Ampere-hour
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An ampere-hour (abbreviated as A·h) is a unit of electric charge. It is a common measurement of how long a battery will last (or in the case of a rechargeable battery, how long it will last when fully charged). The ampere is the SI unit of electric current. One ampere-hour is equal to 3600 coulombs (ampere-seconds), and indicates the amount of electric charge that passes either terminal of the battery when it provides one ampere of current flow for one hour. The commonly-seen milliampere-hour (mA·h) is equal to 3.6 coulombs.
However, in reality, the available capacity of a battery depends on the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected. Therefore, a battery rated at 100 Ah (360000 coulomb) will deliver 20 A (20 coulombs per second) over a 5 hour period, but if it is instead discharged at 50 A (50 coulombs per second), it will run out of charge before the theoretically expected 2 hours. For this reason, a battery capacity rating is always related to an expected discharge time, which is typically 5 or 20 hours.
The relationship between current, discharge time and capacity is expressed by Peukert's law.
In general, the higher the ampere-hour rating, the longer the battery will last for a certain device. Installing batteries with different Ah ratings will not affect the operation of a device rated for a specific voltage.
The Ah rating of a battery is related to, but not the same as, the amount of energy it stores when fully charged. If two batteries have the same nominal voltage, then the one with the higher Ah rating stores more energy. It would also typically take longer to recharge.
The energy E available from a battery is approximately given by
where- Q is the charge in Ah, and
- V is the nominal voltage.
Other units
The SI unit of electric charge is the coulomb. One ampere-hour is equal to 3600 coulombs.See also
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