Cell balancing is a battery management process that keeps the individual cells in a battery pack at a similar state of charge (SoC) and voltage levels. In EVs and stationary storage, balancing is controlled by the Battery Management System (BMS) to improve usable capacity, maintain safety margins, and extend battery life.
What Is Cell Balancing?
A battery pack contains many cells connected in series and parallel. Over time, cells drift apart due to small differences in capacity, internal resistance, temperature, and aging. Cell balancing corrects these differences by:
– Reducing high-cell voltage/SoC to match lower cells
– Ensuring the pack can charge and discharge safely without one cell hitting limits early
– Improving the accuracy of SoC and state of health estimation
Without balancing, one “weak” or “high” cell can limit the entire pack’s usable energy.
Why Cell Balancing Matters in EV Charging
Cell balancing affects how an EV behaves during charging and why charging may slow down near full SoC:
– If some cells reach maximum voltage early, the BMS reduces charging current to protect them
– This can cause earlier or stronger CV tapering in a CC-CV charging profile
– Balancing helps maintain consistent charging performance over the vehicle’s lifetime
– Poor balance can reduce usable range and increase charging time variability between vehicles
For charging operators, it explains why two identical sessions can have different charging speeds depending on battery condition.
How Cell Balancing Works
Cell balancing is typically managed by the BMS using one of two methods:
– Passive balancing
– The BMS bleeds energy from higher-voltage cells through resistors
– Simple and common, but energy is dissipated as heat
– Often occurs near the top of charge when cells are close to full
– Active balancing
– Energy is transferred from higher cells to lower cells using converters
– More efficient and can work across a wider SoC range
– More complex and typically used in higher-performance systems
Balancing can occur during charging, after charging, or during rest periods depending on vehicle strategy.
When Cell Balancing Happens
Balancing is most noticeable:
– Near high SoC (e.g., above ~80–90%), where cell voltage differences matter most
– After a full or near-full charge, when the BMS has time to equalize cells
– In older packs with higher cell-to-cell variation
– After repeated partial charging cycles where cells drift without a full top balance
Some vehicles recommend occasional full charges specifically to support balancing (vehicle-dependent).
Key Benefits of Cell Balancing
– Higher usable capacity and more consistent range
– Improved charging consistency and reduced early tapering due to cell limits
– Better safety margins by preventing overvoltage on individual cells
– Longer battery lifetime through reduced cell stress
– More accurate pack monitoring and diagnostics
Limitations to Consider
– Balancing cannot restore lost capacity from aging; it only equalizes cells
– Passive balancing wastes energy as heat and can extend charge time near 100%
– Active balancing adds cost and complexity
– Heavy fast charging and high temperatures can increase imbalance over time
– Cell balancing behavior is controlled by the vehicle, not the charger
Related Glossary Terms
Battery Management System (BMS)
State of Charge (SoC)
State of Health (SoH)
CC-CV Charging Profile
Charging Curve
Battery Aging
Battery Impedance
Battery Thermal Limits
Battery Degradation Modeling
Battery Pack