BMS-11 — Cell Balancing

1. The Weakest Cell Decides the Pack’s Fate

Series-connected cells end up with different SOC/voltage values because of manufacturing tolerances, temperature differences, and uneven aging. Here’s the problem: during charging, the highest-SOC cell hits the upper voltage limit first, and charging has to stop there — even if the other cells aren’t full yet. The reverse happens during discharge: the lowest-SOC cell hits the lower limit first.

In a 96-cell string, if 95 cells could reach 100% SOC but one hits its upper limit at 97% due to manufacturing tolerance, charging stops there and the whole pack only fills to what that “97% cell” allows — the remaining 95 cells’ potential capacity goes to waste. In other words, the pack’s usable capacity is set by its most imbalanced cell.

2. Passive or Active?

Passive balancing turns a high-SOC cell’s excess energy into heat through a resistor (bleed resistor) — energy isn’t transferred, it’s spent:

I_bal = V_cell / R_bleed
P_loss = V_cell × I_bal = V_cell² / R_bleed

With V_cell = 4.1 V and R_bleed = 33 Ω, I_bal ≈ 124 mA and P_loss ≈ 0.51 W per cell. That looks small, but the total heat from balancing many cells at once has to be accounted for in the thermal design.

ASSUMPTION — R_bleed = 33 Ω is a representative value; the real resistance is chosen based on the target balancing current and the heat budget.

Active balancing transfers energy from a high-SOC cell to a low-SOC one instead (capacitor-, inductor-, or transformer-based). More efficient and faster, but the hardware/control is far more complex and expensive.

There’s no “best” — the choice depends on cost, efficiency, and pack requirements. Passive balancing is common in production, because in most designs the imbalance is already small and low-current passive balancing is enough.

3. When Does Balancing Start and Stop?

Balancing kicks in once the voltage/SOC difference between cells crosses a threshold, and stops once the difference falls back below it. The threshold isn’t arbitrary: if the measurement chain’s accuracy budget is ±5 mV (BMS-03), the threshold needs to sit clearly above that (30-50 mV, say) — otherwise measurement noise alone can trigger and stop balancing even with no real imbalance (oscillation).

Because passive balancing generates heat, it adds to warming at high temperature; the BMS coordinates it with thermal management (BMS-12), keeping it within a temperature window.

4. The Target Is SOC, Voltage Is Just a Proxy

The goal is to equalize SOC — voltage is only a proxy for it. Two cells can show the same OCV but behave differently under load if they have different internal resistances. And in chemistries with a flat OCV-SOC plateau like LFP, voltage can barely move while SOC swings across a wide range — balancing based purely on voltage can miss the real SOC gap in that case.

5. When Things Go Wrong

A balancing MOSFET stuck closed (continuous discharge, unwanted heating), an open-circuit bleed resistor (balancing becomes ineffective), and mismatched cell wiring are typical faults. The BMS can catch these by comparing the balancing current/effect against expected behavior (plausibility).

6. How It’s Verified in Production

EOL testing confirms each channel is wired to the correct cell and produces the expected balancing current. HIL testing injects MOSFET-stuck-closed/open-circuit scenarios to confirm the BMS detects these faults (BMS-19, BMS-20).

7. How It Connects to Other Systems

The balancing decision relies on cell voltage measurement (BMS-03) and SOC estimation (BMS-08); it operates in coordination with thermal management (BMS-12), and it’s monitored by fault management (BMS-13).

Interactive Tool

Use the Cell Balancing Calculator below — enter the voltage/SOC difference between cells and a resistor value, and it computes the balancing current, duration, and heat dissipation.

Summary

• Passive turns energy into heat, active transfers it; both aim to reduce the SOC gap.
• The threshold should sit well above measurement noise; temperature requires balancing to be coordinated with thermal management.
• The target is SOC equalization — voltage is only a proxy.

Sources

• Gregory L. Plett, Battery Management Systems, Volume II — balancing.