In all desktop PCs there is a battery - what is the purpose of this battery and how long does it last before it has to be replaced? Does the replacement of this battery affect the BIOS of the computer? I've had a PC for 6 years now and never replaced the battery. • Actually all PCs, desktop and laptop alike, have a battery like this, typically referred to as the "CMOS" battery. It serves an important function, even though it's actually possible on most machines to run without one ... which quickly becomes annoying. The good news is that, as you've seen, they typically last for years. • "CMOS" stands for "Complementary metal-oxide-semiconductor", the original technology that was originally used in some of the circuitry that the battery was used to power. While no longer the case, the term "CMOS" or CMOS battery lives on anyway. The purpose of the CMOS battery most often today is simply to allow your computer to remember what time it is. I think of it as a glorified watch battery. If the battery dies or is removed, then when your computer boots it will have forgotten the current date and time.