Marius,
STEP1: Use the Keyfob first. Look at the batteryService.c profile to see how they are measuring the voltage. The Keyfob has P_06 and P_07 available. Test out your code on that before you attempt to do anything with a custom board.
STEP 2: Read the sections of the HAL that deal with the ADC and the CC254x users guide that I mentioned in the first post that covers the ADC. AVDD is the analog voltage. The person who designed the board would have to know that, unless of coarse it was left blank...
It will save you a lot of waisted time. You are playing with too many variables here. If you can at least be sure your code works on a proven and working device like the Keyfob, then you will be able to determine if your new hardware is working correctly.