If your drill is no longer holding a charge, it can be difficult to get the jobs done that you need to finish. There could be several reasons for the lack of power. It's possible that the battery charger has gone bad. However, you won't know for sure until you do some basic troubleshooting steps. The problem could also be in the battery pack or in the connections between the drill's battery pack and the charger.
Plug the battery charger into an outlet. Use the Volt-Ohm meter to test that power is reaching the contacts for the battery. If not, try another outlet that you know is working. If there is still no power, the battery charger is bad. If there is power, proceed to the next step.
Check for any debris around the contacts on the charger and on the battery pack. Remove it with the compressed air canister.
Plug the battery charger and battery in and attempt to recharge. If the battery charges, the problem is resolved. If the battery does not charge, the problem may be in the battery pack.
Open the battery pack with the Phillips or flathead screwdriver. Remove all pieces and set any springs or plastic casing aside. Use the Volt-Ohm meter to test whether the battery pack has a charge. If it does, the charger is working fine. If it doesn't, you have bad battery cells.
Sketch the existing battery cell configuration with the pencil and paper, noting how the positive and negative ends of the cells are connected to each other. Disassemble the battery back with the wire cutters, reserving the thermostat wire, the cardboard layers and the connector.
Recreate the cell bundle by taping the new cells into the correct configuration with the paper tape. Use the needle-nosed pliers to help you adjust the position of the battery-connection straps.
Put on your gloves and glasses. Starting with the bottom layer, solder the battery straps to form the connections you noted in your diagram. Repeat for additional layers, being sure to solder the connections between the layers as well.
Insert the connector and thermostat wire into the new battery pack, taping them in place. Solder the connector as indicated in your diagram. Check that you have a charge with the Volt-Ohm meter. If not, double check your soldered connections.
Reassemble your battery pack and insert it into the charger. Wait overnight and test the battery pack in your drill. If you have full power, your problem is solved. If not, replace the battery charger.
Do not attempt to install the new cells into the battery pack unless you have experience with electric soldering.