Lithium battery check with ESP32: voltage divider problem

Tagged: ai tools

This topic has 3 replies, 2 voices, and was last updated 1 year, 1 month ago by Sid.

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- 5 March 2024 at 03:24 #29890
  Sid
  Participant
  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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- 7 March 2024 at 15:23 #29925
  Renzo Mischianti
  Keymaster
  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
- 7 March 2024 at 15:33 #29926
  Renzo Mischianti
  Keymaster
  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
  - 8 March 2024 at 18:47 #29934
    Sid
    Participant
    Thank you Renzo, very helpful.
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