In nicad and nimh batteries crystals form to shorten life or kill a battery completely. They can be revived with a jolt of higher DC voltage and amperage.
I was given a very nice makita drill kit. The neighbor said both batteries are dead and they cost more than a new drill. I tried charging them and they would trip the circuit breaker in the charger and read zero volts. I jolted them with my truck battery and now they both charge to 11 volts and perform as new. Okay, more than a jolt, more like 2 minutes and the battery got only slightly warm. I'm sure battery size and jolt size has correlation. On smaller batteries, a battery charger might waken them.
I highly recommend goggles and long sleeves in case of explosion.
