I've tested mine at 6 volts input from a 4 cell alkaline battery pack and it worked fine...might be able to go up to 9 without too much heating of the transistor...but if you go that high, you might need a voltage regulator...not sure. I am in the process of using a LM317T for that purpose.
My bench supply uses one and I turned up the voltage to 8 volts for a few seconds and got alot of heat from the transistor. You have to consider that the 1n4007 on the timer board (original design) takes about .5 volts away from the input voltage..so at a fully charged nicad pack you'd have 5.3 - 5.5 volts ending up with about 5 volts.. and at that voltage, I still got almost 2 hours run time. Spark was great though either way.

John