Ampere-Hour (Ah)
An ampere-hour (Ah) is a unit of electric charge. One ampere-hour is the charge transferred by a current of one ampere flowing for one hour, equal to 3,600 coulombs.
Charge is not energy
Ampere-hours describe electric charge, while Kilowatt-Hour (kWh) describes energy. A battery's Ah rating cannot be compared meaningfully with another battery's energy capacity unless voltage is also considered.
A simplified conversion is:
Energy (kWh) = nominal voltage (V) × capacity (Ah) / 1,000
For example, 100 Ah at a nominal 400 V corresponds to 40 kWh by this calculation. The result uses nominal voltage, however, while real battery voltage changes with state of charge, current, temperature, chemistry, and ageing. A measured energy result also depends on the test boundaries and losses. The formula is therefore a useful relationship, not a substitute for a manufacturer's rated or tested energy value.
Cell and pack ratings
Cell capacity is commonly specified in Ah. How cells are connected changes the pack rating:
- Cells connected in series add voltage; the ideal string retains the Ah rating of one matched cell.
- Parallel cells retain the same voltage and add their Ah capacities.
Real packs are limited by cell variation, temperature, resistance, and the boundaries enforced by the battery management system. Multiplying a cell's nameplate rating by a cell count can therefore describe the intended configuration without proving the energy available to the driver.
See Series and Parallel Cell Configuration for series and parallel notation and Battery Basics for the relationship between charge, voltage, power, and energy.
Do not confuse these units
- A is electric current.
- Ah is electric charge capacity.
- kW is power, or the rate of energy transfer.
- kWh is energy.