Here I’d like to share a simple Lipo voltage monitoring setup, that I’m using in my Bixler with an FrSky D8R-XP receiver. No sensors or other electronics are required - only a single servo cable and two resistors, total value of about 50c + some soldering.
The FrSky D-series receivers have a great feature: the analogue telemetry inputs, called A1 and A2. On some receivers both are freely accessible (D8R-XP, D8R-Plus), on others the A1 port is hardwired to the internal receiver voltage, but the A2 port is always accessible for external connections (D4R-II). On those ports, a voltage of up to 3.3V can be connected. It is measured internally (0-3.3V = 0-100%) and transmitted via the telemetry uplink to your transmitter. Using a simple voltage divider soldered from two resistors, one can monitor the main battery voltage of a model (or any other voltage).