The battery in your Dell Dimension desktop computer is the CMOS battery. The CMOS battery is what supplies power to your motherboard when your computer is turned off. Your CMOS battery makes it possible for your computer to retain the BIOS settings and time clock when your computer is not connected to an additional power source. Your CMOS battery looks like a large watch battery.
Turn off your computer and remove the power cord and any other peripheral components.
Remove the side of your computer case by removing the two screws on the back of your computer.
Locate the CMOS battery on your motherboard. It will look like a large, silver watch battery.
Grab the edge of the battery and gently pry it out of its chamber.
Place a new CMOS battery into the battery chamber, then replace the side of your computer case.