What do the CMOS BIOS batteries mean

How to recognize an empty BIOS battery

Friedrich Stiemer

There are a number of reasons why a PC won't start or crash with error messages. One reason could be an empty Bios or CMOS battery. In this tip, we'll tell you how you can tell.

EnlargeIf the computer does not want to start up, there may be a simple solution, especially with old mainboards.

So that the central settings and routines of your PC are retained even when the power connection is cut, your motherboard has a separate button cell battery. As a rule, the Bios battery lasts three to five years, but the time can be significantly reduced under poor conditions such as low ambient temperatures.

You can recognize an empty button cell by the following characteristics:

  • When the system starts, you will receive a message such as “Mismatch CMOS” or “CMOS Read Error”.

  • You will receive a message such as "Invalid Date and Time", or the BIOS clock and the date show incorrect data, for example 01/01/1970.

  • Your user settings have been lost, such as your boot order or your settings for fans or controllers. The message "Configuration Error" can also appear.

If you receive one of the above messages, you should switch off your computer and insert a new button battery. Usually this is a CR-2032 button cell with 3 volts. Which battery you really need can be found in the manual for your motherboard. You do not need any special tools for the change; a (flat) screwdriver is usually sufficient. Just make sure that you have completely disconnected the computer from the power supply and ideally have yourself grounded. Then reconnect all of your peripherals and go to your BIOS to make your settings again.