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Replacing Your Computer's CMOS Battery

CMOS battery replacement information

When your CMOS battery goes bad, you'll notice some strange behavior out of your computer. Your PC needs to remember several important pieces of information in order to operate correctly: the current date and time, the amount of memory installed, the number of disk drives (and their configuration), etc. This information is stored in CMOS memory, which is retained with the help of the CMOS battery when you turn off the power.

With earlier computers, either the user entered this information manually each time the computer booted up, or it was set using DIP switches or jumpers. Today's computers store this information in a special CMOS chip that uses a tiny battery to retain the information when power is removed from the computer.

CMOS (complementary metal oxide semiconductor) is a type of chip that consumes a miniscule amount of  power. While the computer is turned off, the CMOS battery discharges very slowly over time. But the battery will discharge completely if you leave your PC turned off for a long period of time. Even if you use your computer every day, the battery will only last two or three years. And sometimes a CMOS battery will simply fail prematurely.

Plug-and-play functionality built into modern computers can detect most of the required settings, but if your CMOS battery has died, your PC will not be able to retain the current date and time. It will also lose any custom settings, such as the boot sequence. And if you have set a CMOS password, you may even be locked out of the computer.

If the CMOS battery does fail, you may receive the message
"System Configuration Lost" as the computer is booting. As a precaution you should keep a record of all the CMOS settings.

To make a record of the CMOS settings, watch for an on-screen prompt when you first start your computer. The prompt will tell you to press a key (often the Del key) to enter the CMOS setup utility. Press the designated key while this message is displayed on the screen. After the CMOS setup screen appears, follow the instructions provided to view all the screens and record each of the settings. That way you'll be prepared when your CMOS battery does fail (and it will fail eventually).

If your PC's clock loses its setting overnight, the CMOS
battery is most likely not holding a charge. The CMOS battery is located inside the case on the system board (motherboard). It usually looks like a small "barrel", or it may be similar to a watch battery. If the battery is a flat, round, watch-type battery, you can probably change it yourself. CMOS batteries come in different voltages, so be sure to get an exact replacement. If the battery is of the "barrel" variety, you probably need to take the computer to a qualified PC service technician to have it replaced.

If you want to try to locate the CMOS battery yourself, be
aware that a static electricity discharge from your hands can cause severe damage to the components inside the computer. After taking proper precautions (use an anti-static wrist strap), open the case and look for a battery on the system board. If you have a diagram of your system board, locating the battery will be easy.

Sometimes the battery is attached with a small clip. Others are actually soldered to the system board. Replacing the latter type of CMOS battery is usually a job best left to a PC technician. A person who is inexperienced in soldering can cause severe  damage to the system board. If you want to try to replace a soldered-in battery yourself, first practice on an old, unused circuit board. You'll likely change your mind.

If the CMOS battery has failed due to old age, you might wish to consider just replacing the system board. If you do, you need to make sure that your new system board is the correct form factor for your computer's case, and that it has the correct bus connector slots for your expansion boards.

Stephen Bucaro is the owner of Bucaro TecHelp. Visit him at: http://bucarotechelp.com

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