Every laptop computer has at least two batteries--a removable one that you charge through a power supply and then discharge when your laptop is in use without being plugged into a power supply, and one attached to the laptop motherboard that maintains the CMOS settings when your laptop is not connected to any power supply and is turned off. Most internal batteries (3-volt Lithium "coin cell" type) are designed by the manufacturers to last at least 5 to 7 years under normal use. However, if you wait for the internal battery to die before you replace it, you risk losing your CMOS settings (date, time, and configuration information). Even though the process may vary by laptop make and model, the general steps for replacing the internal battery are fairly simple.
Use the manufacturer's laptop service and repair manual to review the procedure for removing the laptop casing and replacing the CMOS battery, and purchase a replacement battery that the manufacturer recommends.
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Boot your laptop into its setup mode according to the user manual instructions, write down the settings for each page in the setup menu on a piece of paper, and then exit the setup mode.
Power down the laptop, unplug it from the power supply, and use the manufacturer's instructions to remove the external battery.
Put on the anti-static wrist strap and attach the clip end to something metallic to discharge static electricity before you open the laptop casing.
Lift the old internal battery from its slot and replace it with the replacement battery.
Follow the manufacturer's instructions for reassembling your laptop, restore the external battery and plug the laptop into the power supply.
Boot your laptop into its setup mode again, and enter the settings you noted on the piece of paper on each page of the setup menu.
Exit the setup mode, and let the laptop boot normally.