If your cordless drill battery is no longer working, you don't have to throw out the drill. Instead, you can repair the battery pack by replacing the cells that comprise the cordless battery. However, you must have experience and knowledge about electrical soldering, including knowing how to identify and join the "entrance" and "exit" straps on the battery cells and to solder them using a small amount of solder. If you do not have this knowledge or experience, you will not be able to do this on your own. Instead, you will need to contact your drill's manufacturer to determine if a replacement battery pack is available.
Put on your protective glasses. Unscrew and open the battery pack. Remove all the cells and the springs and any other pieces from the plastic casing and set them aside.
Note the position of each cell in the pack and observe how they are connected to each other. Locate the connector and determine which cells are connected to the cathode (negative) and anode (positive) sides of the connector. Diagram the connections between cells and to the connector to help you recreate them.
Disassemble the layers of the old cell bundle so you can recover the cardboard inserts between them, and so you can locate the correct pathway for the thermostat. Carefully cut the old cells away from the connector.
Take the new cells and begin to connect them, taping them together with paper tape to help form the bundle correctly. Be sure you position the cells so that the straps are where they need to be to be able to solder the cells together according to the sketch you made.
Once all the cells in the bottom layer are taped together, solder the connections. Replace the cardboard insert, making sure you thread the thermostat through as needed. Repeat step 4 with the second layer of cells. Solder the connection between the two cells that are connecting the first and second layer.
Fit the connector in place on the last layer and tape it in place. Solder any remaining connections between the cells and then solder the last two cell straps to the connector according to the sketch you made of the old battery pack.
Use the Volt-Ohm meter to test that the newly assembled battery pack is holding a charge. If not, recheck all of your soldered connections.
Reassemble the battery pack in the reverse of how you took it apart. Screw the plastic casing back together.
Do not attempt this project if you are not familiar with electric soldering.