A button cell, watch battery, or coin battery is a small single cell battery shaped as a squat cylinder typically 5 to 25 mm (0.197 to 0.984 in) in diameter and 1 to 6 mm (0.039 to 0.236 in) high — resembling a button. Stainless steel usually forms the bottom body and positive terminal of the cell, and an insulated top cap is the negative terminal.
Button cells are used to power small portable electronics devices such as wrist watches and pocket calculators. Wider variants are usually called coin cells. Devices using button cells are usually designed around a cell giving a long service life, typically well over a year in continuous use in a wristwatch. Most button cells have low self-discharge and hold their charge for a long time if not used. Relatively high-power devices such as hearing aids may use a zinc–air battery which has a much higher capacity for a given size, but dry out after a few weeks even if not used.
Button cells are single cells, usually disposable primary cells. Common anode materials are zinc or lithium. Common cathode materials are manganese dioxide, silver oxide, carbon monofluoride, cupric oxide or oxygen from the air.