In October 2024, the operator of a commercial battery storage system in the Stuttgart region commissioned an independent expert assessment. The system — a 500 kWh LFP unit with 250 kW rated power, commissioned in August 2023 — had been showing gradual capacity loss since June 2024, which was registered by the overarching energy management system (EMS) as reduced charge/discharge cycles.
The storage system was used for self-consumption optimization and PV surplus storage (coupled with a 320 kWp rooftop PV system). The operating data showed:
| Parameter | Target (datasheet) | Actual (Oct 2024) | Deviation |
|---|---|---|---|
| Usable capacity | 500 kWh | 385 kWh | -23.0% |
| Maximum discharge power | 250 kW | 192 kW | -23.2% |
| Round-trip efficiency | 94.5% | 87.3% | -7.2 pp |
| Full cycles (total) | — | 412 | Expected: <5% degradation |
The on-site investigation comprised three measurement days with the following program:
Evaluation of BMS protocols over 14 months of operation (approx. 2.3 million data points). From March 2024 onwards, increasing deviations in cell voltages within individual modules became apparent. The analysis of balancing events revealed that in Racks 3, 7, and 11, the passive balancing frequency was 4.7 times above the normal level — a clear indication of defective balancing boards.
Controlled discharge of all 12 racks at 0.5C with simultaneous individual cell voltage measurement. Results:
| Rack | Min. cell voltage (EOD) | Max. cell voltage (EOD) | Delta | Assessment |
|---|---|---|---|---|
| Rack 1 | 2.81 V | 2.84 V | 30 mV | ✓ Normal |
| Rack 3 | 2.52 V | 2.83 V | 180 mV | ⚠ Critical |
| Rack 7 | 2.61 V | 2.84 V | 155 mV | ⚠ Critical |
| Rack 11 | 2.58 V | 2.82 V | 168 mV | ⚠ Critical |
The voltage differences in Racks 3, 7, and 11 exceeded the manufacturer-defined threshold of 50 mV by a factor of 3 to 3.6. This indicates a systematic failure of the passive balancing function.
Opening the affected modules confirmed the suspicion: the balancing resistors on the BMS boards showed thermal discoloration and solder joint cracks. Serial number analysis revealed that all three defective boards originated from production batch Q3/2023.
Commissioning of the 500 kWh storage system, acceptance test without objections
First BMS warnings: "Cell Voltage Imbalance" in Rack 3 (initially interpreted by the operator as a sensor glitch)
EMS registers reduced usable capacity (-12%), warnings also in Racks 7 and 11
Capacity drops below 80% — manufacturer support recommends "firmware update" (without success)
Commissioning of independent expert assessment
On-site measurements, BMS board inspection, root cause determination
Assessment completed, warranty claim against supplier enforced
| Item | Amount |
|---|---|
| Lost revenue from self-consumption optimization (Jun–Dec 2024) | EUR 28,400 |
| Lost grid service revenue (frequency regulation) | EUR 19,800 |
| Replacement of 3x BMS boards incl. installation | EUR 8,200 |
| Recalibration and capacity test (3 days) | EUR 4,800 |
| Assessment costs | EUR 18,000 |
| Total damage | EUR 79,200 |
The assessment recommended the following to the operator:
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Request a consultationLast updated: 2026-06-16 | Author: Christoph S. Prestele, TUV-certified expert assessor | PV-BESS-Assessor.com