• Many PCs still have replaceable CMOS batteries on the motherboard.Computer system motherboard image by Timur Anikin from Fotolia.com
  
  As of 2010, many PCs still rely on a complementary metal-oxide semiconductor (CMOS) battery to operate properly. This battery usually connects directly to the motherboard. The battery provides some power to the computer whenever the machine is off so that the computer can remember basic settings when you restart. Failure of the CMOS battery can cause serious problems for your machine. Once you recognize the symptoms of failure, replacing the battery constitutes a simple process.
  
  

Date and Time

• Your computer relies on the CMOS battery in order to display the date and time properly. If the CMOS battery starts to fail, your computer may not remember the actual date and time. This lapse usually points to a failing CMOS battery; do not ignore this symptom. Incorrect date and time might not seem like a huge issue, but because your computer relies on the date and time for functions such as email and chat timestamps and producing information about the creation and modification of files, it can cause major headaches., even causing some programs to stop running if the computer thinks your software license has expired.
  
  

Failure Finding Data

• Your computer uses the CMOS battery to store information about the machine's installed memory and drives. If the battery stops working or becomes weak, the computer may not be able to find the information you need. This may mean just some specific files or software programs, but it also can mean the inability to access entire drives. If you receive error messages about CMOS RAM, drive configuration or the size of your memory, the CMOS battery is likely on its last legs and requires replacement as soon as possible.
  
  

Failure to Boot

• The CMOS battery helps the computer store information about your hard drive. Your hard drive stores your everyday files, but it also stores the primary operating system. If the computer forgets how to access the drive with the operating system, the computer won't be able to boot up. At this point, you won't be able to use your machine until you change the battery. Because hard drives can fail on their own, you need to assess whether the boot failure is an isolated event or whether it has happened in addition to other CMOS battery failure symptoms.