@ejlane @eiten problem is that before any calculations sensorValue from analogRead is 127. So if I use 127 in any formula it's doesn't make and reasonable value.

1000000/(1000000+470000)*127 = 86,3945578231

I took the formula from https://www.mysensors.org/build/battery#external-measurement. R1 and R2 names (not values) are swapped in my schematic

Btw, board was meant to be power by 1,2,3 or 4 AA batteries. It's powered by TPS61025DRCR. But I am not sure I did the power management correctly.

Hi,
I've built my very first PCB with atmega328p-au. Everything works fine (MCU, sensor, radio) except battery measurement. It always give me 0.4V regardless of what battery is connected (1,5V 1xAA or 3V 2xAA).

const float VOLTAGE_DIVIDER_R1 = 1e6;
const float VOLTAGE_DIVIDER_R2 = 470e3;
uint8_t BATTERY_SENSE_PIN = A1;

void setup() {
  // use the 1.1 V internal reference
#if defined(__AVR_ATmega2560__)
  analogReference(INTERNAL1V1);
#else
  analogReference(INTERNAL);
#endif
}
...
// gives me 127
int sensorValue = analogRead(BATTERY_SENSE_PIN);


// 3.440426e+00
const float voltageDivider = ((VOLTAGE_DIVIDER_R1+VOLTAGE_DIVIDER_R2)/VOLTAGE_DIVIDER_R2)*1.1;

// 3.363075e-03
const float voltsPerBit = voltageDivider/1023;

// 0.43 V
float batteryV = sensorValue * voltsPerBit; // calculated

R1 = 470K
R2 = 1M
C3 = 100nF
C3 is connected to A1

Any idea what could be wrong?