Battery capacity is a measure of how much electrical charge or energy a battery can store and subsequently deliver under specified conditions. Capacity is commonly expressed in ampere-hours or milliampere-hours when describing electrical charge, while the total amount of stored energy is expressed in watt-hours or kilowatt-hours. These measurements are related but are not interchangeable. Ampere-hours describe the quantity of electric charge that can be delivered at a specified operating condition, whereas watt-hours incorporate voltage and therefore describe energy. For an electric vehicle, battery capacity is usually discussed in kilowatt-hours, such as a pack having a stated energy capacity, because this is more directly related to the amount of energy available for propulsion and auxiliary systems.
Ampere-hour capacity can be understood conceptually as current multiplied by time. A battery that could theoretically deliver one ampere for ten hours under specified test conditions would have a ten-ampere-hour capacity, although real batteries do not necessarily behave linearly under every discharge rate. Energy capacity is approximately related to charge capacity multiplied by voltage, but the actual relationship requires integrating voltage over the discharge because voltage changes as the battery operates. A battery with a particular ampere-hour rating can therefore store different amounts of energy depending on its operating voltage. This is why comparing batteries solely by ampere-hours can be misleading when their nominal voltages differ.
Battery capacity is not necessarily one immutable physical quantity. Manufacturers specify capacity using particular test procedures involving factors such as temperature, discharge current, cut-off voltage and battery condition. A battery discharged slowly may deliver more measured capacity than when subjected to a very high discharge current, depending on its chemistry and internal resistance. Temperature can also alter available capacity, with cold conditions often reducing usable energy or power. Age causes capacity to decline as electrochemical degradation accumulates. Consequently, a battery's rated capacity, measured capacity and currently available capacity can represent different quantities.
Electric vehicles distinguish between total or gross battery capacity and usable or net capacity. Gross capacity represents the energy associated with the full battery under defined conditions, whereas usable capacity is the portion the vehicle's control system makes available to the driver. Manufacturers may reserve energy at the upper and lower ends of the cell operating range to protect the battery, preserve power capability and provide buffers for management functions. A vehicle can therefore have a battery whose physical energy capacity is greater than the energy accessible through the displayed state-of-charge range. The exact definitions and terminology vary between manufacturers, so capacity figures should be compared using equivalent measurement conventions.
Capacity is closely related to range but does not determine range by itself. An electric vehicle's range depends on usable battery energy and energy consumption per unit distance. Consumption varies with vehicle mass, speed, aerodynamic drag, tyre characteristics, temperature, terrain, heating and air conditioning, traffic and driving style. Two vehicles with identical battery capacities can therefore travel substantially different distances. Likewise, the same vehicle can achieve different ranges from the same battery depending on conditions. Capacity should consequently be regarded as the available energy resource, while efficiency determines how effectively that resource is converted into distance travelled or other useful work.
Battery capacity also differs from power capability. A battery with large energy capacity can store a substantial amount of energy without necessarily being capable of delivering or accepting extremely high instantaneous power. Power is measured in watts or kilowatts, while energy is measured in watt-hours or kilowatt-hours. Fast charging is primarily constrained by power and the battery's ability to accept that power safely, whereas driving range is more directly related to usable energy capacity and vehicle efficiency. A battery can therefore have high capacity but relatively conservative charging limits, or lower capacity with a design capable of relatively high charging power.
State of charge and state of health affect the capacity available at a particular time. State of charge indicates the estimated amount of usable energy remaining relative to the relevant capacity reference, commonly expressed as a percentage. State of health is a broader measure of ageing or remaining performance relative to a reference condition and can include capacity as well as power capability. A battery-management system estimates these quantities using voltage, current, temperature, historical operation and battery models. The displayed percentage is therefore an estimate rather than a direct measurement of stored energy. Its accuracy can vary, especially as the battery ages or operating conditions change.
Battery capacity is consequently best understood as a specified measure of stored electrical charge or energy under defined conditions, rather than simply the size of a battery. For consumer electronics, capacity may be quoted in milliampere-hours, while electric vehicles generally use kilowatt-hours because energy is the more useful system-level quantity. Capacity declines with degradation and varies with temperature and operating conditions. Comparing batteries accurately requires considering usable versus gross capacity, nominal voltage, measurement conditions, power capability, efficiency and age. Capacity explains how much energy is available, but it does not independently determine charging time, acceleration, range or overall battery performance.
