Most cordless tools are powered by rechargeable nickel cadmium (NiCad) batteries. The storage capacity of any battery technology will degrade over time, though NiCad batteries can be repaired at home. There are methods to extend the lifespan of NiCad batteries, but it's better to rebuild a damaged battery with upgraded power cells.
NiCad batteries have an estimated lifespan of around 250 recharge cycles. If a NiCad battery is frequently recharged before it is completely drained, it can succumb to a "memory effect": the battery can no longer hold a full charge. In some cases, these batteries can be rejuvenated. Charge the battery fully and then completely discharge it several times. Crystal growth within a NiCad battery can also cause an internal short. These crystals can be shattered by placing the batteries in a freezer overnight or by a sharp rap with a hammer. It's also possible to break these crystals by "zapping" a charged battery with a source of electricity, such as a 12-volt automotive battery. Identify the positive and negative terminals of your cordless tool's battery with a multimeter and put on protective gloves, glasses and a facial shield. Use jump leads to run current through the battery for a 10th of a second. If the battery remains dead, it will need to be rebuilt.
Although brands of tools have differently shaped batteries, common industrial power cells known as "sub-C" type batteries power the tool. Open the battery pack. Inside you'll see a number of smaller batteries wired together. A single dead cell will prevent the entire pack from functioning, so check each cell with a multimeter. You may replace a single cell, but it's wiser to upgrade the entire pack to cells with a higher milliamp-hour (mAh) rating for longer battery life.
Don't replace NiCad cells with more modern Nickel Metal Hydride (NiMH) cells: the tool won't charge properly. Scratch any oxidation from the terminals of the cells and apply a thin coat of solder to maximise electrical conduction. Solder the cells together with their attached tabs or commercially-available battery bars. Reuse wiring and fuses and maintain the same sequence as the original pack to maintain the proper voltage.
Test your rebuilt pack with a multimeter to ensure it's providing proper current for your tool before reassembling the battery pack. Because the spent cells contain heavy metals, recycle them rather than throwing them away.