Remove the cover from your 14.4-volt NiCad battery pack. Use a small screwdriver to unscrew the screws, but keep them safe, as they are small and easy to lose. Put a hand on either side of the cover and lift it off. Put it to one side. If the battery is a sealed unit, don't attempt to force open the cover. It could be dangerous, and it's unlikely you'll be able to refit the cover.
Check the type of NiCad cells in the battery pack. The size and type are labeled on the battery cell. However, to ensure you get exact replacements, take the complete pack to the electrical store. You need 12 battery cells, as a 14.4-volt NiCad battery pack consists of 12 cells, each producing 1.2 volts.
Check how the cells are fitted into the battery pack. Some clip in, so use a screwdriver and your fingers to pry out the 12 NiCad cells from the battery pack. Others may be soldered in place. In that case, heat a soldering iron. Touch a cell terminal using the soldering iron and let the solder on the terminal and connector melt. Remove the soldering iron, and slide the screwdriver between the battery cell terminal and the connector before the solder solidifies. Lever the battery upward, then carefully remove it, using your fingers. Repeat the process until all the cells are removed. Turn off the soldering iron.
Put the 12 NiCad cells you've removed from the battery pack into a clear bag. You need to dispose of the battery cells in a battery recycling unit once you've completed the task of rebuilding your battery pack. Check out methods of disposal using the link in the resources section.
Put the 12 replacement cells into the battery pack compartment. Use your fingers to push each one into place. If they need soldering, heat a soldering iron. Touch the end of the soldering iron on the terminal and connector then introduce the solder. Let a little solder melt so it covers the terminal and the connector, then remove the soldering iron and solder. Allow each terminal to cool and the molten solder to harden. Repeat the process on each cell until all the cells are secure.
Replace the battery cover. Place the screws back in the holes and tighten them, using a screwdriver.