The batteries in the ZZMT are meant to be directly charged. The resistance in the batteries is what causes the current to flow and charge them.

Originally, remember that 4 cells were being used to charge 2. Charging 4 cells with 5 is going to significantly reduce the time it takes to charge the batteries.

To have equal time, you would need to use 8 cells to charge 4, though I wouldn't recomend it. Personally, I would use the switch idea and triple charge the 4 cells when they are in 2S2P configuration or charge with atleast 6 cells for probably around 75+ seconds.