Measure battery with adc internal

[quangminh.le0701]

my circuit to measure battery right now is esp32, nrf24l01, and 18650. i powered up esp32 with 18650 3.6v norminal/4.2 max battery. run a voltage divider 100k, 27k then it's 0.893v a measure point. at 3.6v measure point like ~0.72.

i use internal 1.1v adc but why i only receive ~3v at monitor

Code: Select all

adc1_config_width(ADC_WIDTH_BIT_11);
adc1_config_channel_atten(ADC1_CHANNEL_0,ADC_ATTEN_DB_0);
float battery_read()
{
    //read battery voltage per %
    long sum = 0;                  // sum of samples taken
    float voltage = 0.0;           // calculated voltage
    float output = 0.0;            //output value
    const float battery_max = 4.2; //maximum voltage of battery
    const float battery_min = 3.0; //minimum voltage of battery before shutdown

    float R1 = 100000.0; // resistance of R1 (100K)
    float R2 = 27000.0;  // resistance of R2 (10K)

    for (int i = 0; i < 100; i++)
    {
        sum += adc1_get_voltage(ADC1_CHANNEL_0);
        delayMicroseconds(1000);
    }
    // calculate the voltage*/
    
    voltage = sum / (float)100;
    Serial.println(voltage);
    voltage = (voltage *1.1) / 2047.0; //for internal 1.1v reference
    // use if added divider circuit
    voltage = voltage / (R2/(R1+R2));
}

My circuit here