Motherboards come with batteries, usually replaceable lithium coin cells. These batteries are in place primarily to keep the time/date clock running and maintain BIOS setup program settings. The batteries usually last at least six years. If a computer loses its BIOS settings and time/date memory every time it is shut down, you’ll need to replace the battery, if possible. The POST might show an error message if the battery has died. Moreover, if someone has set a supervisor password in the setup program, and now has forgotten it, and the computer cannot boot, the only way to reset or cancel the password is to remove and replace the battery. You might be able to use a battery tester to test the voltage of the battery without removing it. Make sure the polarity of the probes matches the polarity of the battery, and check to see if the battery is still good. Double-check the motherboard documentation to make sure the battery is the right type.
Battery Replacement
If the BIOS settings are still valid and you need to replace the battery, know that the motherboard might have a capacitor that will hold a charge just long enough for you to replace the battery without losing your settings. However, whether or not it has such a capacitor, it is recommended to write down all non-default BIOS settings before changing the battery. Changing a replaceable battery is a simple but delicate operation. Carefully remove the old battery and insert the new one.
It means the CMOS battery is dead ,this is the motherboard battery that keeps the BIOS settings when the computer is turned off. The biggest problem is that it shows the wrong time and date so when you start the computer the operating system will have the wrong date and time.This will cause the antivirus to fail to contact the server to download updates and windows updates as well
