A CMOS battery is a small item which powers the BIOS memory of a motherboard in a computer. The battery is necessary because it supplies the power required for the motherboard to store short-term data, even after you have turned the computer off. A CMOS battery is a small, 3-Volt, 1/4-inch size battery. Replacing a CMOS battery is a straightforward process and you do not need expert help.
Unplug the computer and disconnect all electric cables and wires.
Unscrew the top and back cover of your desktop or laptop computer using a screwdriver. The screws are located at the back of the desktop computer and sometimes at the bottom of the main box.
Locate the motherboard. The motherboard is the mainframe of the computer to which other cards, such as the visual and sound cards, are attached. If you can see the CMOS battery is in a location which you can easily reach, you do not need to remove the motherboard. The CMOS battery looks like a silver coin, about the size of a shirt button or a dime.
Remove all additional cards that are on the motherboard, if you cannot remove the CMOS battery otherwise. Unscrew the screws holding the additional cards and pull them out from their slots.
Unscrew the motherboard. Locate the battery. Unflip the fastener or retaining clips that hold the battery in place. Take care not to break any of the plastic parts that keep the battery from falling out.
Insert a new battery and lock the fastener or retaining clips. Restart the computer.
Never attempt to change a battery while the computer is powered up.