A card of four Energizer 2032 lithium coin batteries (CR2032). Lithium coin cells power small electronics such as car remotes and keyless entry fobs, bathroom and kitchen scales, glucose and heart-rate monitors, watches, calculators, garage door openers and computer motherboards. Each cell gives 3 V and measures 20 mm across and 3.2 mm thick, the most widely used coin-cell size. The Energizer pack is child-resistant, which matters because swallowed coin cells are dangerous to small children; keep spare and used cells out of reach. The batteries contain zero mercury and hold their charge for years in storage, so a four-pack covers several devices or keeps spares on hand.