amperehour

An ampere-hour (Ah) is a unit of electric charge commonly used to express the capacity of a battery or other energy storage device. It represents the amount of charge transferred by a steady current of one ampere flowing for one hour. In coulombs, 1 Ah equals 3600 coulombs.

Mathematically, charge Q equals the integral of current over time: Q = ∫ I dt. If the current is constant, Q = I t. For example, a battery rated at 2 Ah can deliver 2 amperes for 1 hour, or 1 ampere for 2 hours. The energy stored, however, also depends on voltage: energy in watt-hours (Wh) equals capacity in ampere-hours times the operating voltage (E = Ah × V). Thus a 3.7 V, 2 Ah cell stores about 7.4 Wh.

Usage and caveats: The Ah rating measures charge capacity, not energy alone, and two batteries with the same Ah rating but different voltages store different amounts of energy. Usable capacity varies with discharge rate (the C-rate), temperature, age, and chemical composition. Higher discharge rates often reduce the available capacity. Batteries are frequently labeled with milliampere-hours (mAh) for smaller devices (e.g., 2000 mAh).

Relation to SI units: The ampere-hour is not an SI base unit; the coulomb is the SI unit of charge. Nevertheless, Ah is widely used in battery specifications and makes it easy to compare charge capacity, while Wh is commonly used to compare energy storage across different voltages.