CMOS battery

CMOS batteries are used to power BIOS memory and the real-time clock to provide a constant power signal, even when the system is off, so as not to lose important start-up information. CMOS was the original term and is still commonly used to refer to Nonvolatile BIOS memory on most PC motherboards. CMOS stands for Complementary metal-oxide-semiconductor SRAM. The original design has been modified from the original, but the name remains, and battery backup power remains.

Technical Details
The BIOS memory and real-time clock on a PC motherboard are typically powered by a CR2032 lithium coin cell battery. CR2032 on motherboards tend to last 2-10 years depending on quality, motherboard draw, ambient atmospheric conditions, and amount of time the system is not powered by an external power supply. Some motherboard may use different cells, such as CR2016, but generally still use 3V lithium coin cell batteries. Some older boards have built in rechargeable batteries, although these have been largely abandoned due to cost, longevity, complexity of managing electronics, and reliability.

Removing a CMOS battery with no power present to the motherboard will typically reset the motherboard to default values the battery is removed. This can be alleviated by keeping the motherboard power-supply attached (and thus still providing power to the circuit) while providing power to the battery.