DIY Arduino Smart BMS: 3S Li-ion Cell Monitor-Automatic Charger

 

1. Introduction

A standard charger is "blind"—it doesn't know if one cell is overheating or failing. This project creates a "Smart BMS" that monitors the individual health of every cell in a 3S (11.1V nominal / 12.6V max) Li-ion pack. It automatically manages the charging process and cuts off power if a "problamatic" cell is detected, keeping your battery pack healthy and safe.

2. Components

• Arduino Uno

• 12V Relay Module (To control the main charger)

• ACS712 Current Sensor (5A or 20A version)

• Voltage Divider Resistors: (6 Resistors: 10kΩ x 4, 20kΩ x 1, 30kΩ x 1)

• 1N4007 Diode (For reverse protection)

• 15V DC Power Adapter (Recommended for 12.6V Li-ion charging)

3. Circuit and Connections

Pinout Diagram of Arduino Uno

Pinout Diagram of Single Channel Relay Module

Pinout Diagram of ACS712

3.1. Relay Module (Single Channel)

Relay Module Pin	Arduino Pin	Connection Description
VCC	5V	Power for the Relay Coil
GND	GND	Ground Reference
IN / Signal	D8	Control Signal from Arduino
COM (Common)	—	Connect to Charger (+)
NO (Normally Open)	—	Connect to Diode Anode

3.2. ACS712 Current Sensor

ACS712 Sensor Pin	Arduino Pin	Connection Description
VCC	5V	Power for the Sensor
GND	GND	Ground Reference
OUT	A3	Analog Current Reading
Terminal 1	—	From Diode Cathode
Terminal 2	—	To Battery Pack (+) Main

 3.3. Voltage Divider 1 (Cell 1 Monitoring)

Divider 1 Connection	Arduino Pin	Connection Description
Resistor Junction	A0	Middle point of 10k/10k resistors
Top Resistor	—	Connect to Cell 1 (+) Tap
Bottom Resistor	GND	Connect to Common GND

3.4. Voltage Divider 2 (Cell 2 Monitoring)

Divider 2 Connection	Arduino Pin	Connection Description
Resistor Junction	A1	Middle point of 20k/10k resistors
Top Resistor	—	Connect to Cell 2 (+) Tap
Bottom Resistor	GND	Connect to Common GND

3.5. Voltage Divider 3 (Cell 3 Monitoring)

Divider 3 Connection	Arduino Pin	Connection Description
Resistor Junction	A2	Middle point of 30k/10k resistors
Top Resistor	—	Connect to Cell 3 (+) Tap
Bottom Resistor	GND	Connect to Common GND

3.6 1N4007 Protection Diode

Diode Leg	Connection Point	Connection Description
Anode (No Stripe)	Relay NO Pin	Receives power from the charger
Cathode (Silver Stripe)	ACS712 Terminal 1	Sends power toward the battery

3.7. 12V/15V DC Power Adapter

Adapter Wire	Connection Point	Connection Description
Positive (+)	Relay COM Pin	The "Input" power for charging
Negative (-)	Common GND	Connect to Arduino GND and Battery (-)

4. Detailed Step By Step Circuit working

• Voltage Tapping: Each cell's positive terminal is connected to an Analog pin. Because each tap has a higher voltage, we use different resistor ratios to keep the signal below 5V.

• Current Monitoring: The ACS712 sits in the main charging line. It tells the Arduino exactly how many Amps are flowing into the cells.

• Relay Switching: The Relay acts as the "Gatekeeper." If the Arduino detects a fault or a full charge (4.2V), it opens the relay to stop the flow of electricity.

• Reverse Protection: The 1N4007 diode ensures that if the power adapter is unplugged, the battery does not discharge backwards into the circuit.

5. Libraries to be included

• No external libraries required (Uses built-in Serial and Analog functions).

6. Code

const int relayPin = 8;

const float cellMax = 4.20; // Max voltage for Li-ion cell

const float cellMin = 3.00; // Min voltage for Li-ion cell

void setup() {

  pinMode(relayPin, OUTPUT);

  Serial.begin(9600);

  digitalWrite(relayPin, LOW); 

}

void loop() {

  // 1. Read Raw Values and reverse Divider Math

  float v1 = analogRead(A0) * (5.0 / 1023.0) * 2.0; // Cell 1

  float v2 = analogRead(A1) * (5.0 / 1023.0) * 3.0; // Cell 2

  float v3 = analogRead(A2) * (5.0 / 1023.0) * 4.0; // Cell 3

  

  // 2. Calculate Individual Cell Health

  float c1 = v1;

  float c2 = v2 - v1;

  float c3 = v3 - v2;

  float totalV = v3;

  // 3. Current Sensing (ACS712 Calibration)

  int rawCurrent = analogRead(A3);

  float amps = (rawCurrent - 512) * 0.0488; 

  // 4. Print Status to Serial Monitor

  Serial.print("C1:"); Serial.print(c1);

  Serial.print("V | C2:"); Serial.print(c2);

  Serial.print("V | C3:"); Serial.print(c3);

  Serial.print("V | Total:"); Serial.print(totalV);

  Serial.print("V | Amps:"); Serial.println(amps);

  // 5. Logic for Charging and Faults

  if (c1 >= cellMax || c2 >= cellMax || c3 >= cellMax) {

    digitalWrite(relayPin, LOW); // STOP: Full

    Serial.println("ALERT: CHARGE COMPLETE");

  } else if (c1 < cellMin || c2 < cellMin || c3 < cellMin) {

    digitalWrite(relayPin, LOW); // STOP: Problamatic

    Serial.println("ALERT: PROBLAMATIC CELL FOUND!");

  } else {

    digitalWrite(relayPin, HIGH); // SAFE: Charging

  }

  

  delay(2000);

}

7. Detailed Step By Step Code working

• Reading and Scaling: The code reads three analog pins and uses multipliers to find the real voltage at each battery tap.

• Cell Isolation: It subtracts the lower tap values from the higher ones to isolate the voltage of Cell 2 and Cell 3.

• Current Logic: It reads the ACS712. If the amperage is correct, it continues; if there is a short, the relay can be programmed to cut off.

• Safety Decision: The if statements monitor every cell. If any cell hits 4.2V, the relay stops the charger. If any cell is dangerously low, it flags a "Problamatic" cell.

8. Tips

• Star Grounding: Keep all Ground wires connected to a single point to avoid "noise."

• Calibration: Always verify the voltage with a multimeter and adjust the multipliers in the code (2.0, 3.0, 4.0) to match your specific resistors.

• Li-ion Only: This project is strictly for Li-ion 3S packs. Do not use it for Lead-Acid batteries.

9. Uses

• Building high-quality 12V Li-ion battery packs.

• Real-time monitoring of solar battery banks.

• Safety system for DIY electric vehicles or drones.

10. Conclusion

By combining monitoring and charging, you have created a system that is safer than most cheap commercial chargers. You can now identify a failing cell before it causes an accident, saving you time and money.