The batteries on cordless power tools are usually the first part to fail. If you've ever priced a replacement battery pack, you'll know that it is probably not a cost-effective purchase. Most people would rather buy a new tool than spend over half the price of a new tool on a replacement battery pack. The best option might be to rebuild the batteries, but only if you have some experience using a soldering iron.
Remove the battery pack from your cordless tool. Unscrew the screws holding the battery case together. Open the battery case. Remove the battery cells--they will all be wired together as a unit.
Draw a rough diagram of the battery cells on paper, noting the negative (-) and positive (+) terminals of each battery cell, and how each terminal is connected with wires.
Remove the connecting wires by holding them with pliers while melting the solder that holds the wire to the battery terminals, using a soldering iron.
Align the new battery cells correctly, according to your diagram. Solder all the connecting wires to the new battery cell terminals, according to your diagram. Work quickly so as not to overheat the batteries.
Slide the battery cells into one half of the battery case. Replace the other half of the cover. Fasten the screws that hold the case together, using a screwdriver. Charge the battery pack overnight before using it with your cordless power tool.
When purchasing new battery cells, give the make and model number of your cordless tool to the battery provider, and they will give you the correct cells for your tool.