I can think of two options here..
One, you could just hook up a decent battery monitor directly to the wiring, that should beep once the lipos dip to the charging point.
Two, you could looking into how rc pilots do it and get a FrSky telemetry system. With this setup you can send battery, gps, altitude, speed, etc.
Freelance Concept Art & Animatronics, 3D Art and 3D Modelling, as well as Physical Modelling, Mechanical design, and illustration.
Create, Innovate, Inspire.