ampere-hour
Electric chargeexact by definition
The ampere-hour is the charge delivered by one ampere flowing for one hour, equal to exactly 3600 coulombs. It is the customary rating for lead-acid, lithium and other storage batteries, and the conversion is exact because the ampere and the hour are both exactly defined. Larger banks are quoted in ampere-hours the way small cells are quoted in milliampere-hours.
Watch out: Ampere-hours describe charge, so two batteries with the same Ah rating store different energies if their voltages differ. A 100 Ah 12 V battery holds about 1.2 kWh while a 100 Ah 48 V battery holds about 4.8 kWh. Lead-acid ratings also assume a discharge rate, usually the twenty-hour rate, and the usable fraction is often only half the nameplate to avoid deep cycling.
| 1 Ah | 3,600 C |
About the ampere-hour
The ampere-hour survives as a battery unit for a historical reason that is still practical: for a given cell chemistry the voltage is roughly fixed, so charge alone ranks capacity within a product family. It also matches how charge controllers and shunts actually work, since they integrate current over time. Coulomb counting is exactly the ampere-hour done continuously, \(Q = \int I \, dt\), and every battery management system in a solar installation or an electric vehicle is doing that sum.
To compare across chemistries or voltages, convert to energy: \(E \ ext{ (Wh)} = Q \ ext{ (Ah)} \ imes V\). Then remember the two derating factors that separate nameplate from usable. Depth of discharge limits how much you may take out, conventionally 50 percent for flooded lead-acid and 80 to 90 percent for lithium iron phosphate. Round-trip efficiency, roughly 80 to 85 percent for lead-acid and 92 to 96 percent for lithium, tells you what fraction of the energy you put in comes back out.