Complementary Metal Oxide Semiconductor. Have you ever wondered why your computer has a battery? The CMOS RAM (Random Access Memory) maintains a small 64 byte of information to actually start the computer. It tells the computer what it is and gives it basic settings to get to the BIOS and perform the POST (Power On Self Test). The CMOS is maintained by power supplied on various forms of batteries. Older PC’s actually used AA batteries which worked fine until they leaked and caused damaged to the motherboard. Newer PC’s use some form of NiCad batteries that will not leak and will recharge while the computer is in use with an average life-span of about ten to twelve years. If you power up your computer and receive a message that your BIOS (SETUP) is incorrect its good indication that your CMOS battery is dead and will need to be replaced.