ni-cad batteries stop working because of crystals forming between the plates (im told) this can be fixed by putting an excessive charge through them for a very short time.
what you need is a dc supply much higher then the voltage of the battery, such as an arc welder. there is a sort of technique where you kind of strike at it as if striking a match. maybe 2 or 3 times. the battery must be flat at the start.
obviously this is very dangerous and i therefore dont recommend anyone does it but i have and it did work
here watch this – http://www.youtube.com/watch?v=ENmAJ2GOAMY
edit before the stw pedantic police turn up that is a mig but i only have an arc