A single Energizer 2025 lithium coin battery (CR2025). 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. The cell gives 3 V and measures 20 mm in diameter and 2.5 mm thick; it is thinner than a 2032, so check the size marked on your old battery before buying. Energizer coin cells contain zero mercury. Swallowed coin cells are dangerous to small children, so keep spare and used batteries out of reach. Wipe the new cell clean and fit it with the marked positive side as shown in the device.