When the error message 'CMOS BATTERY FAILED' is displayed while booting up the PC, it is required to replace the battery of the motherboard of the PC in question. The CMOS battery of the system needs to be ideally replaced after every 5 years as there is depletion in the battery life of the motherboard. In addition to the CMOS BATTERY FAILED error message, the other messages include NO OPERATING SYSTEM, NEW CPU INSTALLED and DATE & TIME ERRORS. The replacement of the CMOS battery needs to be done in such a way so as to avoid any sort of unexpected issue.
If the following error message is displayed when booting up your PC,
CMOS BATTERY FAILED
The you can tackle this problem simply: change the battery of your motherboard.
Replacing the battery of your motherboard (CMOS battery)