On the 18MZ you can even record a voice message that says whatever you want like"Battery low do not fly" and it will do it as soon as voltage triggers the threshold, but as voltage will drop on some conditions during flight I do not think it will be a good idea to have it that way.

The Rx sends two voltages one will be the one feeded to the Rx the other the external voltage wich uses the micro connectors used by all the Horizon hobby micro batteries, HH sells te conector with wires.