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 jumper cables 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 maximize 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.