By ESS news
Germany and the rest of the European continent were in an intensifying heat wave for several days pv magazine Focus 2026 convened in Munich on June 24. The heat lent a degree of urgency to a theme that runs through much of the session’s discussion: how battery systems, and the warranties behind them, hold up once conditions become extreme.
Factory audits routinely reveal battery quality issues at every stage of production, including at top suppliers, said Garikoitz Sarriegi Etxeberria, head of converter and storage services at German engineering consultancy Kiwa PI Berlin, in the opening portion of the event.
Sarriegi Etxeberria argued that the greater risk often lies not in what manufacturers test, but in what they do not test, citing thermal management systems that are only validated for cooling performance, with heating performance often left unchecked in cold conditions.
He said cell-level voltage and temperature data collected from battery management and supervisory control systems during capacity testing often reveal abnormal distribution patterns across large battery packs.
Sarriegi Etxeberria added that these deviations could affect health status and lifetime performance, and advised buyers to contractually require full disclosure of this data before commissioning, given the 20-year operating horizon involved.
Warranty risk
The panel discussion that followed described an incident where it took six months to replace a damaged battery part, raising the question of whether the costs of the delay should be shared equally between supplier and buyer. The panelists said the answer generally depends on what is already written into the contract, including provisions for damages and insurance coverage. Without these protections, they said, the buyer continues to bear the brunt of the loss.
Johanna Bonilla, BESS senior business development manager EU for Shanghai-based JinkoSolar, said during the first panel discussion of the event that the Chinese company produces some of its own cells, but also qualifies third-party suppliers to the same quality standards. She said the proper integration of cells into a complete system is as important as cell quality itself.
Bonilla also argued that JinkoSolar’s approach of controlling full system integration gives customers a single point of warranty responsibility. She said the company has six spare parts warehouses across Europe to support faster replacement time under the service agreements.
Bonilla said she regularly sees vendors offering guarantees of system availability of 99% or higher, which she considers unrealistic over the life of a project. She distinguished between system uptime and usable energy delivered, arguing that the latter is the most meaningful metric: a system can technically be online and still not meet the energy output required by a toll or offtake agreement.
CEGASA Energía CEO Iñigo Atutxa described the Spanish company’s supplier verification process, which includes batch cell sampling, individual cell voltage testing and cycle testing before approving new suppliers. He said inconsistent quality control practices exist among battery manufacturers, ranging from those who inspect components daily to others who only do so monthly.
Atutxa also raised the issue of replacement economics at the rack level, noting that a single underperforming cell can sometimes force the replacement of an entire rack’s capacity rather than the cell alone, depending on how the system is integrated. He said this risk can be mitigated through integration design choices made before a project is built.
Financial strength
Tim Koenemann, global head of green infrastructure finance at German commercial lender Commerzbank, said lenders rely heavily on independent technical advisors during project development, construction and commissioning, noting that contractual protection against underperformance is more important for financiers than directly assessing supplier quality. He said the financial strength and credit ratings of counterparties are crucial in credit decisions as performance guarantees can last 10 to 15 years or more.
Koenemann said he advises against concentrating an entire storage portfolio with a single integrator, citing a hypothetical 500 MW portfolio as an example of a position better spread across multiple suppliers to limit counterparty risk. He reiterated Bonilla’s distinction between uptime and usable energy, saying the latter is what ultimately determines whether the terms of a tolling agreement are met.
Stephan Rohr, CEO of battery analysis software provider TWAICE, which monitors the performance of more than 130 utility-scale storage projects, said cell-related problems represent about 10% of serious incidents recorded in a database maintained jointly with a research institute.
“My experience over the past few years is that energy storage systems are high-touch systems,” says Rohr.
He explained that this means that even a system that passes site acceptance testing well can still deteriorate significantly if it is not actively and proactively managed afterwards. Small problems like a blocked filter or an unreplaced weak cell tend to worsen over time instead of resolving on their own.
Rohr also noted that approximately 45% of these serious incidents are due to control and communication errors between power management and battery management systems, with the remainder related to components such as HVAC systems.
Fragmented certification
In a separate presentation, Bonilla outlined JinkoSolar’s energy storage activities, which has been operating as a dedicated unit since 2022; in 2024 it became a leading battery manufacturer. She said the company has supplied to 50 countries and operates nine service centers around the world, including one soon in Italy.
Bonilla said certification requirements for BESS remain fragmented across Europe, with different standards applied at cell, battery, system and installation levels, and no single, uniform certification is accepted for all markets. She said EU regulations under Annex V – which outlines safety parameters for stationary battery energy storage systems – focus on risk management rather than prescribing a single certification standard. This allows manufacturers to choose from multiple accepted testing methods, depending on the requirements of individual banks, insurers or network operators.
Bonilla added that some markets are imposing additional national requirements on top of EU rules, citing Germany’s BVES warehousing association certifications as an example.
Benjamin Callam, chief product officer at California-based PVFARM, said battery storage site planning functions less as a pure engineering exercise and more as a collaborative decision-making process involving engineers, developers and engineering, procurement and construction (EPC) contractors, each of which could define a project’s success differently.
He said an anonymized customer told the company that “decisions fail when teams optimize individually.”
Callam described leaving room for future capacity expansion as one of the “dark horses” of battery storage planning, because variable behind-the-meter returns make it difficult to know in advance how much additional capacity might be worth later. He said planning decisions made early – including space reserved for future expansion – can maintain flexibility as the market economy evolves. He said constructability constraints, including crane access and site geometry, are often overlooked in early-stage planning, especially in retrofit projects that add storage to existing solar sites with limited available space.
Focus on co-location
Peik Uhr, senior business development manager technical consulting for German solar software provider Meteocontrol, said the company’s services include support, revenue forecasting and sizing assessments for both standalone and co-located battery storage projects. He said Meteocontrol’s revenue models combine third-party yield assessment software with internal technical calculations to project revenue into the wholesale, balancing and ancillary services markets.
Uhr said Augsburg-based Meteocontrol can also estimate optimal battery capacity and power size for retrofit projects at existing solar sites without conducting detailed technical engineering. He said the company is working to incorporate location-specific data on the likelihood of curtailments into its revenue models to better account for grid connectivity constraints, although he said a reliable database does not yet exist to support forward-looking forecasts of curtailments.
Stefano Alberici, VP technology EMEA for Chinese industrial battery storage integrator HyperStrong, says most developers approach the company with fixed technical specifications already established, although a minority seek advice on optimal system configuration. He said projects involving customers who are open to revising their original assumptions tend to become longer, more involved discussions, but often result in better-fitting systems and more satisfied customers. He mentioned a newly launched battery unit in containers weighing about 24 tons, compared to about 44 tons for a standard 20-foot container, which will facilitate compliance with weight limits for European road transport.
Kai Klingenhagen, head of BESS for German solar developer Belectric, said standalone storage still accounts for the majority of projects, but co-located solar-plus-storage configurations are becoming more common as negative wholesale prices and grid constraints push developers toward DC-coupled hybrid designs. He described the availability of grid connections, especially in Germany, as the main constraint shaping this shift.
Concluding the panel, speakers agreed that whether a project is delivered under a single turnkey EPC contract or split into separate balance-of-plant agreements, reducing the number of contractual interfaces remains critical to keeping battery storage projects viable.
