Amperehours

Ampere-hours (Ah) is a unit of electric charge used to express the capacity of batteries and energy storage systems. It represents the amount of charge transferred by a steady current of one ampere flowing for one hour. Since one ampere equals one coulomb per second, one ampere-hour equals 3600 coulombs.

The Ah rating is related to energy through the battery’s voltage: energy in watt-hours (Wh) equals capacity in ampere-hours times nominal voltage (Wh = Ah × V). This means two batteries with the same Ah at different voltages can store different amounts of energy.

Ah ratings describe how much charge a battery can deliver, but the usable capacity depends on discharge rate, temperature, and aging. Higher discharge currents reduce usable capacity due to the Peukert effect, so the Ah value is most meaningful under specified test conditions, often at a given C-rate (for example, 1C or C/5).

In practice, smaller batteries are labeled in milliampere-hours (mAh), where 1 Ah = 1000 mAh. Larger battery packs, such as those in electric vehicles or stationary storage, are typically specified in Ah alongside their voltage. The energy content of such packs is usually given in Wh or kWh, calculated by multiplying Ah by the system voltage.

Capacity degrades with time and use, influenced by cycle count, depth of discharge, temperature, and manufacturing quality. Manufacturers provide nominal capacities and operating ranges, but real-world performance can vary from the labeled Ah.