Reading the battery voltage of the supply battery

Official Hardware MKR Family MKR WAN 1300
11

try this:

float battery_voltage = 3.01; // use voltmeter

analogReadResolution(10);
  analogReference(AR_INTERNAL1V0); //AR_DEFAULT: the default analog reference of 3.3V // 
AR_INTERNAL1V0: a built-in 1.0V reference
  // read the input on analog pin 0:
  int sensorValue = analogRead(ADC_BATTERY);
  // Convert the analog reading (which goes from 0 - 1023) 
  float voltage = sensorValue * (battery_voltage / 1023.0);
  // print out the value you read:
  Serial.print("Voltage: ");
  Serial.print(voltage);
  Serial.println("V");

  float battery_percentage = ((voltage * 100) / battery_voltage);

  Serial.print("Batery Percentage: ");
  Serial.print(battery_percentage);
  Serial.println("%");

  analogReference(AR_DEFAULT);