Documentation

// CN3791 Battery Voltage Monitor - Arduino Uno / Nano
// Wire BAT terminal to A0 through a 100k/100k divider to GND.

const int BAT_PIN = A0;
const float VREF = 5.0;          // Uno/Nano ADC reference
const float DIVIDER_RATIO = 2.0; // 100k + 100k -> halves the voltage
const float LOW_BAT_THRESHOLD = 3.3; // volts

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  int raw = analogRead(BAT_PIN);
  float vPin = (raw / 1023.0) * VREF;
  float vBat = vPin * DIVIDER_RATIO;

  Serial.print("Battery: ");
  Serial.print(vBat, 2);
  Serial.println(" V");

  if (vBat < LOW_BAT_THRESHOLD) {
    // Blink rapidly as low-battery warning
    digitalWrite(LED_BUILTIN, HIGH);
    delay(100);
    digitalWrite(LED_BUILTIN, LOW);
    delay(100);
  } else {
    digitalWrite(LED_BUILTIN, LOW);
    delay(1000);
  }
}

// CN3791 + ESP32 - reads battery, sleeps if low.
// Wire BAT through 100k/100k divider to GPIO34.

const int BAT_PIN = 34;
const float VREF = 3.3;
const float DIVIDER_RATIO = 2.0;
const float LOW_BAT_SLEEP = 3.2; // volts - go to sleep below this
const uint64_t SLEEP_US = 60ULL * 60ULL * 1000000ULL; // 1 hour

float readBatteryVoltage() {
  // Average 16 samples for stability
  uint32_t sum = 0;
  for (int i = 0; i < 16; i++) {
    sum += analogRead(BAT_PIN);
    delay(2);
  }
  float raw = sum / 16.0;
  float vPin = (raw / 4095.0) * VREF;
  return vPin * DIVIDER_RATIO;
}

void setup() {
  Serial.begin(115200);
  delay(200);

  float vBat = readBatteryVoltage();
  Serial.printf("Battery: %.2f V\n", vBat);

  if (vBat < LOW_BAT_SLEEP) {
    Serial.println("Low battery - deep sleep 1h");
    esp_sleep_enable_timer_wakeup(SLEEP_US);
    esp_deep_sleep_start();
  }

  // Otherwise do normal work: read sensors, post data, etc.
  Serial.println("Battery OK - running normally");
}

void loop() {
  delay(5000);
}

# CN3791 + Raspberry Pi Pico - MicroPython
# Wire BAT through 100k/100k divider to GP26 (ADC0).

from machine import ADC, Pin
import time

bat_adc = ADC(Pin(26))
led = Pin(25, Pin.OUT)

VREF = 3.3
DIVIDER_RATIO = 2.0
LOW_BAT = 3.3

def read_battery():
    # 16-bit ADC on Pico
    raw = bat_adc.read_u16()
    v_pin = (raw / 65535) * VREF
    return v_pin * DIVIDER_RATIO

while True:
    v = read_battery()
    print("Battery: {:.2f} V".format(v))

    if v < LOW_BAT:
        for _ in range(5):
            led.toggle()
            time.sleep(0.1)
    else:
        led.value(0)

    time.sleep(2)