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How To Install A CMOS Battery

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Web Only October 2004 • Vol.15 Issue 10	Add To My Personal Library
How To Install A CMOS Battery

It would be funny if it weren't so annoying: A tiny battery that costs just a couple of dollars can single-handedly bring your high-tech PC screeching to a halt.

The CMOS (complementary metal-oxide semiconductor) battery is located on your computer's motherboard. It's just a little battery, not unlike what you could find powering a handheld calculator. When that battery starts to run out of power, unpleasant things will happen. You may notice that the PC keeps forgetting the date and time, resetting them to midnight on Jan. 1, 1990, perhaps. Or the PC will suddenly be unable to access certain peripherals. Or it may refuse to boot at all, displaying a "CMOS Checksum Error" or "System Configuration Lost" message instead.

First, let's back up and provide some background. Your computer's BIOS (Basic Input/Output System) is the traffic controller for all of the PC's hardware. Part of the motherboard, the BIOS is responsible for booting the computer, keeping track of the time, and reading the keys that you type on the keyboard. It makes sure the CPU can talk to the motherboard and the hard drives can talk to the IDE (Integrated Drive Electronics) bus.

The settings for the BIOS are stored in special memory called CMOS. A battery backs up the CMOS, which retains these essential settings when the computer is off. When that battery runs out of juice, the CMOS forgets the proper settings and things stop working properly.

A typical CMOS battery should last for years—perhaps a decade—but numerous factors can reduce that time. If your computer is showing the symptoms of a bad battery, go ahead and replace it.

Replace The Battery

1. Save your current BIOS settings. When you remove the old battery from the motherboard, the information stored in the CMOS will be lost, so you should write down the BIOS settings.

If the battery is already completely drained, your CMOS settings have already been forgotten. So you'll only need to take this step if you think the CMOS battery still has some life left in it.

Turn the PC on. As soon as it powers up and beeps, press the appropriate key to enter the BIOS setup utility. This varies between computers but is often the DELETE, ESC, or F2 key. The information screen that appears when you first turn on the PC will tell which key to press on your system. The simplest route may be to simply check the manual.

You'll see the main menu of the BIOS setup utility. You'll probably have to use the keyboard to navigate, although a few setup utilities include mouse support. Navigate to every setup screen and write down the configuration settings. If you have a print that uses a parallel port rather than a USB (Universal Serial Bus) port, you may be able to save the tedium by pressing the PRINT SCREEN key to print each configuration screen.

Alternatively, some motherboards include software—either as part of the BIOS setup utility or as separate software—that saves CMOS settings to a floppy diskette. Check your system manual to find out if yours does.

2. Get access to the motherboard. Unplug the computer and disconnect all of the cables. Remove the side panel to reveal the motherboard.

Move the computer to an area that isn't crackling with static electricity. Static electricity can wreak havoc with your motherboard, so don't continue while wearing stockings on a plush carpet as the cat rubs herself on the computer. It's a good idea to use an antistatic wrist strap whenever you're working inside a computer.

This old battery has got to go.

3. Locate the battery. You'll need to know what type of battery you need. Check the PC's manual or just examine the motherboard itself. The most common type of CMOS battery is CR2032, a round, silver-colored battery about the size of a dime. If your motherboard uses a CR2032, you're in luck. They're inexpensive, easy to replace, and widely available. You can buy one for about $4 at the photo counter of your local drug store, RadioShack, or an online vendor.

If your motherboard doesn't use a CR2032, things could be trickier. With any luck, it will use another easily replaceable battery, but some older PCs use barrel-shaped batteries that are soldered to the motherboard. If you are handy with electrical equipment, you can get out a soldering gun, remove the old battery, and solder in the replacement. If not, find a computer repair shop to do it for you. (Then again, chances are that any computer that uses a soldered battery is well past its prime. This might be a good excuse to get a new machine.)

The replacement battery does not need to be from the same manufacturer as the original, as long as it is the same type of battery. Just like when you're buying flashlight batteries, a D battery is a D battery, whether it's made by Energizer or Duracell.

4. Remove the old battery. Next, remove the old battery. Different motherboards have various configurations of clasps that keep the battery clipped to the motherboard. Figure out how the battery is attached and gently remove it. Be careful not to break the retaining clips, or anything else, on the motherboard.

An empty battery socket.

In the several motherboards we checked, it was possible to remove and replace the battery without taking out any peripherals. But it's possible that on your system, an optical drive, add-on card, or other internal component blocks easy access to the battery. If you're not comfortable removing and reinstalling those components, bribe a friendly neighborhood computer geek to help.

5. Test the battery (optional). If you have access to a multimeter, you might want to test the old battery. A fully charged CR2032 should put out 3 volts, but most motherboards will work fine with 1.8 volts or more. If the battery is still putting out at least that much juice, the PC's problem is probably not the battery.

6. Install the new battery. Now install the new battery. Be sure to put the poles in the same position as the original battery: For a CR2032, the positive pole (marked with a plus sign [+]) should be facing up. Use the clips to hold the battery in place.

Now, reattach any internal peripherals that were in the way, replace the PC's side panel, and reattach the cables.

Configure The BIOS

While you were replacing the battery, the CMOS probably lost all of the BIOS settings. Before you can boot the PC, you need to reconfigure the BIOS.

Reset the date, time, hard drive configuration, and processor settings with the BIOS (Basic Input/Output System) configuration utility.

Turn the PC on and press the key to start the BIOS setup utility. Methodically go to every setup screen and set the values to the settings you wrote down or printed. Or, if you backed up the BIOS settings with the motherboard's backup software to floppy disk, use it to restore the settings. Either way, don't forget to reset the date and time.

If you didn't write down the settings beforehand (or they were already lost due to the dead battery), look for the Restore Default Configuration item, which should reset the BIOS to reasonable, although not necessarily optimized, settings. It could be called Load Fail-Safe Defaults, Load Best Performance Settings, or something similar.

After you've reconfigured the BIOS, use the Save And Quit command. The BIOS settings will be written to the CMOS and backed up by your fresh, new battery. The computer will reboot and load the operating system. You're back in business, fully charged and ready for another 10 years of computing.

by Kevin Savetz

Clear The CMOS

It's possible that a dead or dying battery can cause the CMOS (complementary metal-oxide semiconductor) data to become corrupted. In this case, the BIOS (Basic Input/Output System) utility's Load Fail-Safe Defaults or Load Best Performance Settings command should set things back to normal. If that doesn't work or the computer is so befuddled that you can't even get to the BIOS setup utility, your motherboard will have another way to reset the CMOS data using a special jumper or DIP switch setting. Generally you simply move a jumper to a special position, wait a few seconds, and then move it back. Check the motherboard manual to find out the procedure for your system.

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