On my wireless transmitters I use 3v batteries... I then connect a voltage divider consisting of 2 10k resistors to the battery terminal to ground and read middle... BUT!! I use and internal reference of 1.5v.. (pic16f1825 has a 5 bit DAC) My system needs 1.8V to function so my percentage is 1.8v -> 3.2v. I just read the voltage which should be 1.5v max to 0.9v min.