I must have at least a half dozen of the DeWalt batteries, but I never thought of rebuilding using cells from one for the other. How do you know which are the bad cells? thanks
You just give the pack a charge and start using the tool while checking the voltage pretty regularly until it starts "collapsing". Like for example an 18V pack might start out at 20 volts straight after charge, but if it's a "flogged" pack it will start dropping pretty fast once you start using it.
So for example I might use it for 10 minutes and see that it's now only 16 or 17 volts or something. Then when you pull it apart all the really weak cells will be zero already, so you mark those ones as bad straight away.
Then you just leave it for a week or so and measure the cells again, the ones that are still showing good readings at this stage are probably ok, so mark those with a pen for reuse.
Make sure however that you leave the tabs attached to the good reusable cells (you can just rip them from the dead cells with long nose pliers). This is important because you can't solder directly to the battery terminals, so you need a spot welder if you lose the tabs.
Where two adjacent cells are both good and you want to separate them, you can split the tab between them with a sharp pair of tin snips. Half a tab is enough to solder to, though I find it works best to slice them at an angle (so that both halves are trapezoid rather than rectangular) to get the easiest join when reusing.
BTW. If you've got two flogged packs then usually one is flogged worse than the other. I find it works best to rebuild the worst pack with new cells and patch up the other.
