I have a 6 cell side by side GP4300. This bad boy used to be great till I had a little accident that caused the battery to be ejected and one of the battery cells vented. I replaced that bad cell, but evidently one of my other cells is bad also. After a charge the pack doesn't have the punch it used to and only lasts about 6 minutes (used to last 15). I can't visibly tell which one it is (could be more than one?). How should I go about checking to see which cell is bad? I have a multimeter that tells me that they all have roughly the same voltage when not under load. Any ideas would be appreciated.