Passive balancing is a method used in a battery management system (BMS) to equalize the state of charge (SoC) of individual battery cells by bleeding off excess energy from higher-voltage cells as heat through resistors. The goal is to keep cell voltages aligned, which helps protect the battery and maintain usable capacity over time.
Passive balancing is common in lithium-ion battery packs because it is simple, reliable, and cost-effective compared to active balancing.
Why Passive Balancing Matters in EVs and Charging
In EV batteries, cells naturally drift apart in voltage due to manufacturing tolerances, temperature differences, and aging. If cells are unbalanced:
– The pack can hit charge cut-off early (reducing usable capacity)
– Some cells can be overstressed (reducing battery life)
– Safety margins shrink under high load or fast charging
Passive balancing helps the BMS keep the pack within safe operating limits during charging and at high SoC.
How Passive Balancing Works
A typical passive balancing process looks like this:
– The BMS measures cell voltages across the pack
– When certain cells exceed a voltage threshold, the BMS switches on a bleed resistor for those cells
– The higher cells discharge slightly, converting energy into heat
– Lower cells continue charging normally until voltages converge
– Balancing may run mostly near the top of charge (e.g., during the final phase of charging)
Where Passive Balancing Is Used
– EV traction battery packs (within the vehicle BMS)
– Stationary battery storage systems (ESS/BESS)
– Smaller lithium-ion packs in chargers, UPS systems, and auxiliary power systems
In EV charging, the charger does not usually perform cell balancing; balancing is handled inside the vehicle by the BMS during the charging process.
Key Benefits
– Simple hardware and lower cost than active balancing
– Proven and widely used for lithium-ion battery packs
– Reliable control strategy with predictable behavior
– Effective for correcting small-to-moderate cell drift over time
Limitations and Considerations
– Inefficient: energy is dissipated as heat rather than redistributed
– Balancing speed is limited by resistor power and thermal constraints
– More heat generation at high SoC can increase cooling needs
– Less suitable for very large packs or severe imbalance compared to active balancing
– Balancing often requires longer time at high SoC, which can slightly extend full-charge duration
Related Glossary Terms
Battery Management System (BMS)
Cell Balancing
Active Balancing
State of Charge (SoC)
Lithium-Ion Battery
Constant Current (CC) / Constant Voltage (CV) Phases
Battery Degradation
Thermal Management