# Ampere hour

An ampere hour (abbreviated Ah or amp hr) is a unit of electric charge, usually used for batteries. This unit combines the amount of current with how long that current can be sustained until the battery completely discharges.[1] Large batteries have several ampere hours but cell phones and other small devices have batteries with a total charge measured in milliampere hours. This measured quantity is called battery capacity.[1]

The number of amp-hours in a battery is found by multiplying the amount of current (in amperes) by the time (in hours) a given current could flow before the battery runs out. This calculation gives the total amount of charge transferred. This unit is useful since a battery will be able to supply current for a longer period of time, than say a capacitor. Chemical reactions inside the battery are limited by the amount of material that can be eaten away, and how quickly, this means that to test the ampere hours of a battery one should use a low current relative to the battery's peak current capability.

1 Ah = 3,600 Coulombs

Often the amp hour is written on the side of a battery, which has a voltage (often 1.5 V). An ampere hour at 1 volt is a unit of energy, specifically the watt-hour (1/1000th of a kWh).