A laptop backup battery (CMOS, or complementary metal oxide semiconductor) is a small cell that is used to preserve system settings like time, date, boot order and hard drive type, even when the computer is powered off. IBM (Lenovo) Thinkpad backup batteries are easy to replace, thanks to the detailed assembly guides and user's manuals that are provided by IBM for each of their laptop models.
Refer to the user's manual for your particular IBM Thinkpad (see resources) to determine what kind of replacement CMOS battery you need. Purchase the battery through IBM, or a local or online computer parts dealer.
Turn the Thinkpad off and unplug it. Turn it over (bottom facing up), then touch something metal to remove static charge.
Slide the main battery release button to the "Unlock" position. Remove the main battery pack and lay it aside. If the CMOS battery is located in the memory compartment, unscrew the memory cover and lay it aside.
Locate the CMOS assembly in the main battery pack bay or memory compartment (refer to your user's manual or assembly guide if necessary). Thinkpad CMOS batteries are typically covered in a yellow rubber-like material, and are housed in a black plastic guide.
Remove the screw that secures the plastic guide, then lift the CMOS assembly up slightly on the rear end. Pull the assembly away from the laptop, keeping it parallel to the laptop case.
Hold the new CMOS assembly parallel to the laptop case, then plug the battery connector in, making sure it is securely connected to the motherboard. Push down on the rear end of the plastic guide, then replace the screw.
Replace the main battery pack or memory cover, then plug in the laptop. Restart the computer, then enter the BIOS (usually by pressing the "F1" or F3" key). Reset your system time and date when prompted, then save the settings.
Depending on your Thinkpad model, you may be able to reuse the plastic CMOS housing and lead wires, by soldering them to a standard CR2025 lithium cell. Only use this method if you cannot find a genuine IBM CMOS battery assembly for your particular model.