Date and Time

• An indication that your internal PC battery has died is when the date and time display are incorrect. Because the CMOS controls the date and time function, when your battery is completely dead, your date and time become inaccurate after you've shut down and rebooted your computer.
  
  

Custom Settings

• If your PC has custom settings, those settings may fail if your CMOS dies. Some PCs display a message when started up that states "System Configuration Lost." In order to prevent losing your custom settings entirely, keep a written record of your BIOS settings so that you can easily reset them once your battery has been replaced.
  
  

Error Messages

• Some PCs display error messages when the internal battery fails. These messages will read "CMOS Read Error" or "CMOS checksum error." Your system will not run until you either replace or recharge the internal battery. Try to recharge the battery by leaving the computer on for 24 hours. In some cases this is all it takes to get the CMOS working again. If this does not work, you need to replace the battery.
  
  

Replacing the Battery

• To get your system functioning optimally again, you need to replace the battery. It is not advisable to change your own CMOS. Electrostatic discharge occurs when you transfer static electricity to a motherboard; it may cause some or all of your system to short-out. In addition to this risk, some internal batteries are soldered into place and cannot be replaced without soldering. Rather than risk damaging your computer, take it to a computer professional. Replacing the CMOS is a simple task for any computer professional and should not be expensive.