A CMOS battery--short for complementary metal oxide semiconductor--is necessary to retain your computer's hardware settings. Symptoms of a dying CMOS battery include system warnings, lost time settings, and sometimes even unexpected shutdowns. Once you've recognized the symptoms, you can check the battery itself.
Check for Problems
Look for incorrect time settings. One jobs of the CMOS is maintaining the system time, and if the CMOS battery is failing, the computer's clock will generally not remember the time between restarts. If the time resets after each reboot, you should replace the battery.
Watch for unexpected computer shutdown or invalid hardware errors. The CMOS helps retain settings for your system's hardware, including hard drives and RAM. If these stored setting are not available, the computer can unexpectedly shut off. Check that all cables are firmly attached. If cables are attached but symptoms continue, replace the CMOS battery.
Check the CMOS voltage. If you have a voltmeter or multimeter, the best method of determining the health of the CMOS battery failure is to check its voltage directly. New CMOS batteries produce 3 volts. If the voltage drops below 2.5, problems can appear. If the battery's voltage drops below 1.8 volts, CMOS battery failure is imminent.
Replace the battery
Record the CMOS settings before you remove the battery. When your CMOS battery is removed, hardware settings stored in the CMOS will be lost. To record these settings, boot up your computer and enter the BIOS utility. Pressing the "Delete" or "F10" key during startup on most systems will start the BIOS utility; if this doesn't work, consult your computer's manual for the correct procedure. Write down any customized settings, such as hard drive configuration, CPU speed adjustments and external port deactivation.
Shut off the computer and then remove the external cover. Attach an antistatic wrist-strap clip to the case housing. This will discharge any static electricity and preserve the computer's delicate electronics. Unplug the AC power cable from the power supply.
Locate the coin-sized CMOS battery on the motherboard. Remove any peripheral cards or cables that obscure access. Using a jeweler's flat-blade screwdriver, slowly pry back the metal holding clip while lifting the CMOS battery with your fingernail.
Install a new CMOS battery by pressing it firmly into the battery casing until the metal holding clip snaps into place. Replace any removed peripheral cards or cables. Replace the computer case and reattach the power cable.
Restart the computer and enter the BIOS. Select "load default BIOS settings" and then reconfigure the customized settings you wrote down earlier. Save settings and exit the BIOS utility.