In 1972, Popular Electronics had an article about how to fix NiCad batteries that had developed an internal short and would not take a charge. Last week I had two Dewalt battery packs (6 batteries in each) that wouldn't charge (charger gave the fast blink error). I hauled out my high-current power supply and zapped each cell 20 times. The packs both work fine now.
Note that you have to disassemble the battery pack to zap Ni-Cads because you must zap each individual cell, not the entire pack at once. Also this is not for NiMH cells or Li-Ion cells, only NiCad cells.