I added a seventh battery in series to my '92 Marathon. By moving the controller/solenoid mount back there was room for it in the center. I am using a Alltrax AXE 4844 controller and a Delta QuiQ 48 charger set for 42v. My 36v speed was 10 mph and on 42v it is 15 mph. I kept the solenoid activation coil on 36v.