Battery (electricity)

:For other uses, see battery (disambiguation).

Battery capacity

The capacity of a battery to store charge is often expressed in ampere hours (1 A·h = 3600 coulombs). If a battery can provide one ampere (1 A) of current (flow) for one hour, it has a real-world capacity of 1 A·h. If it can provide 1 A for 100 hours, its capacity is 100 A·h. Likewise, 20 A for 2 hours equals 40 A·h capacity. But...

Related Topics:
Ampere - Coulomb

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While a battery that can deliver 10 A for 10 hours can be said to have a capacity of 100 A·h, that is not how the rating is determined by the manufacturers. A 100 A·h rated battery most likely will not deliver 10 A for 10 hours. Battery manufacturers use a standard method to determine how to rate their batteries. Their rating is based on tests performed over 20 hours with a discharge rate of 1/20 (5%) of the expected capacity of the battery an hour. So a 100 ampere-hour battery is rated to provide 5 A for 20 hours at room temperature. The efficiency of a battery is different at different discharge rates. When discharging at 5% an hour, the battery's energy is delivered more efficiently than at higher discharge rates. This is Peukert's Law.

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To calculate the 5% discharge rate of a battery, take the manufacturer's ampere-hour rating and divide it by 20. For example, you have a AA cell rated at 1300 mA h (milliampere hours). The 5% discharge rate from which this rating was derived would be 1300 mA·h / 20 h = 65 mA.

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