Trying to measure individual voltages of two battery cells connected in series using voltage sensors and Arduino

Question

I am trying to use two voltage sensors connected to an Arduino Uno to measure the individual voltages of two battery cells connected in series.

When I connect a sensor with only one cell, it gives the individual voltage correctly. But when I connect both sensors to their individual cells, I get 0 V reading from one sensor and the combined series voltage of the two cells on the other. I have tried swapping the voltage sensors and batteries but the same thing happens.

Is there a way to measure the individual cell voltages without getting the series voltage? Diagram :

Here is the code that I am using:

#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address

const int voltageSensor1Pin = A3;          // sensor pin
float vIn1;                                // measured voltage (3.3V = max. 16.5V, 5V = max 25V)
float vOut1;
float voltageSensor1Val;                   // value on pin A3 (0 - 1023)
const int voltageSensor2Pin = A2;          // sensor pin
float vIn2;                                // measured voltage (3.3V = max. 16.5V, 5V = max 25V)
float vOut2;
float voltageSensor2Val;                
const float factor = 5.128;               // reduction factor of the Voltage Sensor shield
const float vCC = 5.00;                   // Arduino input voltage (measurable by voltmeter)

void setup() {
  pinMode(8, OUTPUT);    // sets the digital pin 13 as output
  pinMode(12, OUTPUT);
  lcd.begin(20,4); 
  lcd.setCursor(0,0);
  lcd.print("Voltage Measurement");
  lcd.setCursor(0,1);
  lcd.print("CELL1: ");
  lcd.setCursor(0,2);
  lcd.print("CELL2: ");
  lcd.setCursor(8,3);
  lcd.print("****");

  Serial.begin(9600);
}

void loop() {
  voltageSensor1Val = analogRead(voltageSensor1Pin);    // read the current sensor value (0 - 1023) 
  vOut1 = (voltageSensor1Val / 1024) * vCC;             // convert the value to the real voltage on the analog pin
  vIn1 =  vOut1 * factor;                               // convert the voltage on the source by multiplying with the factor

  voltageSensor2Val = analogRead(voltageSensor2Pin);    // read the current sensor value (0 - 1023) 
  vOut2 = (voltageSensor2Val / 1024) * vCC;             // convert the value to the real voltage on the analog pin
  vIn2 =  vOut2 * factor;          
  
  Serial.print("Cell 1 Voltage = ");             
  Serial.print(vIn1);
  Serial.println("V");

  lcd.setCursor(8,1);
  lcd.print(vIn1);
  lcd.setCursor(12,1);
  lcd.print("V");

  Serial.print("Cell 2 Voltage = ");             
  Serial.print(vIn2);
  Serial.println("V");

  lcd.setCursor(8,2);
  lcd.print(vIn2);
  lcd.setCursor(12,2);
  lcd.print("V");

  delay(1000);
}

Answer 1

It is possible to measure individual battery cells. According to the circuit diagram, the GND signal of the upper module should not be connected to the Aurdino.

First, measure the voltage of the lower battery cell based on 720mV (720mV > 3.6V). The value calculated in this way (3.6V) must be subtracted from the second measured value (4.34-3.6= 740mV). Calculate the voltage of the second battery cell based on 740mV (740mV > 3.7V).

Answer 2

Use the negative side as your reference for both sensors. Then measure the first battery, that will give you battery 1 voltage. Then the second, that will give you B1 + B2. Subtract the B1 from Bw to get the voltage of the actual voltage of B2.