The CMOS is usually powered by a CR2032 cell battery, referred to as the CMOS battery.
Most CMOS batteries will last the lifetime of a motherboard, up to 10 years in most cases, but will sometimes need to be replaced.

Incorrect or slow system date and time and loss of BIOS settings are major signs of a dead or dying CMOS battery.
Replacing them is as easy as swapping out the dead one for a new one.