When i used brushed in my B4 with lipos i just bought one of those voltage alarms that plugs into the balance connector of the lipo, and so when it got low it would beep. I mounted it on the battery hold down piece and had it plugged in the whole time i ran the car. They are plenty loud enough for you to hear them from the rostrum.
Something like this
Low voltage alarm
Worked a treat and cheap solution.