Ampere-Hours & Battery Capacity

Quick Answer

An ampere-hour is a quantity of charge: one ampere flowing for one hour, or 3600 coulombs. Cells are rated in ampere-hours, or in milliampere-hours for small ones, because that maps onto run time at a known current. It measures charge, not energy, and the rating holds only under its test conditions.

Multiply current by time and you have charge, which is why the unit is built the way it is: an ampere for an hour. Portable cells use the milliampere-hour because the numbers land in a friendlier range, and the two are the same quantity a factor of a thousand apart. Electric charge covers the conversion to coulombs.

What the unit cannot tell you is how much energy the cell holds. That takes the voltage as well: multiply ampere-hours by volts and the answer is watt-hours. A 2000 mAh lithium cell at 3.7 V holds 7.4 Wh, while the same 2000 mAh on a 1.2 V nickel-metal-hydride cell holds well under half of it. Comparing two chemistries on the mAh figure alone compares nothing. Electrical energy works the distinction through.

The printed capacity is also a test result rather than a promise. It was measured at a stated discharge current, down to a stated cutoff voltage, at a stated temperature, and the same cell returns less when any of those move against it: drawn harder, cut off higher, or run cold. Manufacturers publish discharge curves for that reason, and the headline number is the top line of a table.