Lithium battery check with ESP32: voltage divider problem

[Sid]

Hi Renzo,

This is off-topic but I don’t know where else to turn.

I want to monitor the battery voltage of this LoRa node. My understanding is that the Li-ion batteries charge at 4.2v so I’m sending that max voltage through a voltage divider to a (3.3v) ADC pin on the ESP32. Strangely (to me) the ADC is returning 4095 always, no matter what the source voltage is. Can you see what I’m doing wrongly? Thanks.

`int ADCvalue;
const int ADC = 25; // battery voltage pin

void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
delay(500);
pinMode(ADC, INPUT);
pinMode(ADC, INPUT_PULLDOWN);
}

void loop() {
ADCvalue = analogRead(ADC);
Serial.print(ADCvalue); Serial.print(” “);
float batVoltage = ADCvalue * 4.2 / 4096; // get battery voltage
Serial.println(batVoltage);
delay(1000);
}’

• This topic was modified 1 year, 1 month ago by Renzo Mischianti.
• This topic was modified 1 year, 1 month ago by Renzo Mischianti.
• This topic was modified 1 year, 1 month ago by Renzo Mischianti. Reason: Add the image also as attach

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[Renzo Mischianti]

Hi Sid,
for my projects, I use this function

float getBatteryVoltage(){
    //************ Measuring Battery Voltage ***********
    float sample1 = 0;
 
    for (int i = 0; i < 100; i++) {
        sample1 = sample1 + analogRead(A0); //read the voltage from the divider circuit
        delay(2);
    }
    sample1 = sample1 / 100;
    DEBUG_PRINT(F("AnalogRead..."));
    DEBUG_PRINTLN(sample1);
    float batVolt = (sample1 * 3.3 * (BAT_RES_VALUE_VCC + BAT_RES_VALUE_GND) / BAT_RES_VALUE_GND) / 1023;
 
    int bvI = batVolt * 100;
    batVolt = (float)bvI/100;
    return batVolt;
}

Where
BAT_RES_VALUE_VCC is the resistor value of VCC part
BAT_RES_VALUE_GND is the resistor value of GND part
3.3 is the reference voltage of the ESP8266
and 1023 is the divider of the analog pin.

I use 20k and 10k resistor.

I do multiple reads to be sure that the reading is correct without voltage oscillation.
Bye Renzo

[Renzo Mischianti]

But I think it’s not safe to connect the output battery directly to the ESP32. It’s better to use a step-up and connect to the 5v (and use this for the ESP32 and LoRa module).

I usually do this: I use a TP4056 to charge the 18650 battery, a step-up to convert the battery’s voltage to a fixed 5v, and from there, I power both the Arduino and the LoRa device (do not power at 3.3v as it reduces the range).

Here you can find a detail.
Emergency power bank homemade

Bye Renzo

[Sid]

Thank you Renzo, very helpful.

[Author]

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