All desktop and laptop computers use a special battery called a Complementary Metal Oxide Semiconductor (CMOS) battery which stores information about your computer, such as the date and time, when the machine is powered off. Normally the battery re-charges itself whenever the computer is turned on but if it stops working or becomes damaged you will need to replace it manually.
Unplug the power cord from the power supply on the back of your computer. Remove all of the extra cables for the mouse, keyboard, monitor, printer and speakers.
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Use a screwdriver to unscrew the case screws on the side of your computer. Put pressure on the side panel and slide it off the case.
Locate the small circular battery, which will usually be either color or silver-colored, near the center of the motherboard. Check the model number on the top of the battery and then purchase a replacement through an online electronics store or a local computer supply store. Make a note of the model information on the battery as you may need it to complete the installation.
Snap off the battery cover if your motherboard has one and then remove the CMOS battery by simply pulling it out of its holder.
Push the replacement battery back into the battery slot and pop the cover back down. Put the side panel back on the case, re-attach all of the cables, and then power on your computer to ensure the battery was correctly installed. Wait to see if the computer changes to a prompt asking for the battery information or if it boots directly to the desktop.
Enter in the battery model information you wrote down earlier if the prompt asks for it and then hit "enter" to finish the installation.
Things You'll Need
Replacement CMOS battery
The main cause of a CMOS battery failure is that it has lost it's charge. You can attempt to re-charge the battery by turning on your computer and leaving it on for a full 24 hours or longer. Make sure to turn off any "hibernation" or "sleep" settings on your computer so that it doesn't automatically shut itself down after a set period of time. Your computer's BIOS driver settings are stored in the CMOS battery so you may need to navigate your web browser to the website for the manufacturer of your motherboard and download a new BIOS driver.
Make sure to ground yourself by touching a metal object connected to the floor before touching the internal components of your computer or the electrostatic discharge could ruin your computer.