BMS cell balancing — passive versus active balancing in lithium battery systems, and what it means for inspection

The lithium-ion BESS thermal-runaway/fire-safety guide covers what happens once a cell has already failed. This article covers an earlier-stage, preventive function of the battery management system (BMS) that exists precisely to reduce the likelihood of a cell ever reaching that failure point through overcharge: cell balancing.

Why balancing is needed at all

A lithium battery pack is built from many individual cells connected in series to reach the required system voltage. No two cells are perfectly identical: small manufacturing variations in capacity and internal resistance, combined with slightly different temperatures across the pack, mean that during charge and discharge cycles individual cells drift apart in state of charge (SoC) over time. Left uncorrected, this drift compounds: the highest-SoC cell in the string reaches its upper voltage limit before the others, which either forces the whole string's charging to stop early (wasting the capacity of the lower cells) or, if charging continues without the BMS intervening, pushes that individual cell into overvoltage — the precondition for accelerated degradation and, in the worst case, thermal runaway. Cell balancing exists to keep the whole string's cells close enough in SoC that neither of these outcomes occurs.

Passive balancing

Passive (resistive bleed) balancing is the simpler and by far more common approach in cost-sensitive applications. Each cell has a small bleed resistor and a switch (typically a MOSFET) controlled by the BMS; when a cell's voltage rises above its neighbours during charging, the BMS switches its bleed resistor into circuit, dissipating the excess charge as heat until that cell's SoC falls back in line with the rest of the string. Typical bleed currents are modest, commonly in the tens of milliamps range, which means passive balancing corrects drift slowly, over multiple charge cycles, and only during charging (it does nothing to correct imbalance that has developed during rest or discharge). Because it discards energy as heat rather than moving it, passive balancing is inherently lossy, but its simplicity, low component count, and low cost make it the default choice for many consumer and light industrial lithium packs.

Active balancing

Active balancing instead moves charge from higher-SoC cells to lower-SoC cells, using inductive elements (flyback or buck-boost converter topologies), capacitive charge-shuttle circuits, or small DC-DC converters between adjacent cells or from each cell to a shared bus. Because energy is transferred rather than burned off, active balancing can correct significantly larger imbalances and can do so faster, and it can operate during discharge as well as charge — an advantage in applications like electric vehicles or stationary storage where cycling is frequent and imbalance can develop continuously. Active balancing hardware is more complex and more expensive per cell than a simple bleed resistor, and its own switching components introduce an additional potential failure mode that a BMS's diagnostic functions must monitor, but well-implemented active balancing achieves markedly better efficiency (commonly cited in the 85-95% range for the energy actually redistributed) than the reject-as-heat approach of passive balancing.

What this means during inspection

For anyone inspecting or maintaining a stationary or mobile lithium battery system under NEN 3140-style periodic inspection, the balancing architecture is not just a specification detail — it shapes what a developing fault actually looks like. On a passively balanced pack, a cell that will not stay in step with its neighbours despite repeated balancing cycles (rising SoC divergence, or the BMS log showing that particular cell's bleed resistor active on almost every charge cycle) is an early warning sign of that cell's declining capacity or rising internal resistance, well before it becomes a safety event. On an actively balanced pack, unexpected imbalance growth despite the balancing circuit actively working points either to a failing cell that active balancing genuinely cannot keep up with, or to a fault in the balancing hardware itself (a failed switching element or converter channel) — a distinction that matters for whether the correct response is cell-level investigation or BMS-hardware investigation.

Practical relevance

Reviewing BMS balancing logs and any available per-cell voltage history — where the monitoring system provides it — as part of periodic inspection of stationary battery installations gives an early indication of cell-level degradation long before a voltage or temperature alarm is triggered, and helps distinguish a normal, gradually converging balancing cycle from a persistent, worsening imbalance that warrants escalation to the battery supplier or a cell-replacement decision.

Common mistakes

Assuming any lithium pack with a functioning BMS is self-correcting regardless of imbalance severity — passive balancing in particular has limited bleed current and cannot correct large or rapidly developing imbalances within a normal charge cycle.
Not checking which balancing topology is fitted before interpreting SoC-divergence symptoms, since the same symptom (one cell drifting from the others) points to different likely causes and different urgency depending on whether the system is passively or actively balanced.
Treating balancing-circuit activity as inherently abnormal, when modest, cycle-to-cycle balancing operation is the normal, expected behaviour of a healthy pack; the abnormal pattern is persistent, worsening, or unusually large imbalance, not balancing activity itself.
Overlooking that passive balancing only operates during charging when diagnosing a pack that shows imbalance mainly appearing after deep discharge cycles — that pattern is expected for passive systems and is not, by itself, evidence of a fault.