Originally Posted by trailranger

Delta-Peak chargers know when to stop charging the cells even when charging at a fast rate. There is minimal difference in total capacity in the cells when charging at 1C or charging at up to 6amps. There will be lower voltage under load when charging at low C rates. Only charge at high amps if the cells are in good working order.

Since you said that you have 3300 packs, I would have to assume these are old cells or stick packs which would limit the charge rate becuase of the chances of one of the cells not being fully functional. I suspect the pack that only charges for 20min has a dead cell or the remaining capacities in each cell vary greatly.

If you can tray your pack on an equalizer before charging your packs would not false peack as much and should take a full charge. If you can not discharge on a tray, discharge the pack untill the no load voltage is 5.4V
peak charge at 4A. Once peak allow the pack to trickle charge @ 100mA untill the total MAH reaches 3600~4000. This is about the only way to equalize a sealed pack.