I have a pixhawk cube,
I have connected pixhawk via supplied power module, in order to get the voltage telemetry I did following:
In initial setup section, > battery monitor>
Monitor: voltage and current (also tried Battery volts)
Sensor:(3DR power module/other)
APM Ver: cube or pixhawk
I am getting 0 At measured battery voltage
And also 0 at flight data screen.