In order to have an accurate battery measurement you need to compare the battery voltage to a reference voltage.
The MKRFOX1200 has an internal reference voltage of 1V.
The ADC_Battery pin is connected to a resistor network that divides the battery voltage. So in case the battery voltage is 3.3V, the voltage at battery equals 1V. You need to tell the ADC to compare the ADC_Battery with the internal reference voltage of 1V. I believe this can be done using the analogReference() instruction.