How to Tell If a CMOS Battery Is Low


If your computer's clock is falling behind or never seems to be correct no matter how often you reset it, you may be experiencing the effects of a low CMOS battery. This battery powers the clock and certain on-board memory, such as the BIOS. Eventually, it wears out, and if not replaced, your BIOS will begin to lose track of important settings on your computer. Know when your battery is low so that you can replace it in a timely manner.

  • The CMOS or Complementary Oxide Metal Semiconductor controls the BIOS (Basic Input Output System) on your computer. It prevents the computer from losing information about the motherboard and hard drives attached to the PC each time you shut down. It also keeps the clock running when the computer is turned off.

  • When the CMOS battery begins to run low, it won’t usually pose a serious problem initially. The clock slowing down is an annoyance, but you’ll have time to open your computer and replace the CMOS battery or take your machine to the repair shop.

  • Look for error messages at startup. Some computer models will display an error message that the CMOS battery is low. The message varies by manufacturer. You may also see a prompt when the computer is booting asking you to "Press F1 for setup or F2 to load default values." The function keys vary from one computer to another. When the computer begins losing data during the boot cycle, the CMOS battery is nearly depleted and should be replaced as soon as possible.

  • Watch for random events when the computer is booting. Your PC might not show signs of failure every time you turn it on. It might be more random at first; or the computer may get to a certain point in the boot process and then freeze. If rebooting does not resolve the issue, a low CMOS battery may be the cause.

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