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Do you have a desktop computer, or a laptop or netbook ?
I'm assuming you have a desktop computer, gong by the sparse information you're provided.
Cmos batteries for desktop mboards typically last about 5 years.
It's a coin shaped battery on modern mboards that is installed in a metal or metal lined socket that is either flat on, or vertical to, the mboard
If you haven't replaced it, replace it.
There is a clip or lever on one side of the socket you move toward the outside of the socket to release it from it's socket.
Local places that have all sorts of batteries probably have the same battery "number" or a clone of it. Even some "dollar" stores have them.
If you HAVE replaced it, if you are still getting messages about a failed battery....
- the battery must be installed so that it's polarity is correct. The + sign on the battery's surface should be visible when it has been installed. If the battery is upside down in the socket, the moard's bios will "think" the battery has not been installed or is dead.
- the contact in the metal socket that touches the top (+) surface of the battery may have been bent such that it's getting a poor connection or no connection. If that contact is bent, remove the battery, bend the contact toward the bottom of the socket, then install the battery again so that the + shows when it's installed.
( If the battery is in a vertical socket, you may not be able to see the + when the battery is installed - that + side must be toward the side of the socket where you can see a contact in the middle on the inside of the socket.)
- rarely, there may be corrosion in the socket that is making the connection of the battery to it's socket poor - usually in that case you will see whitish or grayish deposits on the metal of the socket. If you clean off the corrosion, the battery not being detected properly should go away. Usually if you use a bit of vinegar on something such as a cotton swab (q-tip) to clean the socket and the underside of the top contact, that will work fine - remove any excess vinegar - the socket must be dry when you install the battery.
- if you have moved the clear Cmos jumper on the mboard to the position for clear,or have removed it, you must move it back to the position for normal or similar, or install it again, otherwise you MAY get a message about the battery. .
After you have removed the battery and have installed a good one in the right direction, or after you have cleared the Cmos by moving a jumper on the the mboard and have moved it back to the "normal" position, you will get a "Cmos checksum error" or similar when you boot the computer. The message is generated because bios defaults have been loaded, including the default original date and time. To get rid of that error, you must go into the bios Setup and set at least the current date and time, Save bios settings. In some cases, you also have to change default bios settings for other things as well.