Did someone put a supercap in parallel whit the battery?
I have some intresting stuff to show you...
how to do....giving many benefit
at first have back more power whit a best is possible regenerative braking energy (encreasing the mileage too)
second, handling power peak from regenerative and successive launch assist whitout stress for a battery pack, then give a very long life to battery.
http://www.slidefinder.net/p/presentation_sc_ultima/32833045