What is a CMOS battery, and why does my computer need one?

All computers (80286 processor) or later require a small battery on the system board that provides power to the Complementary Metal Oxide Semiconductor (CMOS) chip, even while the computer is turned off. This chip contains information about the system configuration (e.g., hard disk type, floppy drive types, date and time, and the order in which the computer will look for bootable disks). The CMOS battery allows the CMOS to preserve these settings.The memory and real-time clock are generally powered by a CR2032 lithium coin cell. These cells last two to ten years, depending on the type of motherboard, ambient temperature and the time that the system is powered off, while other common cell types can last significantly longer or shorter periods. Higher temperatures and longer power-off time will shorten cell life. When replacing the cell, the system time and CMOS BIOS settings may revert to default values. This may be avoided by replacing the cell with the power supply master switch on. On ATX motherboards, this will supply 5V standby power to the motherboard even if it is apparently "switched off", and keep the CMOS memory energised.
If you do not know the model battery you require, we recommend you try to obtain the specification first from the manufacturer or consider making a visual comparison between our batteries and the battery currently installed in your computer. The proper course toward replacing your CMOS battery is to determined the model required. You will need to open your computer, locate the battery and compare it to the models we have on display.