If you are only racing for 5 minutes, the chances are you won't come close to the low voltage, especially if you are running 2wd.
However, these are available:
http://www.cmldistribution.co.uk/cml...tId=0000006331
They solder to the battery side of the ESC and plug in line with the reciever and esc signal wires that would normally be plugged into the esc (hence the 2 receiver wires, one is a plug, one is a socket).
When the voltage gets too low, they pulse the throttle and flash an led. If the voltage gets really low, the cut the throttle completely.
At around a third of the cost of a reasonable esc whether it's worth it is up to you.