Opening the Laptop
Open up your laptop. To do this, you will have to unscrew five or six screws that hold the laptop together. Turn the laptop off and unplug it. Then turn it over and find the screw holes in the bottom of the laptop. Unscrew them and then remove the bottom portion of the laptop. Now, locate the CMOS battery.
Finding the CMOS
The hardest part of replacing a CMOS battery may be finding the CMOS itself. Usually it is located right in the center of the motherboard (main circuit board of the computer). Look carefully around the motherboard until you find the CMOS. It is round and silver. Typical CMOS batteries are smaller than a quarter and about a 1/4 inch thick. It might say 3V CRV032 on the battery as well.
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Removing and Replacing
To remove most CMOS batteries, you need to undo a small silver clasp that fits across the width of the battery. Just pull this clasp up and slide the battery out. Take note of how the battery was set into the clasp. The new battery needs to go in exactly the same way. Once you have the battery out, you can insert the new one. Just lift up the clasp and slide in the battery.
Buying a New Battery
CMOS batteries are sold at Radio Shack and other electronics stores. You need to match the part number on your current CMOS to the new one. The part number is written right on the side of the battery package. Most are 3 volt lithium batteries, but the size may vary. Check the part number to be sure.