[jhautz]

Bad juju to combine old and new cells. The old cells are not as powerful as the new cells and will hit the low voltage cut off before the new cells except the new cells will keep the pack voltage higher and you will over discharge the old cells. Plus every time you charge you will likely have a pack that is pretty far out of balance.

My recomentation would be to turn your old pack into a smaller pack by removing thr bad cell/cells and get yourself a new pack AND a charger that can monitor all cells individually while it charges. Most modern chargers would have avoded the problem you are describing.