The ignition LED on the new ignition modules will flash indicating a low battery voltage. While the ignition is rated to run at 6.0 volts, the module will not allow the engine to start unless the battery voltage is at or above 6.5 volts. (Typical full charge on a 5 cell pack is 6.7 - 6.8 volts). The reliable, and lighter answer, is to use a 2S lipo pack.

Pete