Assessment: SOH Due Diligence for 8 MWh Used Storage System — Purchase Price Reduction of EUR 340,000 Recommended

Assessment type: Valuation assessment / Due diligence System size: 8 MWh / 4 MW NMC (second life) Region: North Rhine-Westphalia Period: Feb – Apr 2025

Assessment summary:

Planned purchase of an 8 MWh NMC storage system (3 years prior operation in frequency regulation market, 2,840 full cycles)
Seller declares SOH 87% — independent testing reveals actual SOH of 79.3%
Impedance spectroscopy (EIS) shows increased internal resistance: +42% compared to new condition
Remaining life prognosis: 4.2 years to EOL (instead of 6.8 years per seller's claim)
Recommended purchase price reduction: EUR 340,000 (difference in useful life + capacity deficit)

Why was an independent SOH assessment required?

A municipal utility in North Rhine-Westphalia planned the acquisition of a used 8 MWh container-scale storage system (NMC technology, 4 MW rated power, first commissioned in 2022) for deployment in the frequency regulation market (FCR/aFRR). The seller (project developer) declared an SOH of 87% based on BMS-internal data. Given the transaction volume (purchase price: EUR 2.1 million), the buyer commissioned an independent SOH assessment.

Test program (4 days on-site)

Day	Test	Method
1	Reference capacity test	Full discharge 0.33C at 25 °C (climate-controlled), 3 cycles
2	Impedance spectroscopy (EIS)	10 mHz–10 kHz, sample of 48 cells from 4 containers
3	Power test	Pulse load 1C / 30 s, voltage drop analysis
4	BMS log forensics	Operating history: cycles, temperatures, deep discharge events

What did the independent SOH measurement reveal?

Capacity test (reference measurement)

Container	Nominal capacity	Measured capacity	SOH
Container 1	2,000 kWh	1,612 kWh	80.6%
Container 2	2,000 kWh	1,574 kWh	78.7%
Container 3	2,000 kWh	1,598 kWh	79.9%
Container 4	2,000 kWh	1,562 kWh	78.1%
Total	8,000 kWh	6,346 kWh	79.3%

Deviation from seller's declaration: The seller's declared SOH of 87% was based on the BMS-internal SOH algorithm, which uses only Coulomb counting data and is known to overestimate in NMC cells after >2,000 cycles. The difference between the declared (87%) and actual SOH (79.3%) amounts to 7.7 percentage points — corresponding to a capacity difference of 616 kWh.

Impedance spectroscopy (EIS)

The EIS measurement on 48 representative cells showed:

Parameter	New value (datasheet)	Measured (average)	Increase
Ohmic resistance R0	0.8 m ohm	1.14 m ohm	+42%
Charge transfer resistance Rct	1.2 m ohm	2.1 m ohm	+75%
Warburg impedance (diffusion)	0.4 m ohm s^0.5	0.7 m ohm s^0.5	+75%

The increased impedance correlates with the capacity degradation and indicates SEI layer growth and loss of active lithium — typical aging mechanisms in NMC after intensive cycling operation.

Remaining life prognosis

Based on the semi-empirical degradation model (Arrhenius + cycle aging), continuing the previous operating profile (1.2 full cycles/day, FCR) yields:

Scenario	Remaining life to EOL (70% SOH)
Seller's claim (SOH 87%)	6.8 years
Expert prognosis (SOH 79.3%)	4.2 years
Conservative (worst case)	3.4 years

What purchase price reduction was recommended?

Valuation and price recommendation:

Factor	Value reduction
Capacity deficit 616 kWh x market value	EUR 154,000
Shortened remaining life (2.6 years less x annual revenue)	EUR 156,000
Increased degradation rate (risk premium)	EUR 30,000
Recommended purchase price reduction	EUR 340,000

Recommendation: Reduce purchase price from EUR 2.1 million to a maximum of EUR 1.76 million — or withdraw from the purchase.

Expert assessment: This case illustrates a growing problem in the secondary market for BESS: BMS-internal SOH values are not a reliable basis for transactions. The 7.7 percentage point deviation is typical for NMC systems after intensive cycling operation. An independent capacity test with EIS validation should be standard for every BESS transaction.

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Last updated: 2026-06-16 | Author: Christoph S. Prestele, TUV-certified expert assessor | PV-BESS-Assessor.com