The CMOS battery (complementary metal-oxide semiconductor) is a small battery that provides power to a motherboard while it is powered off. This power source is used to store the BIOS configuration data (but not the BIOS firmware itself) and powers the Real Time Clock (RTC). The BIOS firmware is â€œhardcodedâ€ in the motherboard itself and is not lost or deleted if a CMOS is removed, but the configuration data will be.
A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of time.
Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time of day.