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Old 02-07-2012, 11:51 PM   #1 (permalink)
Daox
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DIY open source lithium BMS (battery management system)

I've made enough progress with my lithium BMS that I think its time to post up what I have so far. I am posting the info for others to use as well as get some feedback.

Let me preface this all with the fact that I am by and far no expert in this. I know some programming and a little bit about electronics. That and a lot of googling and help from others has lead me to developing this.

The BMS is currently setup to work with the plugin kit for my Prius. So, right now its very simple. For example, where you see dc-dc converter it would be the controller in an EV. However, it was developed while keeping in mind that I'd like to use a similar version on an EV.

With that I'd like to go over generally how it works without having to dive into code:

Charging protection is pretty simple. All the cells voltages are individually monitored via a series of celllog devices. The celllog can measure up to 8 cells at once. If more cell monitoring is needed you need another celllog and the tiny circuit that accompanies it. My circuit diagram shows 2 celllogs which is what I am using. The celllog has a user settable high voltage alarm that is used. Once the alarm goes off, the arduino cuts power to the charger.

Discharge protection is taken care of via a current sensor. You have to enter in the amp hours your batteries are rated for and the max DOD you want them to see. From there the current sensor will keep track of amp hours used. For my plugin kit it automatically shuts the kit off after I hit my max DOD. For a full EV you could simply not use that part of the circuit. I have added an 8 LED bar graph that tracks SOC of the pack as a 'fuel gauge'.

Balancing is currently not done with the BMS. It is designed to have the batteries balanced at the low end of their charge and then put into use. I have yet to find a good way to keep track of the SOC of each cell and balancing makes that much more complicated.

Thermal protection is not currently done either. The plugin kit doesn't even pull 1C on the batteries so its not an issue. However, I would be interested in adding this.

Here is the schematic:



And here is the code (NOT UP TO DATE):
Code:
/*
  Lithium BMS
  This is a simple BMS system for lithium ion batteries.
*/

#include <avr/interrupt.h>
#include <avr/power.h>
#include <avr/sleep.h>

// Pins
const int OnSwitchPin = 2;  // pin to read on switch status
const int ChargerRelayPin = 3;  // pin to control the charger relay
const int Pin110V = 4;  // pin to read if charger is plugged in
const int OnSwitchRelayPin = 5;  // pin to control the on switch (dc-dc pwr)
const int AlarmPin = 6;  // pin to read celllog alarm signal
const int latchPin = 8;  // pin to connect to latch pin on shift register
const int dataPin = 11;  // pin to connect to data pin on shift register
const int clockPin = 12;  // pin to connect to clock pin on shift register
const int CurrentPin = 1; // analog pin to read the current sensor

// Variables
volatile float CurrentRead = 0;  // holds CurrentPin analog read
volatile float AHused = 0; // holds amp hours used
float BatteryAH = 0; // holds usable battery amp hours
int AlarmPinStatus = 0;  // holds AlarmPin status
int OnSwitchPinStatus = 0;  // holds the OnSwitchPin status
int Pin110VStatus = 0;  // holds the Pin110V status
unsigned long DelayTimer = 0;  // variable to hold disconnect time
float SOC = 0;  // holds state of charge

// User defined variables
const int RatedBatteryAmpHours = 78;  // rated amp hours of the batteries
const float DOD = .7;  // maximum depth of discharge allowed


void setup() {
  pinMode(AlarmPin, INPUT);
  pinMode(ChargerRelayPin, OUTPUT);
  pinMode(Pin110V, INPUT);
  pinMode(OnSwitchRelayPin, OUTPUT);
  pinMode(OnSwitchPin, INPUT);
  pinMode(CurrentPin, INPUT);
  digitalWrite(AlarmPin, HIGH);  // set AlarmPin high
  digitalWrite(ChargerRelayPin, LOW);  // turn charger relay off
  digitalWrite(OnSwitchRelayPin, LOW);  // turn on switch off
  
  BatteryAH = RatedBatteryAmpHours * DOD;  // calculate usable Ah
  
  // initialize Timer1
  cli();          // disable global interrupts
  TCCR1A = 0;     // set entire TCCR1A register to 0
  TCCR1B = 0;     // same for TCCR1B
  // set compare match register to desired timer count
  OCR1A = 16001;
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // Set CS10
  TCCR1B |= (1 << CS10);
  // enable timer compare interrupt
  TIMSK1 |= (1 << OCIE1A);
  // enable global interrupts
  sei();
}

void loop() {

  // SOC gauge code
  // calculate SOC
  SOC = (BatteryAH - AHused) / BatteryAH;
  // write SOC to led bargraph
  if (SOC > .875) {
    // 8 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 255);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > .75) {
    // 7 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 127);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > .625) {
    // 6 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 63);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > .5) {
    // 5 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 31);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > .375) {
    // 4 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 15);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > .25) {
    // 3 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 7);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > .125) {
    // 2 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 3);
    digitalWrite(latchPin, HIGH);
  }
  else if (SOC > 0) {
    // 1 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 1);
    digitalWrite(latchPin, HIGH);
  }
  else {
    // 0 leds lit up
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, 0);
    digitalWrite(latchPin, HIGH);
  }

  
  // Charger connect code
  // read pin110v status
  Pin110VStatus = digitalRead(Pin110V);
  // if 110V power is plugged in
  if (Pin110VStatus == HIGH) {
    digitalWrite(ChargerRelayPin, HIGH);  // turn charger on
  }
  
  
  // Charger disconnect code
  // read alarm pin status
  AlarmPinStatus = digitalRead(AlarmPin);
  // if alarm turns on set a timer to detect false alarms
  if (AlarmPinStatus == LOW && millis() - DelayTimer < 10) {
    DelayTimer = millis();  // set timer
  }
  // if alarm stays on
  if (AlarmPinStatus == LOW && millis() - DelayTimer > 2000) {
    digitalWrite(ChargerRelayPin, LOW);  // turn charger off
    AHused = 0;  // reset amp hours
    DelayTimer = 0;  // reset delay timer
  }
  
  
  // DC-DC connect code
  // read on switch status
  OnSwitchPinStatus = digitalRead(OnSwitchPin);
  // if dc-dc switch is on && battery has capacity left
  if (OnSwitchPinStatus = HIGH  && AHused < BatteryAH) {
    digitalWrite(OnSwitchRelayPin, HIGH);  // turn dc-dc converter on
  }
  
  
  // DC-DC disconnect code
  // if amp hours used exceeds usable amp hour capacity
  if (AHused > BatteryAH)
  {
    digitalWrite(OnSwitchRelayPin, LOW);  // turn dc-dc converter off
  }
  
  
  // DC-DC power down code
  // if dc-dc converter switch is off enter sleep mode
  if (OnSwitchPinStatus = LOW) {
  // enter sleep mode
  sleepNow();
  }
}


// sleep mode code
void sleepNow() {
  attachInterrupt(0,loop, RISING);  // set interrupt to on switch
  set_sleep_mode(SLEEP_MODE_PWR_SAVE);
  sleep_enable();
  digitalWrite(latchPin, LOW);  // turn off SOC gauge
  shiftOut(dataPin, clockPin, MSBFIRST, 0);
  digitalWrite(latchPin, HIGH);
  sleep_mode();  // enter sleep mode
  sleep_disable();  // resume code after exiting sleep mode
}


// interrupt to count amp hours used
ISR(TIMER1_COMPA_vect)
{
    CurrentRead = analogRead(CurrentPin);
    // Convert currentread to amps
    CurrentRead = (CurrentRead - 512) * .78125;
    // Convert to AH & add to total
    AHused = AHused + (CurrentRead * .001);
}
Here is a list of the components:


I've been using the charging protection for a few months with my plugin kit and it has worked great. I have just finished working on the discharge protection and haven't yet assembled the circuit.

Anyway, thanks for reading if you got this far. Its a lot to look at, but I'd really appreciate some feedback.

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Last edited by Daox; 02-20-2012 at 04:20 PM.. Reason: updates
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