RC Model Reviews

Mine doesn't work on a 4-cell battery (measured 5.23V)

Are you taking the voltage from the board or direct from the pack?

I tried with independent 4-cell supplies to the coder and to the FrSky module and measured the voltage at the end of the module wires, while visualising the PPM signal on a scope.
The LED doesn't light and the receiver doesn't respond (Binding had been performed previously on a higher voltage).

Interesting, I know the person who did the mod was using a 5V BEC to regulate the Voltage. That being said he was using it to step down the voltage from a 3S lipo.

Will let FrSky know and see what there response on this one is.

I did more tests tonight.
My module does require higher than 5.6V to start working. It continues to work down to 5.2V, maybe lower, I did not try any further.
Possibly your friend's BEC output is higher than 5.5V?
I would not operate this on 4 cells, even if it does start on a freshly charged pack.
It should be okay on 5 cells, but I would prefer 6.
So far, I tend to trust FrSky's product specifications.

Good information. I wanted to hack a FlySky module to my DX6i so I could use in with my little Syma X1 quadcopter but ran into the voltage issue. The DSM2 transmitter module in the DX6i requires 3.3v while the FlySky module has a 7805 regulator and needs an additional ~3v of headroom to reliably regulate. I'll probably replace the 7805 with a low-dropout equivalent to help conserve power or I might even see if I can get a ~6ish volt to 5v switcher for the FlySky module and another with 3.3v out to replace the 3.3 linear regulator in the DX6i box.

However, feeding the module with 8ish volts will allow me to swap in other modules, including the HK orange DSMX module.

The DX6i feeds out a 3.3v ppm signal and the FlySky module obviously was designed to see a 5v ppm signal. I may want a level shifter to do things right, though the 3.3v signal will theoretically work...