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Yes, you can have them rebuilt. It’s a pretty simple process as, was already mentioned, Festool uses the same cells as everyone else does. In the older NiCd or NiMh they use standard Sub-C cells which are around $1.75 – 2.50 per cell. Each cell is 1.2 volts so the in my older CDD’s there are 10 cells in the battery. They’re wired in series (+ > -) so the voltage adds.
The difficulty of doing it yourself is that the best way of connecting the cells is to capitative weld them and that requires a special and expensive machine. Unless you’re doing a lot of these sort of things it’s not very cost effective to buy one as even the cheap ones are $200-500. You will read that you can scuff the cells and solder them but it’s a very poor method as it’s not very secure or sturdy and worst of all solder requires heat to be applied longer to the cell and that can damage the cell.
I was planning on buying one of these welders as I’ve had uses for this before but when I added up the cost of the cells and the cost of just dropping the battery off at Batteries Plus, a local company that offers to re-cell the battery, it worked out to about $10-15 more than just buying the batteries. So for $40 I am having the batteries rebuilt and as a bonus the cells will have a higher capacity than they did new due to the march of technology. Also, there’s nothing wrong with NiCds, they’re very well suited to use in power tools. Typically when a battery is failing it’s not the whole thing but one out of the 10-15 cells that has shorted. This creates a real strain on the rest of the pack and the performance suffers. You can replace just that one cell and the pack will spring back to life.
LiOn is certainly a better technology but for $40 to get another 2-3 years I think it makes sense to re-cell the batteries of older drills.
Once I’m back home I’ll pick the packs up, test them and open them up so you (and I) can see the quality of the work and how the batteries are connected and laid out.