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The purpose of this article is to provide a rigorous, step-by-step method to diagnose and fix battery cell balancing failures in lithium-ion packs used in EVs, ESS, and industrial applications.
1. Understand How Balancing Works
Cell balancing equalizes state of charge across series cells so pack limits are not dictated by the weakest cell.
Passive balancing. Bleeds energy from high cells through a resistor and a switch until they match the lower cells.
Active balancing. Moves charge from higher cells to lower cells using inductive or capacitive transfer circuits to reduce heat and energy loss.
Caution: Never force-balance or jumper cells across the harness. High transient currents and wiring damage can occur.
2. Common Symptoms and What They Mean
| Observed Symptom | Likely Root Cause | First Diagnostic |
|---|---|---|
| One cell stays ~30–80 mV above others at full charge. | Bleed resistor open or MOSFET not switching. | Measure balance-node voltage drop when balancing flag is active. |
| Balancing reported active yet no temperature rise at shunt. | ADC miscalibration or disabled channel in firmware. | Inject known reference voltage and compare reading to meter. |
| Balancing stops early at mid SOC. | Incorrect start or stop thresholds or thermal derate. | Read BMS parameters for Vstart, Vstop, Tlimits, and timeout. |
| Balancing oscillates every few seconds. | Hysteresis too tight or high harness resistance. | Check hysteresis setting and measure line resistance end to end. |
| Only a block of cells never balances. | Cell monitor IC or daisy-chain comms fault. | Scope the iso-SPI or UART chain and verify frame CRC. |
| Pack hits OVP or UVP before completion. | Severely imbalanced or degraded cell capacity. | Run capacity test on outlier cell and compare to nameplate. |
3. Quick Calculations to Set Expectations
Estimate whether the hardware can realistically correct the imbalance in the available time.
# Passive bleed current and time I_bleed = V_cell / R_bleed # Example: 4.1 V cell, 100 Ω resistor → 41 mA. # Time to remove ΔAh from one high cell: t_hours ≈ ΔAh / I_bleed(A)
Using voltage delta at similar SOC:
ΔAh ≈ C_nominal(Ah) × (ΔV / (dV/dSOC near current SOC))
Rule of thumb near 3.6–3.9 V for NMC: ~1% SOC per 10–12 mV per cell.
Example: 60 Ah pack, ΔV=60 mV → ≈5–6% SOC → 3.0–3.6 Ah.
With 41 mA bleed → 3.2 Ah / 0.041 A ≈ 78 h.
Caution: Passive balancing above 30–50 mA cannot quickly correct multi-ampere-hour deltas. Expect multi-day equalization if ΔSOC is large.
4. Step-by-Step Diagnostic Workflow
4.1 Verify Measurement Integrity
Calibrate the voltage measurement path first. A bad reading masks true behavior.
1. Disconnect charger and loads. Let the pack rest for 30 minutes to reduce polarization. 2. Measure each cell with a calibrated DMM at the sense pins. 3. Compare to BMS-reported voltages. Note offset per channel. 4. If offset >±5 mV typical spec, run ADC calibration procedure or update gain/offset registers. 5. Inspect sense-line continuity and connectors for corrosion or loose crimps. 4.2 Check Balancing Hardware Paths
1. Command balancing ON for a known high cell at safe SOC (e.g., 40–60%). 2. Probe across the bleed resistor. Confirm gate drive at the MOSFET. 3. Record I_bleed = V_cell / R_bleed. Verify within ±10% of design. 4. Use a thermal camera. A balancing cell's shunt should warm by 3–10°C in minutes. 5. If no heating, test the FET and resistor for opens and check the driver enable pin. 4.3 Validate Firmware Thresholds and Logic
1. Read parameters: V_start, V_stop, ΔV_target, hysteresis, T_low/high, timeouts. 2. Confirm start triggers only above minimum SOC and within cell temperature window. 3. Increase hysteresis to 10–20 mV to prevent chattering in noisy packs. 4. Set ΔV_target to 5–10 mV for EV-grade packs or as required by use case. 5. Confirm balancing disable on charger present if your topology requires it. 4.4 Assess Pack and Cell Health
1. Run a controlled charge/discharge cycle at C/10 and log cell voltages. 2. Identify chronic outliers. Calculate capacity via Coulomb counting. 3. If capacity loss >20% on any cell, replacement is indicated. 4. Check cell internal resistance via DCIR or pulse test. Compare to spec. 5. If DCIR spread >30% among cells, balancing will struggle. Investigate aging and temperature gradients. 5. High-Leverage Fixes by Failure Mode
| Failure Mode | Corrective Action | Design Improvement |
|---|---|---|
| Open bleed resistor or MOSFET failure. | Replace component. Verify gate driver and snubbers. | Use higher power resistors, derate to 50–60%, and include current sense. |
| ADC offset or gain error. | Recalibrate with precision references. | Add periodic self-cal routine and stored coefficients per channel. |
| Harness resistance and ground bounce. | Rework crimps and route returns properly. | Use twisted pairs, Kelvin sense, and star grounds near monitor ICs. |
| Balancing disabled during charge. | Allow charger-coordinated balancing or enable at taper current. | Implement CAN handshake between charger and BMS for safe co-balancing. |
| Thermal derate halts balancing. | Improve airflow or heat sinking of shunts. | Place shunts on copper pours with thermal vias and monitor local NTCs. |
| Cell capacity mismatch. | Rebin or replace weak cells. | Match cells by capacity and DCIR at build. Enforce tighter screening bins. |
| Active balancer transfer fault. | Test inductor paths and driver timing. Replace faulty channel. | Add per-channel fault flags and soft-fuse monitoring. |
6. Parameter Settings That Work
Use these starting points and tune with data from your pack.
| Parameter | Typical Value | Rationale |
|---|---|---|
| V_start for balancing. | Cells >3.70 V and ΔV >15 mV. | Avoid low-SOC noise and start when voltage maps to SOC reliably. |
| V_stop or ΔV_target. | ΔV ≤5–10 mV. | Minimizes overwork and heat while achieving practical equality. |
| Hysteresis. | 10–20 mV. | Prevents rapid on and off cycles under ripple. |
| Thermal limits. | Start <45°C, stop >55°C local NTC. | Protects shunt components and cells. |
| Bleed current. | 30–100 mA passive. 0.5–3 A active transfer. | Balance time versus heat and cost trade-off. |
7. Verification Test After Fix
1. Charge pack to 80% SOC. Hold at rest for 20 minutes. 2. Enable balancing with logged telemetry: V_cell_i, T_shunt_i, I_bleed_i, flags. 3. Confirm each targeted channel heats modestly and ΔV decays exponentially. 4. Continue until ΔV ≤10 mV for 30 minutes without oscillation. 5. Perform a discharge at C/5 and verify ΔV remains ≤15 mV across 20–80% SOC window. 6. Save parameter set and calibration data with date and pack ID. 8. Prevent Recurrence
Specify matched cells and enforce incoming inspection for capacity and DCIR bins.
Derate shunts and MOSFETs by temperature and ensure airflow over the balancing zone.
Schedule periodic recalibration with a traceable reference module.
Log ΔV, ΔT, and balancing duty cycle over life to detect drifts early.
Coordinate the charger to allow balancing during taper for faster equalization.
FAQ
Is passive or active balancing better?
Passive is simpler and cheaper for small ΔSOC corrections. Active is better when capacity spreads are large or turnaround time matters.
How much heat should I expect during passive balancing?
Power is approximately P = I_bleed × V_cell. A 50 mA bleed at 4.0 V dissipates about 0.2 W per active channel.
Can I balance at low temperatures?
Reduce or disable balancing below 0°C. Lithium plating risk increases and component ESR rises.
What ΔV is acceptable in operation?
Keep ΔV within 10–15 mV in the main SOC window and within 5–10 mV at top-of-charge for EV-grade packs.
Why does balancing stop when the charger connects?
Some BMS block balancing under charge to avoid measurement error. Enable taper-mode balancing with correct logic and safeguards.