A button cell, watch battery, or coin battery is a small battery made of a single electrochemical cell and 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; insulated from it, the metallic top cap forms the negative terminal.
Button cells are used to power small portable electronic devices such as wrist watches, pocket calculators, and remote key fobs. 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, holding their charge for a long time if not used.