How to Check the CMOS Battery

By Contributing Writer

A charged complementary metal oxide semiconductor battery is necessary to retain your computer's hardware settings and for the proper functioning of its operating system. Symptoms of a depleted CMOS battery can include system warnings, lost time settings or even unexpected shutdowns. Once you've recognized the symptoms, you can check the battery itself.

Things You'll Need

  • Phillips screwdriver
  • Jeweler's flat-blade screwdriver
  • Voltmeter or multimeter
  • Antistatic wrist strap

Checking for Problems

Step 1

Watch for a BIOS warning on startup. If your system is equipped with a CMOS battery monitor a warning message will appear when it's time to replace your CMOS battery. Most standard or home computer systems do not have this feature, however.

Step 2

Look for incorrect or changing time settings. One function of the CMOS is maintaining system time. If the CMOS battery is failing, you may notice invalid times showing within the operating system, such as on the clock at the bottom right of your monitor. This symptom is confirmed if time resets after the computer system is restarted. Under these circumstances, the CMOS battery should be replaced.

Step 3

Watch for unexpected computer shutdown or invalid hardware errors. The CMOS retains settings for your system's hardware, including hard drives and RAM. If these stored setting are not available during operation, the computer system can unexpectedly shut off. Check that all cables, both inside and outside the computer, are firmly attached. If cables are attached but symptoms continue, replace the CMOS battery.

Step 4

Check the CMOS voltage. If you have a voltmeter or multimeter, the direct method of determining CMOS battery failure is to check the CMOS battery's voltage. Most new CMOS batteries produce 3 volts. When this voltage drops below 2.5, problems can appear. If the battery's voltage drops below 1.8 volts, CMOS battery failure is imminent.

Replacing the Battery

Step 1

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.

Step 2

Shut off the computer and then remove the external cover using a Phillips screwdriver. 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.

Step 3

Locate the 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.

Step 4

Check the voltage of the CMOS battery. Determine positive and negative polarity by the plus or negative sign engraved on top of the battery. Set the voltmeter to the lowest range above 3 volts. Apply the positive multimeter lead to the positive side and negative lead to the negative side. Read the voltage. If the meter reads below 2.5 volts, replace the CMOS battery.

Step 5

Install a new or functioning 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.

Step 6

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.

Tips & Warnings

  • CMOS-type batteries are often used in cameras and small electronics. Replacement batteries can be purchased at photo counters, drug stores, electronics stores or online. Compare engraved serial numbers before purchasing.
  • Remove power and discharge static electricity before touching delicate electronics.