still quite a few out there on older systems. Single stage regulators work similar to the on-off type, but generally also do some regulation of the current going to the battery. For many years, single stage regulators were the standard, but in the past few years several excellent 3-stage PWM regulators  have become available at reasonable prices. 3-stage regulators are becoming the standard for most systems. For complete details, see our "Care and feeding of Batteries" page.
One coulomb is the amount of charge accumulated in one second by a current of one ampere. Electricity is actually a flow of particles called electrons, and one coulomb represents the charge on approximately 6.241 506 x 1018 electrons. The coulomb is named for a French physicist, Charles-Augustin de Coulomb (1736-1806), who was the first to measure accurately the forces exerted between electric charges.
A "cycle" is a somewhat arbitrary term used to describe the process of discharging a fully charged battery down to a particular state of discharge. The term "deep cycle" refers to batteries in which the cycle is from full charge to 80% discharge. A cycle for an automotive battery is about 5%, and for telephone batteries is usually 10%.
Cycle Life
How many times a battery can be cycled before it reaches the point where it can only be charged up to (usually) 80% of it's original capacity. This depends on how deep the battery is cycled. Cycle life ratings are not commonly published - especially for the lower quality batteries, and in many cases may not have been done, as it can be quite time consuming. A battery with a cycle life of 900 would take 3 years just to test (although accelerated testing is commonly done).
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- D -

Direct Current (DC)
The only type of electrical current which a PV module or battery can supply. Most home systems and some RV and boats use an inverter to convert the DC into 117 volts alternating current (AC), which is the standard household power.
Discharge and over discharge
Discharging is the process of using power from the battery. As power is drawn from the battery, chemical changes take place between the Lead, the acid, and the Lead Sulfate. Charging reverses this chemical change. Next to overcharging, the worst thing for a battery is to remain fully or partly discharged for long periods of time. This causes Sulfation, which is a hard Lead Sulfate coating on the plates. Sulfation can reduce battery capacity dramatically.
Depth of discharge. How much of the available charge has been used compared to 100%. SOC (state of charge is similar - it is how much is left). Most deep cycle batteries are considered to be at 0% SOC, or 100% DOD, when cell voltage is 1.75 volts, or 10.5 volts for a 12 volt battery.
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- E -

Effective internal resistance
Re- The apparent opposition to current within a battery that manifests itself as a drop in battery voltage proportional to the discharge current. Its value is dependent upon battery design, state-of-charge, temperature, degree of sulfation, and age.
The conductive chemical (such as acid), usually fluid or gel, in which the flow of electricity takes place within the battery, and which supports the chemical reactions required.
Equalization is a "supercharge" which is applied to the battery at intervals of from 2 to 8 weeks. This charge voltage is about 10% higher than the normal float or trickle charge. This ensures that the cells are all equally charged, and in flooded batteries makes sure that the electrolyte is fully mixed by the gas bubbles. Gelled and sealed batteries in general should be equalized at a much lower rate than flooded - usually the final charge cycle on a 3-stage charger is sufficient to equalize all the cells.
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- F -

The voltage at which the battery is "floated", or just enough current is supplied to equal the self-discharge of the battery. This is typically about 14.2 volts for a 12 volt battery.
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- G -

Gassing occurs when more current is being fed to the battery than it can use. The excess current produces Hydrogen and Oxygen gasses. Some gassing is normal, but excessive gassing can indicate that the batteries are being overcharged. The gasses released are explosive if a spark or flame occurs, so adequate ventilation must be provided. Batteries normally start gassing at about 80-90% of full charge. A common fallacy is that you should stop charging as soon as the battery starts gassing. Most batteries start gassing at about 80% SOC, so if you quit charging at that point, you will never get a full charge. Most better chargers cut back on the current when the battery reaches this point to prevent excess gassing.
Gassing and sealed gel cells
Gelled cells will withstand much less gassing than AGM or flooded batteries. The gel can develop large bubbles or "pockets", which reduce battery capacity due to poor contact with the plates. It can also cause the gel to dry out from water loss, making these pockets permanent. Gelled batteries are charged at a slightly lower voltage than flooded, .1 to .3 volts less to avoid over-gassing. Some advertisements and product brochures have stated that gelled cells have a "high" capacity for taking a charge - this is NOT correct, as it is 1/2 to 1/4 the maximum current that a flooded or AGM battery can take. We have seen this statement in inverter and charger literature, not just battery product brochures.
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- H -

An instrument used to check the specific gravity (strength) of the electrolyte in the battery. Most Lead-Acid batteries will be in the range of 1.1 to 1.3 specific gravity, with most fully charged batteries being about 1.23 to 1.30 (some hydrometers multiply this number by 1000, so 1.23 would read as 1230.) Hydrometers are inexpensive and can be purchased at any auto parts store. Some batteries manufactured for use in very hot or very cold climates may have stronger or weaker acid. If so, it is usually marked on the battery. See also Specific Gravity.
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- I -

Heavy cables, usually 10 to 20 inches in length used to connect 2 or more batteries into banks. These vary from #4 wire for smaller battery banks up to #4/0 (0000) for larger banks with large inverters.
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- J -

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- K -

1000 watts.
Kilowatt-hour, or kWh
Watts times Hours. If you have something that uses 1000 watts, and you use it for one hour, it will use 1 kWh. If it uses 10 watts for 100 hours, that is also 1 kWh.
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- L -

Low Voltage Disconnect, or LVD
The voltage at which most load controllers and inverters will disconnect from the battery to avoid totally draining the battery too far down. This is usually at about 10.5 to 11.2 volts for a 12 volt system. Also called Low Voltage Cutoff.
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- M -

Marine Cranking Amps. Essentially the same as CA (cranking amps). How many amps the battery will supply for 30 seconds at 32 degrees F (0 C) before the voltage drops to 1.2 volts per cell (7.2 volts for a 12 volt battery). See also CCA. This was originally an advertising term to make marine batteries look better.
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- N -

Nickel-Iron (NiFe)
A battery constructed of Nickel and Iron plates. Extremely long life, but low efficiency (as low as 60%). Note: some batteries with a "NiFe" label are NiCad. Nickel-Iron batteries were formerly made by the NiFe company, but apparently the only source now is imports from Hungary.
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- O -

Overcharging is one of the most destructive elements in battery life (the other is long term undercharging). Most batteries don't die a natural death, they are murdered - usually by overcharging. Overcharging causes the plates to disintegrate and shed. These particles end up on the bottom of the cell. Eventually, the cells will short out, fall apart, break apart, or generally die. Overcharging also increases water loss tremendously, causing even more problems. Gelled cells can be damaged faster than flooded, and flooded can be damaged faster than AGM batteries by overcharging. Water loss is a particular problem with sealed gel cells, as the water cannot be replaced. In some cases, severe overcharging can also cause considerable heat in cheaper batteries with high internal resistance, causing plates to buckle and cases to warp and break. See also discharge. Some poorly designed charge controls compound the problem by both overcharging and undercharging.
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- P -

Parallel Connection
Batteries connected in parallel means that all the Positive (+) terminals are connected together, and all the Negative (-) terminals are connected together. Batteries wired in parallel supply the same voltage but higher current. The amp-hour ratings add for each battery, but the voltage stays the same. New batteries should not be paralleled with old ones. (See also Series Connection).
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- Q -

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- R -

Batteries are rated in several ways. Automotive and marine starting batteries are rated in CCA, or Cold Cranking Amps. However, batteries used in photovoltaic systems are rated in Ampere Hours (AH). This rating tells you how many amps the battery will put out if discharged over a specified period of time, usually 8 or 20 hours. A 100 AH battery will give you 1 amp for 100 hours, or 100 amps for one hour. AH is a the measure of capacity for deep cycle batteries.
Reserve Capacity (RC)
Reserve capacity is sometime used to rate deep cycle batteries. It is the number of minutes that a battery can maintain a useful voltage (over 1.75 VPC) at a constant 25 amp discharge rate at 80 degrees F. Reserve capacity is probably a more useful measure than AH for batteries that run heavy loads, although most batteries also have tables that show the AH capacity at different discharge rates. AH is approximately equal to RC X .60.
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- S -

Self Discharge
All batteries will "self discharge" if sitting idle, even with no load. The rate can vary considerably with the type of battery and the age. A brand new AGM deep cycle will self discharge at about 2% a month, while an old conventional golf cart battery may be as high as 2% per day.
Series Connection
Batteries connected in series have the Positive (+) terminal of one battery tied to the Negative (-) terminal of the next battery. Power is taken from the two terminals at the end of the series string. Batteries wired in series supply the same current but the voltage is higher - for example four six-volt batteries in series will supply 24 volts. The amp-hour rating will be that of the smallest battery if different types are connected.. If old and new batteries are used together, the maximum current will be that of the weakest battery. See also Parallel Connection.
Short Circuit
A condition in which a short electrical path is unintentionally created. An example would be setting a wrench on top of a battery touching the terminals. Batteries can supply thousands of amps if short circuited, melting the wrench, the terminals, and showering sparks and molten metal. This is not a good thing, and may be harmful to people and other living things.
Even though Lead Sulfate is created in the materials of plates during normal discharging, this term is used to describe the generation of a different form (large crystals) of Lead Sulfate which will not readily convert back to normal material when the battery is charged. Sulfation occurs when a battery is stored too long in a discharged condition, if it is never fully charged, or if electrolyte has become abnormally low due to excessive water loss from overcharging and/or evaporation. Often sulfation can be corrected by charging very slowly (at low current) at a higher than normal voltage, usually at about 2.4 to 2.5 volts per cell at 1/2 to 8 amps (depending on battery size). This will gradually remove the sulfation in many cases. This term is also often misused to explain almost any battery homicide.
Specific Gravity (SG)
The measurement used to express electrolyte strength. SG compares the weight of the electrolyte to water, which has a SG of 1.000. SG changes somewhat with temperature, so most hydrometers come with a correction chart. A full charge should be about 1.265 at 77 degrees F (25 degrees C). This changes with temperature. This cannot be measured in sealed batteries. Pure acid has a SG of 1.835. A fully discharged battery will have a SG of about 1.12. SG should not be measured right after water is added as the reading will not be accurate until the electrolyte is fully mixed. This could take hours or days - an equalization charge will speed this up considerably. The SG in many AGM batteries may be as high as 1.365, but there is no practical way to measure it. If you get new batteries, you should fully charge them and equalize them and THEN take a specific gravity reading for future reference, as not all manufacturers use exactly the same SG, and SG may also vary for the same battery sold in different climates.
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- T -

Trolling Battery
Another name for deep cycle battery. A trolling motor will use about 20-30 amps, so a 105 AH (group 31)deep cycle battery will run the motor for about 3 to 4 hours. Used for small electric trolling motors.
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- U -

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- V -

The unit of measurement of electrical potential or "pressure". Most batteries come in 6, 12, & 24 volt. A single cell is 2 volts.
Volts per cell - a six volt battery has 3 cells, a 12 volt has 6. All Lead Acid batteries are 2 volts (nominal) per cell.
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- W -

A term used to measure total power. It is amps multiplied by volts. 120 volts @ 1 amp is the same as 12 volts @ 10 amps. It is also amps x amps x resistance. One horsepower = about 750 watts. A battery that can supply 220 AH at 12 volts is equal to 2640 watts. Watt-hours or kilowatt-hours (kWh) is how many watts times the number of hours. If a microwave pulls 1000 watts for 10 minutes, then it has used 1000/6, or 167 watt-hours.
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- X -

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- Y -

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- Z -

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