The recommended supply is 12-15V, with a max safe voltage of 17V.
4s/14.8V lithium polymer (Li-Po) batteries come off the charger at approximately 16.8V. This is absolutely fine.
The system will function with reduced speed/torque down to approximately 8V.
At 5~6V the electronics will remain active but the motors won't attempt to move.
If subjected to a voltage over ~17.5V, the system will attempt to safely block it. It will also block reversed polarity voltages.