The CMOS (Complementary Metal Oxide Semiconductor) is a small amount of memory on a motherboard used to store this information about your computer’s system configuration. The CMOS battery allows your computer to store information for items such as hard drive information, date and time and other BIOS related information when the computer is powered off.
These batteries usually last a long time and most of the time will last the life of the computer. If you notice that your time and date are always off even if you keep correcting it then that that may be a sign that the CMOS battery is going bad. Replacing the CMOS battery is fairly easy but you need to make sure you get the right type with the right specifications to match the one you are replacing.
The CMOS battery is located on the motherboard and looks like a flat silver disk and is similar looking to a watch battery but larger.
With the computer off and unplugged simply pop it out of the holder. Then look on the battery itself for markings that indicate a model number and/or voltage etc. Usually you can find them by model number and Amazon.com is a good place to get them cheap. You may want to see if you can read the number without taking it out so you can have your new one handy when you make the swap. Keep in mind that you will most likely have to go into the BIOS and adjust your date and time and make sure your other settings are correct after replacing the battery.