Back in the eighties it was not usual to MCU-s with ADC-s to the RC transmitters which would have made the low voltage alarms easy. However the Robbe engineers used a small galvanometer to display the battery voltage:

I have decided to repurpose this. I have connected it to a PSU and checked the voltage levels corresponding to the notable poisitions: full scale, red area start:
- RED start: 1.3V
- Full scale: 2.37V