Cell balancing in a lithium BMS — passive versus active, and why it is a safety function

The guide on UPS battery types — VRLA and lithium covers the difference between the two technologies at system level. This article goes one level deeper: how the Battery Management System (BMS) of a lithium battery pack continuously balances the individual cells in a series string, and why that function — unlike with a VRLA battery, where balancing hardly plays a role — is a safety-critical function for lithium rather than merely a means of maximising usable capacity.

Why cells in series drift apart

A lithium battery pack consists of tens to hundreds of cells connected in series to reach the desired system voltage. Even cells of the same type and the same production batch always drift slightly apart in practice in terms of individual cell capacity, internal resistance and self-discharge. Over repeated charge/discharge cycles this difference gradually grows: the cell with the smallest capacity is the first to reach its maximum cell voltage on charging, and the first to reach its minimum cell voltage on discharging — while the remaining cells in the series string still have margin left. Without correction, that single "weakest" cell therefore limits the usable capacity of the entire pack, and — more importantly — that cell runs the risk of overvoltage on continued charging, which for lithium cells can be a direct trigger for thermal runaway.

Passive balancing: bleeding off excess energy as heat

Passive balancing (resistive balancing) switches a small resistor in parallel across cells approaching the upper limit of their voltage window, bleeding off the excess charge as heat until the cell voltage again matches the other cells in the string. This is the simplest and cheapest implementation, but has two inherent drawbacks: the bled-off energy is entirely lost as heat (rather than being used elsewhere in the pack), and the balancing current is typically limited to tens to hundreds of milliamps — with a large capacity difference between cells, passive balancing can therefore be too slow to fully correct within a single charge cycle.

Active balancing: transferring energy instead of wasting it

Active balancing moves charge from higher-voltage cells to lower-voltage cells — via a switching converter (capacitive or inductive energy transfer) — instead of bleeding the excess energy off as heat. This is significantly more efficient and can operate with higher balancing currents, which is particularly relevant for large packs with a substantial number of cells or a large capacity difference between cells (for example after partial cell ageing). The trade-off is significantly greater complexity and cost of the BMS compared with a passive implementation.

Why this is primarily a safety function

For a lithium cell, structural exceedance of the maximum cell voltage leads to accelerated electrolyte decomposition and, under sustained overvoltage, to thermal runaway — a self-reinforcing exothermic reaction that can no longer be controlled by normal cooling. Cell balancing prevents a single, weaker cell in a series string from reaching a dangerous overvoltage during charging while the pack voltage as a whole still appears to be within normal limits — a pack voltage measurement alone does not reveal an individual cell overvoltage. IEC 62619 (safety requirements for lithium cells and batteries for industrial applications) and IEC 63056 (safety requirements specifically for lithium systems for stationary energy storage) therefore impose requirements on both per-cell voltage monitoring and the functioning of the balancing function as part of overall battery safety, not as a convenience feature for capacity retention.

Practical relevance

When assessing a lithium BMS (in a UPS, home battery, or electric vehicle), it must be checked whether the system monitors individual cell voltages (not solely the pack voltage), whether a balancing alarm or fault is logged and reported as a separate event, and whether the balancing power (passive or active) is matched to the size and expected capacity spread of the specific pack — an undersized passive balancing system on a large pack with strongly diverging cell ageing can structurally lag behind in restoring balance.

Common mistakes

Monitoring only the total pack voltage without monitoring individual cell voltages — a single cell can reach a dangerous overvoltage while the pack voltage as a whole still appears normal.
Regarding cell balancing as merely a capacity optimisation rather than a safety function — a long-failed balancing function increases the risk of cell overvoltage on further charge cycles.
Ignoring a balancing fault because the pack still "functions normally" — the degradation of balance between cells is typically gradual and only becomes noticeable in usable capacity over a longer term, while the underlying safety risk is already present earlier.
Treating passive and active balancing as functionally equivalent when assessing a large pack — with a substantial capacity difference between cells, a passive system can be structurally too slow to correct within a charge cycle.