NewerTech 3V Lithium CR2032 Coin PRAM Computer Clock Battery for Mac

Macintosh computers contain an internal backup battery to maintain system settings, such as the date and time, when the computer is turned off . The battery may last up to five years though actual life is determined by shelf life of the battery and usage patterns of the computer. The batteries used in the Macintosh computers listed in the specs tab are lithium 3V "coin cell" batteries.