Glossary / Batteries and storage
Energy density
How much energy a battery stores per unit of weight or volume. The higher it is, the longer a light, small battery lasts.
There are two figures: energy per unit weight (watt-hours per kilogram) and per unit volume (watt-hours per litre). A lead-acid battery holds some 30 to 40 watt-hours per kilogram; lithium-ion batteries exceed 200. The reason is that lithium is the lightest metal and also gives a high voltage[1].
Where the battery has to be carried, as in electric vehicles and robots, energy density sets the range or running time. In stationary storage it matters less, and price and lifetime matter more. For comparison, petrol holds about 12,000 watt-hours per kilogram, far beyond any battery.
Related terms
- Lithium-ion batteryA battery that charges and discharges as lithium ions shuttle between the positive and negative electrodes. Used everywhere from phones to electric cars.
- All-solid-state batteryA lithium-ion battery whose liquid electrolyte is replaced by a solid material. It is expected to be harder to ignite and to store more energy.
- Grid batteryA large battery connected to the power grid that charges when electricity is plentiful and discharges when it is short.