Battery Capacity

Battery capacity is the measure of how much electrical energy an electric vehicle's traction battery can hold when fully charged, expressed in kilowatt-hours. One kilowatt-hour represents the energy of one kilowatt of power sustained for one hour, so a 60 kWh pack could in principle supply 60 kilowatts for an hour, or a smaller draw for proportionally longer. It is the headline specification most buyers look to when assessing how far a car will travel between charges.

The figure reflects the combined storage of hundreds or thousands of individual lithium-ion cells wired into modules and assembled into the pack. Capacity is determined by the cells' voltage and their charge storage in ampere-hours, and manufacturers increase it by adding cells, adopting denser cell chemistries or improving the efficiency with which cells are packaged into the available space beneath the floor. Packs in current vehicles span a wide range, from around 40 kWh in compact models to 100 kWh or more in large, long-range cars.

Capacity is the single largest influence on driving range, because range is essentially the stored energy divided by the rate at which the car consumes it. A larger pack carries more energy and therefore travels further, all else being equal, which is why range and capacity are so often discussed together. However, capacity alone does not determine range; a heavy, inefficient vehicle may travel no further on a big battery than a light, efficient one on a smaller pack.

An important subtlety is the distinction between gross and usable capacity. The gross, or nominal, figure describes the total energy the cells could theoretically hold, but manufacturers deliberately reserve a buffer at the top and bottom of the charge range that the driver can never access. This buffer prevents the cells from being fully charged or fully discharged, both of which accelerate wear, and so protects longevity. The usable capacity, which is what actually powers the car, is consequently a few percent smaller than the gross figure quoted in some specifications.

Real-world range also depends heavily on factors beyond the battery, including ambient temperature, driving speed, terrain, the use of heating or air conditioning, payload and tyre choice. Cold weather in particular reduces both the available capacity and the efficiency of the vehicle, so a nominal figure should be treated as a best case rather than a guarantee.

Battery capacity relates closely to usable battery capacity, which expresses the protected, accessible portion; to the kilowatt-hour, its unit of measurement; to EV range, which it largely governs; and to the high-voltage battery, the physical pack in which that energy resides.