The desktop CMOS battery, also known as the motherboard battery or BIOS battery, is a small coin cell battery that provides power to the complementary metal-oxide-semiconductor (CMOS) chip on the computer's motherboard. This battery is responsible for maintaining certain system settings, including the date and time, BIOS settings, and hardware configurations, even when the computer is powered off.

Over time, the CMOS battery can lose its charge, leading to issues such as incorrect system time, BIOS settings resetting to default, or startup errors. When this occurs, replacing the CMOS battery with a new one of the same type (usually a CR2032 lithium battery) resolves the issue and restores proper functionality to the system.