In order to rebuild an 18-volt battery pack, remove all of the fasteners holding the case together. Once the case is apart, note the physical configuration of the battery and the configuration of the connectors between the positive and negative of each battery. Test the individual cells of the pack, determining which cells need replacement. After locating the bad cells, remove them and solder fresh cells in place. Reassemble the case.
Rebuilding an 18-volt NiCd battery pack can save a ton of money, and it is not a difficult job. Rebuilding a battery pack does require some electrical skills and knowledge in the use of soldering irons, and it is helpful to understand how to use a multimeter in order to identify bad cells. Remember, batteries are dangerous, and the materials used to manufacture batteries, acid and heavy metals, are hazardous. Always use protective gear, and be aware that improperly wired batteries pose a risk of explosion.
Starting with a freshly charged battery pack, remove all of the fasteners holding the plastic case of the battery pack together. Separate the case and remove the pack of cells. Inspect the individual cells for leaks and corrosion, and using a voltmeter, check the voltage of each individual cell. Anything under 1 volt is bad. Note the configuration of the batteries and the straps that make the electrical connections between the batteries. It is helpful to draw a simple diagram outlining the position of all of the components. Remove the tape that is holding the cells together and carefully separate the cells. Using a sharp tool, such as a chisel, break the spot welds connecting the straps to the cell. Solder the new cells in place, arrange the cells in their original configuration, tape the bundle of cells together and reassemble the pack.