Battery Management System *UPDATE

As promised, here’s the rest of the code for the battery management system. The BMS now reads the voltage of each battery cell and balances each cell until the battery voltage across all cells is closely matched; +/-0.03V.

I also included a box with a switch and an LCD display. Some extra components are included such as a small trimmer potentiometer to adjust the LCD brightness, DIP switch with 4 contacts, toggle switch to turn power-off and on and a box to store the components.

This is what the new setup looks like:




And here’s the result:

1.This is the starting position of the 4 lithium batteries.

2. Then the batteries begin to discharge.

3. And at the end, the battery voltages are closely matched.

Here’s the new code:

#include "SPI.h"
#include <LiquidCrystal.h>
// SDO - PIN 12
// SDI - PIN 11
#define WRCFG  0x01 //Write Configuration Registers
#define RDCFG  0x02  // Read config
#define RDCV   0x04  // Read cells

#define STCVAD 0x10  // Start all A/D's - poll status
#define RDFLG  0x06 //Read Flags
#define RDTMP  0x08 //Read Temperatures

#define STCDC  0x60 //A/D converter and poll Status
#define STOWAD 0x20 //Start Test - poll status
#define STTMPAD 0x30// Temperature Reading - ALL

#define address 0x80
//Functions
#define TOTAL  4
#define LIMIT 0.01
byte byteTemp;
byte CFGR1 = 0x00;
byte CFGR2 = 0x00;
float cellVolt[TOTAL];

LiquidCrystal lcd(7,6,5,4,3,2);


void setup()
{
pinMode(10,OUTPUT);
pinMode(11,OUTPUT);
pinMode(12,INPUT);
pinMode(13,OUTPUT);
digitalWrite(10, HIGH);
SPI.setBitOrder(MSBFIRST);
SPI.setDataMode(SPI_MODE3);
SPI.setClockDivider(SPI_CLOCK_DIV16);
SPI.begin();
Serial.begin(9600);
//LCD Setup
lcd.begin(16,2);
lcd.setCursor(0,0);
lcd.print("C1: ");
lcd.setCursor(8,0);
lcd.print("C2: ");
lcd.setCursor(0,1);
lcd.print("C3: ");
lcd.setCursor(8,1);
lcd.print("C4: ");
//lcd.print("Hello world!");
writeReg(); //Initial setup
}

void loop()
{
readV();
//Serial.print("Lowest cell: "); Serial.print(lowCell()); Serial.print(" , Value: "); Serial.println(cellVolt[lowCell()-1]);
//Serial.print("Highest cell: "); Serial.print(highCell()); Serial.print(" , Value: "); Serial.println(cellVolt[highCell()-1]);
startBalance();
//delay(2000);
Serial.println(CFGR1,BIN);
lcd.setCursor(3,0);
lcd.print(cellVolt[0]);
lcd.setCursor(11,0);
lcd.print(cellVolt[1]);
lcd.setCursor(3,1);
lcd.print(cellVolt[2]);
lcd.setCursor(11,1);
lcd.print(cellVolt[3]);
delay(1000);
}

void writeReg()
{
 Serial.println("Writing config...");
  digitalWrite(10, LOW);
  SPI.transfer(address);
  SPI.transfer(WRCFG);
  SPI.transfer(0x01);//0
  SPI.transfer(CFGR1);//1
  SPI.transfer(CFGR2);//2
  SPI.transfer(0x00);//3
  SPI.transfer(0x71);//4
  SPI.transfer(0xAB);//5
  digitalWrite(10, HIGH);
}
void readReg()
{
Serial.println("Reading config...");
digitalWrite(10, LOW);
   SPI.transfer(address);
   SPI.transfer(RDCFG);
   for(int i = 0; i < 6; i++)
   {
   byteTemp = SPI.transfer(RDCFG);
   Serial.println(byteTemp, HEX);
   }
  digitalWrite(10, HIGH);
}
void readV()
{
  digitalWrite(10,LOW);
  SPI.transfer(STCVAD);
  delay(20); // wait at least 12ms as per data sheet, p.24
  digitalWrite(10,HIGH);
  byte volt[18];
  digitalWrite(10,LOW);
  SPI.transfer(0x80);
  SPI.transfer(RDCV);
  for(int j = 0; j<18;j++)
  {
  volt[j] = SPI.transfer(RDCV);
  }
  digitalWrite(10,HIGH);
  cellVolt[0] = (((volt[0] & 0xFF) | (volt[1] & 0x0F) << 8)*1.5*0.001);
  cellVolt[1] = (((volt[1] & 0xF0) >> 4 | (volt[2] & 0xFF) << 4)*1.5*0.001); 
  cellVolt[2] = (((volt[3] & 0xFF) | (volt[4] & 0x0F) << 8)*1.5*0.001);
  cellVolt[3] = (((volt[4] & 0xF0) >> 4 | (volt[5] & 0xFF) << 4)*1.5*0.001);
  Serial.println(cellVolt[0]);
  Serial.println(cellVolt[1]);
  Serial.println(cellVolt[2]);
  Serial.println(cellVolt[3]);
  Serial.println("--------------------");
}

int startBalance()
{
 CFGR1 = 0x00;
 CFGR2 = 0x00;
 int low = lowCell();
 for(int i = 1; i < TOTAL+1; i++)
 {
 if(i != low)
 {
 balanceCell(i);//set each cell individually; discharge if not lowest cell
 }
 }
 Serial.println(CFGR1,BIN);
 Serial.println(CFGR2,BIN);
 writeReg();
  
}

int balanceCell(int x)
{
  double diff = cellVolt[x-1] - cellVolt[lowCell()-1]; //check the difference between current cell and lowestCell
  if(x <= 8 && diff > 0.03)	//discharge if difference more than 0.03V
  {
  CFGR1 += 0x01<<(x-1);
  }
  if(x <= 8 && diff <= 0.03)
  {
  CFGR1 += 0x00<<(x-1);  
  }
  if(x > 8 && diff > 0.03)
  {
  CFGR2 += 0x01<<(x-9);
  }
  if(x > 8 && diff <= 0.03)
  {
  CFGR2 += 0x00<<(x-9);
  }
  
}

int highCell() //find highest voltage cell value
{
  int num = 0;
  float cellTemp = 0;
  for(int i = 0; i < TOTAL;)
  {
    if(cellVolt[i] > cellTemp)
    {
      cellTemp = cellVolt[i];
      num = i+1;
    }
    i++;
  }
  return num;
}

int lowCell() //find lowest cell value
{
  int num = 0;
  float cellTemp = 5;
  for(int i = 0; i < TOTAL;)
  {
    if(cellVolt[i] < cellTemp)
    {
      cellTemp = cellVolt[i];
      num = i+1;
    }
    i++;
  }
  return num;
}

(If WordPress doesn’t display the code correctly here’s a link to the file)
https://www.sendspace.com/file/91rwxs

*A few additional safety features are needed before the BMS is complete. Such as: low voltage warning and action, high voltage warning, temperature sensing and a current sensor to monitor the charging current as well as the SOC (state-of-charge) of the battery pack.