Almost all desktop computers contain a small battery. Believe it or not your computer needs this battery to save its CMOS settings (Complimentary Metal Oxide Semiconductor) .

I bet you didn't know that did you! This small battery (about the size of a 10 pence coin) provides power to the computer motherboard ROM chip where your computer CMOS setup data is stored.

Change CMOS Battery

If this battery goes flat your computer will 'forget' its settings because the battery cannot save the data stored in your ROM chip. (Read Only Memory).


 

Symptoms of a Flat CMOS Battery

Your computer will start to do some strange things if this little cmos battery runs down. You will need to change the CMOS battey from time to time. When you start your computer messages may appear on your screen such as :-

• System date and time incorrect and will not change
• Invalid system settings--Run Setup - unable to change it
• CMOS checksum error - unable to change error

The best sign of a need to change the cmos battery is your system date and time. If you change your CMOS settings using the computer BIOS and they are not saved, then this confirms your battery is low or dead. If your CMOS change is successfully saved then you may have a virus of some sort attacking your BIOS.

System Bios

BIOS, pronounced "bye-oss," an acronym for Basic Input/Output System. The BIOS is built-in software stored on your ROM chip that determines what a computer can do without accessing programs from a disk. On PCs, the BIOS contains all the code required to control the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions.

Change CMOS battery

To change your CMOS battery is quite easy, but make sure you get the correct battery or you may damage your ROM chip. If you damage this chip your motherboard is buggered. It's cheaper to get a new motherboard than try to repair the ROM chip let me tell you.

Most modern motherboards use a Lithium Micro Coin Battery 3v 220 mAh as pictured above. It is retained on the motherboard by simple clips in the round slot where the battery sits. be careful not to break the spring clips when you change the cmos battery.

Typical CMOS Battery location on a motherboard