The correct VBAT_SCALE is 135 (default was 131 before). This has been verified with a variable power supply to be within 0.1 volts accurate through the range of 2-12 volts. I have updated the firmware tool but your saved profile will not automatically update.

I've also discovered a bug in MultiWii 2.2 where a change of the VBAT_SCALE in firmware requires a RESET in the MultiWiiConfig GUI before the VBAT_SCALE takes affect. Until you hit RESET the voltage monitoring will continue to be incorrect.

If you are reading a voltage of 0, then you probably haven't wired up the gray VBATT wire from the EZ-Connect Radio harness. This is the 3rd wire counting from the right of the cable. The black GND wire is on the right and there are two gray wires next to it, VBAT and VIN, you want the 3rd one in. See EZ-Connect Wiring System.