2S LIPO Monitor

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello FEllow Arduinites,

I've been trying for the past several days to come up with a LIPO cell voltage monitor, but failing to do so. I've read a lot of posts and borrowed some code, I'm close but not quite there yet. For example using my multi-meter I measure the voltage at the divider and I get 4.06v ,but when the arduino tells me the voltage on that cell it is 4.71. I know it's erroneous since the maximum voltage any single cell can have is 4.2v only. I'm only monitoring one cell until I figure out what's wrong, or somebody tells me where's my error.

BTW: I'm powering the Arduino with the same 2S Lipo battery.

Here's my code with the schem.

Schem for a 2S LIPO (7.4v/8.4v max):

LIPO (MAIN +) ------- 7.4v -------------------------------(Arduino pwr socket +) 
                                                      ----(Arduino pwr socket -)
LIPO (Cell 2+) ----- 4.2v max ----(Arduino Pin A1)    |
                                                      |
                  10K ohms  10K ohms                  |
LIPO (Cell 1+) -----vvvv--|--vvvv---------------------|
                          |                           |
                          |--4.2v---(Arduino Pin A0)  |
LIPO (GND-) ------------------------------------------|

Code:                                                                             */
#include <SparkSoftLCD.h>      // library to manipulate the SparkFun LCD backpack v2.5 

#define CELL_1_PIN 14   // Analog Pin A0
#define CELL_2_PIN 15   // Analog Pin A1
#define LED_PIN 13      // Digital Pin 13
#define LCD_PIN 2       // Digital pin 2 to LCD

SparkSoftLCD LCD = SparkSoftLCD(LCD_PIN);

void setup(){
  pinMode(CELL_1_PIN, INPUT);
  pinMode(CELL_2_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  pinMode(LCD_PIN, OUTPUT);
  LCD.begin(9600);
  LCD.clear();
//  delay(1500);  
}
void loop(){
  
  cellVolts();
  digitalWrite(LED_PIN,HIGH);
  cellVolts();
  digitalWrite(LED_PIN,LOW);
}

void cellVolts(){
  int cell1Volt, cell2Volt;
  LCD.cursorTo(1,1);
  LCD.print("Cell 1 voltage: ");
  LCD.cursorTo(2,1);
  cell1Volt = (analogRead(CELL_1_PIN)*5);
  LCD.print(cell1Volt / 1024, DEC);
  LCD.print(".");
  LCD.print(cell1Volt % 1024, DEC);
  delay(1000);
}

Any help, as always greatly appreciate it.

2

since you are working with decimals, wouldn't it be more appropriate for cell1volt and cell2volt to be floats?

3

BTW: I'm powering the Arduino with the same 2S Lipo battery.

  cell1Volt = (analogRead(CELL_1_PIN)*5);

The value returned by analogRead is a function of the operating voltage, which may not be 5.000, as your equation assumes.

You'll need to assure that your operating voltage is 5.000 or adjust the cell1Volt calculation.