Battery Management Systems

What a BMS does: protection, SoC and SoH estimation, and cell balancing for safe and optimized battery pack operation.

The Battery Management System (BMS) is the electronic “brain” that monitors and controls the battery pack. Its primary responsibility is to maintain safety while optimizing the life and performance of the cells.

Core Functions

Protection

Ensures the battery never operates outside safe limits for voltage, current, and temperature.

Estimation

Calculates key metrics (State of Charge, State of Health, State of Power, State of Energy) using sensor data and models.

Balancing

Keeps cells at similar states of charge to maximize pack capacity.

State Estimation

The BMS performs real-time calculations to inform the user and control systems:

State	Description
SoC	State of Charge—how much charge remains
SoH	State of Health—how much the battery has degraded
SoP	State of Power—maximum charge/discharge power available
SoE	State of Energy—remaining energy considering efficiency

See the State of Charge and State of Health pages for more details on these metrics and how they are estimated.

Protection Functions

The BMS must protect against several failure modes. These include overvoltage (which can cause electrolyte decomposition), undervoltage (which can dissolve copper from the current collector), overcurrent (causing rapid heating), and over-temperature (accelerating degradation). In high-voltage systems, isolation faults also pose electric shock risks.

These failure modes are interconnected—for example, overcurrent leads to heating, which can trigger thermal runaway if unchecked. The BMS must monitor all conditions simultaneously.

Temperature Monitoring

The BMS monitors temperature sensors distributed throughout the pack and can:

• Command the cooling system to increase flow
• De-rate power output if cells exceed safe limits (typically above 55°C)
• Limit or prevent charging at low temperatures (typically below 0°C to 10°C depending on chemistry) to avoid lithium plating

Isolation Monitoring

In high-voltage systems (400V or 800V), the BMS monitors insulation resistance between the battery terminals and the vehicle chassis. If a fault is detected, it opens the contactors to prevent electric shock.

Cell Balancing

In a series-connected pack, cells inevitably drift apart in their SoC because no two cells are identical. Manufacturing variations, temperature gradients across the pack, and differences in self-discharge rates all contribute to this drift. Without balancing:

• A single “full” cell prevents the rest of the string from charging
• A single “empty” cell forces the whole pack to stop discharging

This means the pack’s usable capacity is limited by its weakest cell, even if other cells have capacity remaining.

Passive Balancing

The simpler and more common approach. During charging, when a cell reaches its upper voltage limit before others, the BMS dissipates its excess energy through a resistor as heat. This allows charging to continue until all cells are full.

Limitations: Only works during charge, wastes energy as heat, and balancing current is typically low (50-200 mA), requiring long charge times for significant imbalances.

Active Balancing

Uses DC-DC converters to transfer energy from higher-SoC cells to lower-SoC cells. This approach is more efficient and can operate during both charge and discharge.

Limitations: Significantly more complex and expensive, requiring additional power electronics for each cell or group of cells.

Passive balancing dominates in most commercial applications due to its simplicity and lower cost. Active balancing is used in high-performance or high-value applications where the efficiency gains justify the added complexity.

BMS Architectures

The physical implementation varies depending on the application scale:

Centralized BMS

A single controller connects to every cell in the pack.

Best for: Small applications (e-bikes, power tools)

Pros: Simple design, lower cost

Cons: Complex wiring for large packs

Distributed (Master-Slave) Architecture

A “Master” controller communicates via a bus (like CAN) with multiple “Slave” boards or Cell Monitoring Units (CMUs).

Best for: Large applications (EVs, grid storage)

Pros: Reduced wiring complexity, improved reliability, modular

Cons: More complex communication protocols

Thermal Management Integration

The BMS controls the thermal management system:

• Cooling: Commands coolant pumps and fans based on cell temperatures
• Heating: In cold climates, may activate heaters before allowing charge
• Thermal modeling: Uses resistive losses ($I^2R$) to predict temperature evolution

Safety Systems

Contactors

Heavy-duty switches that can isolate the battery in emergencies:

• Open immediately upon crash signal from vehicle
• De-energize high-voltage lines to prevent hazards
• Controlled by the BMS based on fault conditions

Fuses

Provide backup protection if the BMS or contactors fail to respond to an overcurrent event.