# 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. 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.

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).