When you say main voltage, what is reading the volt? Are you reading it from the flute controller or from the OSD.
What voltage dividers are you using? The voltage divider takes the 14.1 volts from a 3 cell (or how ever many cells you want to use) and then divides it down to 5Volt that the OSD or FC can read, they can not read above 5V. Then you apply parameters in the OSD to cover the under 5V ready to the correct values.
Have a read of these: They are advanced uses of the voltage reading but you will get the point.