The Thinkpad backup battery
the "clock battery" as most people know it by keeps all CMOS Settings intact when the laptop is without direct power to the motherboard, such as when turned off.
Most often these batteries last about 5-7 years before needing changing, they are of the CR2032 or the CR2025 or equivalent battery. they are lithium ion, the same material the power batteries are made of, but they cannot be recharged.
How to replace the thinkpad battery by yourself.
This tutorial is recommended to be preformed by persons who are capable of handling sharp objects carefully.
Disclaimer: Thinkwiki nor the writer of this article can be held responsible for the following of this tutorial. Take care when removing the welded tabs, you may injure yourself. Danger of explosion is present if the battery is installed incorrectly. Dispose of the battery according to your local disposal laws, failure to dispose of properly may cause injury to children or pets.
An OEM Replacement battery will run you $12+, but you can save a bit of money by preforming the following modifications.
This article assumes you know how to remove the battery from its compartment and have prior knowledge on the dangers of lithium batteries.
Thick gloves made of fabric or thick rubber, most people would have this around the home anyway.
A box cutter (exacto knife) or a craft knife, also as above, most dollar stores have them also.
Masking tape or electrical tape, as above.
CR2302 or CR2305 (Recommended to use CR2302, because they last a year or two longer than CR2305.) these batteries can be purchased from some dollar stores, or places like radio shack or the source or circuit city for $1-$7
A plastic bag, a Ziplock(R) bag works just fine.
First, put the gloves on your hands, this will protect them if you slip with the knife.
Second, you should take the exacto knife and remove the heat shrink plastic around the battery, which will be yellow white or black or another color, careful not to be too rough otherwise you may knick the battery and allow lithium to escape.
Third, you have to remove the tabs from the dead battery, this is the hard part. hold the battery on a surface you can knick (It is recommended to use a cutting board for this purpose.) Hold the battery upright, grasped between your thumb and index finger. slip the exacto knife down the loose edge of the + side tab (red), and move the knife back and forth until you break the welds, BE CAREFUL NOT TO KNICK THE BATTERY OR BREAK THE WIRE!. Repeat for the other side.
Fourth, Now that you have removed the tabs, flatten them out and place them on the new battery, important note, although your assumption that the nubbed end of a battery is positive, its reversed for these types of batteries. so red goes onto the flat shiny part with the writing on it. the black tab goes on the other side, now apply tape to the sides, making sure that it is tight and no metal is showing. you may put the battery back into your thinkpad.
Fifth, Wrap the old battery in some tape also and place in a plastic bag and dispose of according to local law.
Doing this enables you to replace the battery with ease next time it runs out, since you just need to repeat steps 4 and 5.