[werk_ag]

And the other question is how to properly calibrate it to read the correct voltage. Mine reads under by quite a bit so I suspect I need to play around with the resistor values?

I think there is a simpler way...

Indeed the precision of the reading depends mainly of the precision of R4 and R5 resistors, as also the precision of the internal reference of the ADC, which you can't control. Even using 1% resistors they always have some tolerance. However there is a way to calibrate the Bat voltage reading to a very acceptable level, without touching in anything.

-Supposing you have a good precision voltmeter, measure voltage at the +Bat pin.
- Compare the reading with the value given by the Pro2 Plus receiver. They differ by more than 0.1V?
- Then find this line of code on the Pro2 PLUS RX or Pro2 Compact software:

Code:

TX_Unit[TX_UnitID].BatVolt        = (int)(((RX_Data[1] * 20.23) / 1024) * 100.0);

Adjust the value of 20.23, raising or lowering it until you get a Battery Voltage reading as close as possible to the measured with the voltimeter.
Theoretically the value should be 20.00, the value of 20.23 is the one that works better for me, but due to the reasons explained above it may be different for each one.