BMS Active vs Passive Balancing: Topology, Efficiency, and Capacity Gain

Introduction

Battery Management Systems (BMS) are critical for ensuring the safety, longevity, and performance of lithium-ion battery packs. Cell balancing is a key function that mitigates voltage divergence among series-connected cells. Two primary approaches exist: passive balancing, which dissipates excess energy as heat, and active balancing, which transfers energy between cells. This guide provides a detailed comparison of their topologies, energy transfer efficiencies, and practical benefits for long-string traction batteries. EJER Tech's precision environmental control equipment can seamlessly interface with enterprise MES systems to enable real-time data upload and operation log traceability, supporting the holistic management of battery storage systems.

Topology Differences: Passive vs Active

Passive balancing employs a simple topology: each cell is paralleled with a shunt resistor and a switch (typically a MOSFET). When a cell voltage exceeds a threshold, the switch closes, and excess charge is bled through the resistor until voltages equalize. This approach is low-cost and easy to implement, but energy is wasted as heat, and balancing currents are limited (typically 50–200 mA) to avoid excessive thermal stress.

Active balancing uses power electronics to shuttle charge between cells. Three common topologies exist: switched-capacitor (SC), inductor-based, and transformer-based. The SC topology uses a flying capacitor and a network of switches to connect adjacent cells alternately, moving charge from higher‑voltage cells to lower‑voltage ones. Inductor-based designs store energy in a magnetic field and transfer it via a buck‑boost or flyback converter per cell. Transformer-based methods employ a multi‑winding transformer or a push‑pull converter to move energy simultaneously among many cells.

Energy Transfer Efficiency Comparison

Switched-capacitor topologies achieve efficiencies of 85–92% in practice, limited by the resistive losses in the switches and the capacitor's ESR. Their efficiency degrades when the voltage difference between cells is small (below 20 mV) because the capacitive charge transfer is proportional to ΔV. Inductor-based designs can reach 90–95% efficiency due to lower conduction losses and the ability to operate in continuous conduction mode. Transformer‑based schemes offer similar efficiency (90–96%) but at higher cost and complexity; they excel in long‑string packs where galvanic isolation is beneficial. Passive balancing, by contrast, has zero transfer efficiency—all excess energy is dissipated.

The choice of topology also affects balancing speed. Passive balancing typically operates at currents of 50–500 mA (limited by resistor power dissipation). Active balancing can achieve currents of 1–5 A or more, reducing balancing time from hours to minutes. However, the overhead circuits add cost and PCB area, which must be justified by the application.

Impact on Usable Capacity in Long Battery Strings

In a long series string (e.g., 96 cells in an electric vehicle pack), even small capacity mismatches accumulate. Without balancing, the usable capacity is limited by the weakest cell—the one that first reaches its low‑voltage cutoff during discharge or high‑voltage cutoff during charge. Passive balancing can equalize voltages only during the charge phase, and its low current may not fully correct large mismatches within a typical charge window. This leaves significant capacity untapped.

Active balancing continuously redistributes charge during both charge and discharge, keeping all cells near their average state of charge. Studies show that for a 96‑cell pack with 5% initial capacity variation, active balancing can recover 3–5% of usable capacity compared to passive balancing. In applications requiring deep cycles (e.g., grid storage or heavy‑duty EVs), this translates into 10–15% more usable energy over the pack's lifetime, because the active system prevents cells from drifting apart under repeated cycling. EJER Tech's environmental control equipment, when integrated with a BMS featuring active balancing, ensures that thermal conditions remain uniform, further enhancing capacity retention.

Conclusion

For long‑string battery packs, active balancing offers a clear advantage in energy efficiency and usable capacity. Switched‑capacitor topologies provide a good balance of cost and performance for moderate strings, while inductor‑based and transformer‑based designs suit high‑power or safety‑critical applications. Although passive balancing remains viable for small, low‑cost packs, the total cost of ownership (TCO) often favors active solutions when capacity gains, thermal management, and lifespan are considered. Engineers should evaluate the topology efficiency, balancing current, and system integration with equipment such as EJER Tech's precision environment controllers to achieve optimal battery performance.