ok let me clearify something for you all...
this is about the only thing i learned about the robots electronics while programming

8 is correct number for the voltage that the robot controller will stop 'working'
the problem with this statement though is that its not the robot controller that stops working... its the radio controller (ur saying rc but that can stand for robot controller or radio controller so i dont know which ur talking bout) that stops working... when voltage drops below 8 volts the radio controller will lose signal... when the robot controller sees this it will tell you that there is no signal. not sure exactly why you guys bring this up... from what i remember the basic run error will not come on at this point (most likely anyways) you should instead see the no data/radio signal light on. if you do have a problem with power just let me know if i can help... my team has 6 electrical guys which can answer any questions you might have on the electrical workings of the robot controller.