li-po for any set-up using a voltage alarm
as long as the esc you are using can handle the power,(a 30 amp esc equals 15c in li-po output)which most can, you are in luck. i run 2 and 3 cell li-po batteries in my vehicles. the issue with li-po is that they need to be stopped at: 6v for 2 cell and 9v for 3 cell. li-po esc's cut the motor off for you when the voltages drop. this system will activate a device of your choice to notify you when you need to bring your car in for charging. i use a buzzer.
the battery power connects directly to the buzzer and to one coil post. the negative runs to the relay( to the com) and to the center wire of a trim pot. you can find the relay at radio shack, use the 12v. the trim pot you can either buy, or take an old servo apart and use the one inside(that's what i do).either side of the trim pot can be used, connect one wire to the other coil post. and finally connect the buzzer negative to the n/c post.
attach the unit to the battery, also clip on a voltmeter. if the buzzer starts and continues, turn the trim all the way to the side that makes it stop.( mark this setting as start) record the battery voltage. now turn on your model and run it to full throttle, only long enough to record the battery voltage.
example: battery is 8.3v, while running it is 8.0. this would be a 2 cell li-po. the cutoff is 6v for 2 cell. 8v minus 6v is 2v. the 12v relay unlatches at 2.3v. you will notice that as you turn the trim, you are controlling the voltage. take the base of 2.3v and add the difference of 2v, and set the trim at 4.3v. to see this voltage change, connect the positive lead to the battery, and the negative to the trim out to coil wire.
when the battery voltage drops to around 6.3v, the buzzer comes on giving time to bring the car back. the same idea is used for three cell. mark a start and set point for each battery. you either need to use a knob, or the servo gear because the trim is sensitive.
solder the unit to the esc connector incoming side. connect the battery, turn the trim to start, then to set. try it out.