What is a CMOS battery, and what does it do? Seems like a fairly simple, straight forward question, doesn't it? In reality, in order to understand what the CMOS battery does, you have to understand how the computer actually starts. Once you understand how the computer starts, then you will see just how important that one little battery really is.
CMOS is an acronym for Complementary Metal Oxide Semiconductor. CMOS are used as a simple form of memory on a computer motherboard. The CMOS holds user customizable information. Items such as passwords, the time, the date and drive configurations. CMOS is also known as BIOS Memory.
CMOS is used as a form of nonvolatile memory. It needs power supplied to it in order to maintain the data that is stored in it. The CMOS battery is that power source.
BIOS is another one of those pesky computer acronyms. It stands for Basic Input/Output System. The BIOS is started with the power switch. Once you have powered your computer up, the BIOS takes over. It reads information from the CMOS and from, possibly a ROM (Read Only Memory) or EPROM (Erasable Programmable Read Only Memory). It begins to initialize components on the motherboard. It starts the clock generator, the processor(s), the PCI (Peripheral Component Interconnect) slots, and gives them their bus numbers and tells them which resources have been allocated to them, it starts the memory controller, then loads itself into memory and continues on. Next the I/O controller is started. The video is started and then the IDE (Integrated Drive Electronics) and SATA (Serial Advanced Technology Attachment) controllers. It then reads the boot sector on the first start up device in the list and allows that drive to take over control of starting the computer.
Without the CMOS, or its battery, the BIOS would not have enough information to start a computer. If you start losing the date or the time on your computer, or you start getting drive error messages, it may just be time for a new CMOS battery. They are fairly inexpensive and can be purchased even at your local Wal Mart store.
The most commonly used CMOS battery in the desktop or tower computers today is the CR2032 Lithium battery.