Lucas Koltermann

Flooded lead–acid batteries continue to operate in utility-scale battery energy storage systems (BESSs), yet their long-term degradation under grid-frequency regulation remains poorly characterized by field evidence. This paper presents an eight-year, multi-indicator degradation analysis of a flooded Ortsfeste Kupferstreckmetall (OCSM) lead–acid string at the M5BAT hybrid BESS facility (RWTH Aachen University), covering five years of active Frequency Containment Reserve (FCR) operation (2017–2021) followed by three years of low-utilization reserve operation (2022–2025). Four complementary health indicators are derived from a shared one-second-resolution operational dataset: DC internal resistance (DCIR), DC and AC round-trip efficiency, coulombic efficiency, and inverter efficiency. Five years of continuous FCR service produced no statistically detectable degradation across any indicator. In contrast, the transition to prolonged high-state-of-charge, low-throughput standby coincided with a pronounced increase in DCIR and a decline of about 14 percentage points in DC round-trip efficiency (from about 88% in the FCR phase to 74.0% in 2024). Consistent trends across all indicators, together with stable inverter efficiency, attribute the observed deterioration to the electrochemical system rather than the power electronics. These findings indicate that prolonged standby operation, rather than active FCR cycling, coincided with the onset of accelerated aging in this flooded lead–acid string. The published open-access dataset provides a valuable basis for future benchmarking and cross-technology comparisons for stationary battery storage.

Industrial battery storage at commercial and industrial grid connection points without on-site generation represents an underexplored flexibility resource. This paper presents a two-level controller stack built around a mixed-integer linear programme that, to the best of the authors’ knowledge, is the first to couple behind-the-meter services with continuous front-of-meter intraday trading and realistic battery simulation. We apply the framework to a broad techno-economic analysis of German industrial load profiles across two deployment scenarios: batteries procured for peak shaving that additionally participate in intraday trading, and pure trading batteries at industrial grid connection points without active behind-the-meter service. In the peak-shaving scenario, stacking intraday trading raises the mean profitability index to 2.43 at 2024 tariff and price levels, and no profile falls below break-even. The revenue streams are largely uncorrelated with capacity tariff shifts. For the pure trading scenario, granting the right to inject into the grid yields a profitability index above 2.0 for every profile studied. Under the stricter zero-injection constraint, full-load-hour analysis separates the sites into three regimes. Sites below approximately 1 500 h/year stay infeasible at every battery size. Sites with 1 500–3 200 h/year favour a 100kW/ 2 h configuration and reach 1.4–1.8; sites above 3 200 h/year sustain consistent 4 h cycles and converge to roughly 2.1. Replacing perfect price foresight with an adapted same-as-last-week predictor does not erode profitability; even in the peak-shaving analysis, where load prediction is critical, the realistic predictor retains on average 76% of the perfect-foresight value. The full-load-hour thresholds and sizing rules identified here give directly applicable guidance to industrial companies, battery project developers, and policymakers assessing the flexibility potential of existing industrial grid connections.

The rapid growth of battery energy storage systems in Germany poses a challenge for transmission grid congestion management: while grid operators require reliable schedules several hours before delivery to plan redispatch measures, batteries can continue trading on wholesale markets until shortly before real time. This schedule uncertainty forces grid operators to reserve additional redispatch capacity, increasing system costs. Schedule freezes, as regulatory instruments that limit permissible deviations from submitted schedules during grid-critical periods, offer a preventive approach to increase planning security for grid operators. This paper investigates how different policy designs affect battery operator revenues and grid congestion, proxied by redispatch requirements. We extend the open-source ISEA Battery Revenue Index to simulate schedule freezes affecting cross-market operation in frequency containment reserve, automatic frequency restoration reserve, and intraday continuous trading. Using data from 2024 for two stylized locations in northern and southern Germany, we analyze sensitivity to freeze lead time, freeze frequency, and tolerance margins. Results show that revenue losses compared to unrestricted operation remain modest, amounting to roughly 0.18 € per MW and frozen hour per year (≈-0.12%) at short lead times and increasing to about 3.40 € per MW (≈-2%) at 8-hour lead times under a 20% freeze share. The findings suggest that well-designed schedule freezes can substantially enhance planning security for transmission system operators while imposing only limited costs on battery operators, providing empirical guidance for regulatory design in congested power systems.

Large‐scale battery storage systems have become popular for various grid services in recent years. A worldwide market growth for battery storage has led to increased competition in several grid service markets. Modular large‐scale battery storage systems require a safe, highly available, and intelligent energy management system (EMS) in order to be economically competitive. One component of this EMS is the control for distributing the power requests between individual battery units of the large‐scale battery storage system. As the EMS is usually undisclosed intellectual property of the system manufacturers, there is only little information on real‐world operation available. To contribute, we present a rule‐based power distribution algorithm (SPDA) in this paper and validate it through field tests on a 6 MW/7.5 MWh system that is providing frequency containment reserve to the German power grid. The results show that especially when combining different battery technologies, the SPDA can exploit individual technological strengths. In this way, the state of charge of the batteries, energy throughput and power load of the batteries can be controlled to extend the lifetime. Moreover, the SPDA managed to shift nearly 80 % of the energy throughput to one battery unit to protect less cyclic stable batteries and make use of the advantage of cyclic stable battery technologies, while fulfilling all grid service requirements. By shifting those large quantities of the energy throughput to more cyclic stable battery units, the large‐scale battery storage system experienced in sum up to 45 % less cyclic aging with the SPDA than with a symmetrical power distribution algorithm. Furthermore, the operational efficiency of a large‐scale battery storage system can be significantly improved via additional software adaptations of the power distribution, depending on the system layout.

The expansion of renewable energy with its volatile feed-in character places higher demands on the power grid of the future. Large-scale storage systems (LSS) are a promising option for supporting the electricity grid and have been gaining importance in the last years, both on the market for frequency containment reserve (FCR) and in research. The majority of publications investigating the interaction between storage and FCR are based on simulations rather than on field measurements. This paper presents the analyses of multi-year, high-resolution field measurements of the hybrid 6 MW/7.5 MWh battery storage “M5BAT” to address this issue. The influence of FCR operation on the operation and degradation of the hybrid LSS and the individual battery technologies is investigated via a statistical evaluation of the historical operating data between 2017 and 2021. The data-based analysis of the LSS and the individual battery technologies reveals a high availability of the LSS of over 96.5%. Furthermore, the FCR operation results in an average SOC of the LSS of 50.5% and an average C-rate of the battery units of 0.081 C. A capacity test after four years of operation exposes that the lead-acid batteries have experienced a loss of energy capacity of up to 36%, whereas the lithium batteries have only experienced a loss of up to 5%. The calendar ageing predominates in this context. The presented results can be used to investigate and model the influence of FCR on the operation and battery degradation of the LSS and its different battery technologies.