Coin cell battery
Alternatively known as a button cell or watch battery, a coin cell battery is a small, circular, stainless steel disc that provides power to portable devices, like watches and computers.
Coin cell batteries are important in computers as they allow them to retain certain system settings while powered off. For instance, the CMOS battery provides power to the system clock so the computer can remember what day it is next time it boots. The picture is an example of a model CR2032 coin cell battery on a computer motherboard. It is the most commonly-used coin cell battery.
Most coin cell batteries contain alkaline (manganese dioxide), lithium, or silver oxide, and can have a lifespan of several years depending on usage.