How Steag Iqony Group used AI software to detect faulty battery cells before issues arose

Iqony, part of the Steag Iqony Group and a European leader in utility-scale energy storage, had been operating a battery energy storage system (BESS) in southwest Germany for six years when it received alarming news: two battery cells were so badly degraded that the risk of a safety incident was growing by the day. If the cells were not removed, they would be at risk of short-circuiting, which could potentially lead to a thermal runaway event. The BESS battery management system (BMS) did not flag the faulty cells or shut down the racks automatically.  

This is where Iqony's robust security concept and its partnership with ACCURE Battery Intelligence have proved their worth. ACCURE detected weakened battery cells during an initial review of the system in September 2023 and promptly alerted Iqony, enabling early intervention. Using its AI-powered predictive battery analytics software, ACCURE analyzed a year’s worth of historical BESS data and accurately identified the two heavily degraded battery cells within the system. One cell’s capacity was severely reduced and was showing significantly stronger voltage responses than the other cells, due to a substantial loss of active material. Iqony responded swiftly to the alerts, removing the two faulty modules containing the faulty cells—a proactive action to mitigate thermal risk.  

What went wrong?
Internal short circuits in battery cells occur when the positive and negative electrodes unintentionally come into contact, creating a direct electrical connection within the cell. This new connection bypasses the normal circuit, leading to high currents, heat generation, and potentially dangerous situations like thermal runaway. Internal short circuits can be caused by manufacturing defects, dendrite growth, or physical damage to the cell. In this case, the ACCURE team suspected dendrites were the cause.

Dendrites are unwanted needle-like metal structures that form on the anode during charging when metal ions are unevenly deposited onto the surface of a battery cell’s negative electrode. In addition to short circuits, dendrites also cause reduced battery capacity and lifespan. Additionally, ACCURE suspected electrolyte loss in one of the cells due to a pronounced impedance increase. Analysis of temperature gradients of this cell further supported this suspicion, indicating abnormal internal resistance and thermal behavior.

The site’s BMS likely missed the faulty cells because it was looking at the average cell voltage across each battery rack, rather than looking for outliers within each rack.

By contrast, ACCURE’s software looked at the maximum and minimum cell voltages and temperatures in each battery rack, enabling the team to see outlier cell voltages that were misaligned with the others and identify the faulty cells.

Another reason the BESS’ BMS likely missed the warning signs of the faulty cells was the way the BESS was being operated as a frequency containment reserve (FCR) resource. For this particular use case, the system typically cycled only a small portion of its capacity, with the site-wide state of charge (SOC) hovering near 50%. Because the system wasn't using its full charging capacity, and because the BMS was not sophisticated enough to detect single battery cell issues when the site was being run only at partial capacity, the weak cells remained hidden from both the BMS and Iqony.

Iqony replaces the faulty modules and implements continuous battery monitoring
Iqony sent the defective modules back to the supplier, who confirmed the faults ACCURE had identified, providing a detailed report including pictures of the modules and cells. The inspection revealed significant gassing of the affected cells, with one cell having ruptured due to excessive internal pressure.

This rupture caused electrolyte leakage, which, in this instance, did not result in further damage. However, leaked electrolyte may corrode electrical connections and compromise insulation, increasing the likelihood of short circuits between cells or modules. Fortunately, Iqony had spare modules on site that had aged similarly in terms of calendar age, allowing for a seamless replacement. As a result, the affected rack was brought back online quickly, minimizing downtime. This swift and well-prepared response highlights Iqony's proactive approach to asset management and its commitment to operational excellence.

Impressed with the safety benefits and cost savings from using predictive analytics technology, Iqony followed through on ACCURE’s recommended corrective actions and engaged ACCURE to continuously monitor the BESS project and five additional BESS plants under a long-term contract.

Since the initial review, while continuously monitoring Iqony's 140 MWh BESS portfolio, ACCURE’s predictive analytics software has identified several additional battery cells in need of replacement and modules in need of rebalancing, and has helped Iqony navigate preventive maintenance priorities to maintain system health and reliability, and extend useful battery life as much as possible.

Pivoting from one use case to another, while focusing on safety

Based on the expectation that BESS assets will be valuable in the future, not only in the FCR market but also in the short-term power markets, Iqony now wants to change its approach at the BESS site. Instead of operating the system as an FCR resource as in years past, Iqony plans to use the battery system’s full capacity to maximize the asset’s value over the coming years. Using a cross-market approach to adjust the use case, Iqony's asset management team is in close coordination with ACCURE’s predictive analytics team to make sure the batteries are operated within limits that will extend battery performance and lifetimes as much as possible.

ACCURE plans to keep a close eye on BESS’ aging battery cells, watching for imbalances and asymmetric degradation of the system’s battery cells.

Together, Iqony and ACCURE aim to keep the BESS assets operating and supporting the grid well past their 10-year warranty and establish new protocols for operating systems with aged battery cells to increase sustainability and set new standards in the energy storage industry.