Passive vs Active Cell Balancing in a BMS: How to Choose

An EV battery pack is only as good as its weakest cell. Charging must stop when the fullest cell reaches its upper voltage limit, and driving must stop when the emptiest cell reaches its lower limit. If the cells drift apart, the pack loses usable capacity even though every cell is healthy. Cell balancing is how a battery management system (BMS) keeps the cells together.

Why cells drift apart

Cells in a series string carry the same current, yet their states of charge still separate over time because of:

• Capacity differences: cells from the same batch can differ by a few percent in capacity
• Self-discharge differences: each cell loses charge at a slightly different rate while parked
• Temperature differences: cells near a heat source age and self-discharge faster
• Unequal loads: monitoring electronics may draw slightly more from some cells

The imbalance usually grows slowly, over weeks and months, which is why balancing does not need to be fast. It needs to be reliable.

Passive balancing

Passive balancing removes energy from the cells that are ahead. Each cell has a switch and a bleed resistor across it, and the BMS turns on the resistor for cells with higher voltage or SOC, burning their extra charge as heat.

• Simple and low-cost: one switch and one resistor per cell, and it is built into most BMS chips
• Small balancing current: typically tens to a couple of hundred milliamps, limited by heat
• Wastes energy: the extra charge becomes heat instead of driving range
• Usually top balancing: it runs near full charge, where voltage differences are easiest to see

For packs that start well matched and are charged regularly, passive balancing is often enough.

Active balancing

Active balancing moves energy from cells that are ahead to cells that are behind, instead of burning it. Common circuits are:

• Switched capacitors: a capacitor shuttles charge between neighbouring cells
• Inductive converters: small DC-DC converters move energy between cells or between a cell and the pack
• Transformer-based circuits: a multi-winding transformer redistributes energy across many cells

Active balancing can use much higher balancing currents and recover energy that passive balancing would waste. In exchange, it needs more components, more control logic and careful design to keep the circuit's own losses low.

Voltage-based or SOC-based decisions

Whichever circuit you use, the BMS must decide which cells to balance and when:

• Voltage-based: balance the cells with the highest voltage. This is simple, but voltage is a poor guide to SOC in the flat middle of a Li-ion voltage curve, and it is distorted by current flow.
• SOC-based: balance using each cell's estimated state of charge. This is more accurate across the whole range, but it depends on a good cell model and estimator. Estimates are most reliable during rest periods, when the cell voltage is close to its open-circuit value.

Our own four-cell balancing study used SOC estimated during rest to drive the balancing decisions, rather than voltage alone.

How to compare them in simulation

Before choosing hardware, simulate both options on the same pack:

1. Build a cell model for each cell, such as a 2RC equivalent-circuit model, with realistic spreads in capacity and initial SOC.
2. Add the balancing circuit: bleed resistors for passive, or an energy-transfer model for active.
3. Write the balancing logic in Simulink or Stateflow, voltage-based or SOC-based.
4. Run realistic profiles: charging, drive cycles and long rests, over many cycles.
5. Compare outcomes: usable pack capacity, time to balance, energy lost as heat, and the peak temperatures of the balancing resistors.

This turns the choice into numbers. You can see how much capacity each option recovers, at what cost in energy and components.

Quick guide

Question	Passive	Active
Cost and complexity	Low	Higher
Energy efficiency	Burns excess as heat	Moves energy between cells
Balancing current	Small	Can be larger
Best for	Well-matched packs, regular charging	Large packs, high cell spread, second-life cells

Where to go next

Balancing works only as well as the estimates behind it, so it fits together with SOC estimation and cell modelling. If you are designing a BMS and want to compare balancing strategies on your own pack, or want your team trained on BMS modelling in Simulink, talk to us about EV battery modelling and BMS.