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.