Change CMOS Battery?

Dennis Faas's picture

Infopackets Reader 'CusterCr' writes:

" I have been away from home for quite some time. A friend of mine went to use my computer recently, and now she has to hit F10 on the keyboard to save the time and date every time the computer turns on. Any ideas what might be wrong? "

My response:

It is most likely that your CMOS battery has faded away and now your BIOS settings have been lost. The BIOS settings will keep resetting every time power is lost or the computer is unplugged because there is no way to save the information (which is done with the aid of a CMOS battery).

Put in Simple English: CMOS is "an abbreviation of Complementary Metal Oxide Semiconductor [try and say that 10 times!]. Pronounced 'see-moss', CMOS is a widely used type of semiconductor. Personal computers also contain a small amount of battery-powered CMOS memory to hold the date, time, and system setup parameters."

Related to CMOS is the BIOS "(pronounced 'bye-ose') and is an acronym for Basic Input/Output System. The BIOS is built-in software 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, [set time and date], and a number of miscellaneous functions." (Source:

Replacing the CMOS Battery

Most computers today use a lithium CMOS battery. Battery voltages can vary, so you'll need to pop out your old one to find out what type it is (I.E.: model number / voltage) and replace it with a similar kind.

Changing a lithium CMOS battery is relatively simple, although it requires that you access the inside of the computer. The battery itself will cost about $15.00 (or less).

To open up the computer case, you will [most likely] need a Phillips ("Star") screw driver. Once the case is off, touch the metal chassis to ground yourself. Don't touch anything inside the computer (especially circuitry) to reduce the risk of static electricity damaging the chips.

To remove the battery, use a flat head screw driver and gently 'pry' on the release mechanism. The battery should pop out with little effort. One the battery is out, replace it with the new one (shiny side up).

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