According to Terry at C-H, the minimum recommended battery voltage is 4.8. That's pretty much the desired rated voltage for a given amp hour sized nicad battery pack. They are reputed to draw approximately 400 mAh per flight hour on a single cylinder engine. Very recent independant scientific lab testing noted that the above works very well and that the C-H system is extremely reliable for extremely long periods of time. Further specifics regarding the testing cannot be provided due to non-disclosure agreements.

More voltage input will have little additional benefit.