I tried stripping down 2 packs to make one good one, complete waste of time. Then I tried the trick with the high current supply to break the internal short circuits, worked for the first charge only.
I found this company on the web, who will replace the cells in your pack or sell you the cells for you to do yourself, easy if you have a soldering iron.
http://www.batteriesplus.co.uk/acatalog ... es_26.html
Then I found replacement packs on ebay for less than the price of the individual cells, sourced form China though and probably not very good quality.
I believe the problem with nicad packs, and maybe others, is that when one cell short circuits, many chargers will no longer detect full charge. This then overcharges the other cells if you leave the pack in the charger overnight (or for days). This then causes more shorted cells and more frequent recharges. By the time you realise the problem all the cells are useless and cannot be rejuvenated by any of the suggested methods.
One would hope that more expensive tools have good quality chargers that do more than just supply a fixed current, however I suspect that this is often an area where cost can be saved and the user is none the wiser till the tool no longer works.
The lessons I have learned:
Don't leave batteries on charge for to long
If your charger has charging indicators, make sure that they do go out after the expected charge period. If it stays on it's probably destroying the cells. for Nicads you may be able to use the 12V battery trick mentioned earlier (but wear safely goggles).
Measure the voltage after charging, a single nicad cell will measure about 1.3V fully charged, an 18V pack will measure just over 19V, if it measures less than 18V then it's likely you have at least one shorted cell, it may be possible to recover it if detected early.
If replacing cells yourself, replace them all, don't just change the 0V cells as others will short out and you likely destroy the new cells as well as the remaining old ones.
If you fully discharge your batteries before charging, don't leave them on discharge for long periods, if there are damaged cells it's quite possible that this will apply reverse polarity to some cells, causing further damage.