There is no power available to be harvested below 6MPH, so the blades should be turning the motor at slightly less than cut in voltage at 6MPH.
I try to make mine start charging at 7MPH, or even a bit faster, and when I stopped chasing getting power from 6MPH wind, my results improved.
That's awful fast for 12V charging, but not impossible.
It is more efficient to charge all the batteries at the same time, in the same bank.
And the changing the batteries around will become a chore.
Considering the total effort and cost expended, I would recommend looking for a more suitable motor.
G-