Battery Management System (BMS)

The Battery Management System (BMS) is the electronic brain of an EV battery pack. It monitors every cell, prevents dangerous conditions, estimates state of charge, and ensures the pack operates safely throughout its life.

A well-designed BMS can extend battery life by 2-3× compared to unmanaged charging/discharging.

BMS Functions

1. Cell Monitoring

• Voltage: Each cell monitored to ±2mV accuracy
• Temperature: NTC thermistors every 5-10 cells
• Current: Hall sensor or shunt resistor (±0.5%)

2. State Estimation

• SOC: State of Charge (0-100%)
• SOH: State of Health (capacity fade)
• SOP: State of Power (available power)

3. Cell Balancing

• Equalize cell voltages across the pack
• Prevent weakest cell from limiting pack

4. Protection

• Over-voltage protection (OVP)
• Under-voltage protection (UVP)
• Over-temperature protection (OTP)
• Over-current protection (OCP)
• Short circuit protection

5. Communication

• CAN bus to vehicle ECU
• Report SOC, warnings, limits
• Receive charge/discharge commands

BMS Architecture

Distributed BMS

• Module-level slave boards (AFE + MCU)
• Master controller aggregates data
• Isolated communication between slaves
• Common in automotive applications

Centralized BMS

• Single board monitors all cells
• Simpler, lower cost
• Limited scalability
• Used in small packs (two-wheelers)

Key IC Components

Cell Balancing

Why Balancing is Needed

Even cells from the same batch have slight variations:

• Manufacturing tolerances (±5% capacity)
• Temperature gradients in pack
• Self-discharge rate differences
• Different aging rates

Without balancing:

• Weakest cell hits limits first
• Pack capacity = capacity of worst cell
• 10% cell variation → 10% less usable capacity

Passive Balancing

$$I_{balance} = \frac{V_{cell} - V_{target}}{R_{balance}}$$

$$E_{lost} = I_{balance}^2 \cdot R_{balance} \cdot t$$

• Simple, low cost
• Reliable (no active components)
• Easy to implement

• Energy wasted as heat
• Slow (10-100 mA typical)
• Only during charging

def passive_balance(cells, threshold=0.02):
    min_v = min(c.voltage for c in cells)
    for cell in cells:
        if cell.voltage - min_v > threshold:
            cell.enable_balance_resistor()
        else:
            cell.disable_balance_resistor()

Active Balancing

• Capacitor-based: Switched capacitors shuttle charge
• Inductor-based: Flyback converters transfer energy
• Transformer-based: Multi-winding transformer

• Energy efficient (>90%)
• Faster balancing
• Works during charge and discharge

• Complex, expensive
• More components to fail
• Requires careful design

Balancing Time Calculation

For passive balancing:

$$t_{balance} = \frac{C_{cell} \cdot \Delta V}{I_{balance}}$$

$$t = \frac{50 \times 0.050}{0.050} = 50 \text{ hours}$$

This is why balancing happens during every charge, not as a one-time event.

Protection Functions

Over-Voltage Protection (OVP)

Under-Voltage Protection (UVP)

Over-Temperature Protection (OTP)

Over-Current Protection (OCP)

Short Circuit Protection

Safety Architecture

Functional Safety (ISO 26262)

Automotive BMS must meet ASIL-B or ASIL-C:

• Redundant monitoring paths
• Watchdog timers
• Safe state = contactors open
• Hardware and software diagnostics

Two-Level Protection

• BMS MCU monitors parameters
• Can adjust limits dynamically
• Logs warnings and errors

• Independent comparator circuits
• Cannot be disabled by software
• Directly controls contactors

Contactor Control

• Close auxiliary
• Close precharge (through resistor)
• Wait for capacitor charge (voltage check)
• Close main
• Open precharge

State of Health (SOH)

SOH indicates how much capacity remains compared to new:

$$SOH = \frac{Q_{current}}{Q_{new}} \times 100\%$$

Degradation Mechanisms

• SEI growth: Lithium consumed in side reactions
• Lithium plating: Fast charging at low temp
• Particle cracking: Mechanical stress in active material
• Electrolyte decomposition: High temperature exposure

SOH Estimation Methods

• Full charge/discharge cycles
• Compare to rated capacity

• Measure DC resistance
• Higher impedance = lower SOH

• dQ/dV peaks shift with aging
• Research-grade technique

End of Life Criteria

CAN Communication

BMS communicates via CAN bus (typical):

• Pack SOC, SOH
• Max charge/discharge current
• Fault codes
• Cell voltages (if requested)

• Charge enable
• Target current
• Preconditioning request

ID: 0x100 (BMS Status)
Byte 0: SOC (0-100)
Byte 1-2: Pack voltage (0.1V resolution)
Byte 3-4: Pack current (signed, 0.1A)
Byte 5: Max temperature
Byte 6: Fault flags
Byte 7: Balance status

Indian Market BMS

• Centralized architecture
• 8-16 cell monitoring
• Passive balancing
• Basic protection

• Distributed architecture
• 96+ cell monitoring
• Advanced SOC algorithms
• ISO 26262 ASIL-B

• Indian: Grinntech, ION Energy, Celkon
• Global: LG, Panasonic, BYD (integrated)

Key Takeaways

• BMS monitors voltage, temperature, current for every cell
• Cell balancing equalizes cells to maximize usable capacity
• Passive balancing is simple but wastes energy; active is efficient but complex
• Protection includes OVP, UVP, OTP, OCP, and short circuit
• SOH tracks capacity fade over battery life
• Automotive BMS must meet functional safety standards (ISO 26262)

What's Next

In the next lesson, we'll explore Electric Motors — understanding PMSM, induction motors, torque-speed characteristics, and why EVs use motors instead of engines.