Nickel-cadmium (NiCad) rechargeable batteries build up a dendrite crystal substance with continual use and recharging. Over time, they hold less of a charge and their battery life noticeably decreases. Eventually, they no longer hold a charge at all. However, you can restore the battery life (also known as battery charge memory) by removing the dendrite crystals with an electric current.
Test the battery with a multimeter to be sure it is completely charged. You will need to connect the clamps on the charger to the terminals on the battery. Be sure you have charged it to its full capacity before testing it with the charger. If the battery has no life left, it will not work after a full night of charging.
Put on safety goggles, a long-sleeved shirt and gloves. Connect the black clamp from the trickle charger to the negative terminal on the battery. Connect the red clamp from the trickle charger to the positive terminal on the battery. Hold the clamps to the battery for one to three seconds only.
Use the multimeter to test the battery charge. If the battery is not fully charged, repeat the process to remove more dendrite crystals. You may have to repeat the process two to three times if the battery is completely dead, but do not hold the trickle charger to the battery for more than a few seconds because it can be extremely dangerous (electrocution can result).
TL;DR (Too Long; Didn't Read)
After your NiCad battery has been restored, it will eventually die after repeated re-charging and need to be restored again.
Wear protective gear and use caution when testing and connecting the battery to the charger. Mishandling can result in electrical shock and severe injury.
Sparks may issue from the clamps connected to the battery terminals. Perform the battery charging in a well-ventilated location with no flammable objects or substances nearby.