One Energizer CR2 lithium battery. The CR2 is a short 3 V cylindrical lithium cell, about 15.6 mm in diameter and 27 mm long, used in compact film and instant cameras, rangefinders for golf and hunting, some torches, home alarm sensors and smart locks. Lithium chemistry gives strong power for high-drain devices, works well in the cold and keeps its charge for years in storage. Check the size code on the battery you are replacing: the CR2 is shorter and thinner than the CR123A. It is a single-use cell and must not be recharged. Recycle used lithium batteries at a drop-off point rather than in household waste.