Originally Posted by jasmine2501
If the batteries are exactly the same, and they have drastically different behavior, then one of them is probably worn out. Nicad packs can be 'refreshed' by draining them all the way down and then charging them up again - and I mean all the way down to zero power left.
Actually, that will wreak a Ni-Cad as well. You don't want to drain it to below 1v per cell. The best way to recover them is to do a "deep cycle", which involves draining them almost completely (1-1.2v per cell, Ie, about 8v for a 7-cell battery), rapid charging them for about 20 minutes, and then trickle charge until complete. Repeat 3 times. That should recover most Ni-Cads.
Alternatively, investing in a good charger with NPT charging technology and a built in deep-cycle feature is recommended.