A personal computer stores hardware configuration data to the complementary metal-oxide semiconductor (CMOS) chip. Users can modify the CMOS and the behavior of hardware installed on the computer through the CMOS Setup Utility. The CMOS battery -- a lithium coin-cell battery mounted on the motherboard -- provides power to the CMOS chip and maintains the computer's internal clock.
Limitations
CMOS batteries do not come with replacement alarms. CMOS batteries tend to have a long shelf life and do not need to be replaced for many years. A computer can run without a CMOS battery, but you will need to configure the BIOS settings and set the date and time every time you boot the computer.
Symptoms
If the CMOS battery fails, the system clock will display the incorrect date and time each time you boot the computer. Even if you reconfigure the system clock to the correct date and time, the computer will forget the changes and revert to the default date after you power down the system.
Function
The basic input/output system (BIOS) detects the hardware components installed on the motherboard at startup. The BIOS refers to the CMOS data to initialize the hardware as defined in the CMOS settings. The computer stores the CMOS settings in a flash memory chip embedded on the motherboard. The CMOS battery powers the flash memory chip and maintains the computers date and time.
Replacement
Remove the power cord connected to the power outlet on the rear of the system. Unscrew and remove the case cover. Press the "Power" button to drain the motherboard of electrical power. Refer to your motherboard manual to locate the lithium coin-cell battery installed on your motherboard if you cannot find it on your own. Hold down the clip or lever securing the CMOS battery in place and lift the component out of its slot with your thumb and forefinger. Insert a replacement battery into the slot. Orient it in the same exact position as the old battery.
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