RAK811 ADC value stuck

Hello:
I developed an application derived from RAK RUI V2.0 RAK7204, powered by 2xAA batteries.

To read battery voltage, a resistor divider was connected to PIN 20 (PA2).

As the batteries start to discharge, the measured voltage remains the same!. Tried to read de adc value using at+command, but the results is the same: the voltage doesn’t change even when the batteries’ voltage discharged around 0.4V

There is a possibility that RAK RUI is using VCC pin as VREF instead of Internal ref?

ADC initialization and reading functions were copied exactly from the example. just take a look from my repo: