Cell-Level and Stack-Level Aging of 180 Ah Stationary Storage Lfp Lithium-Ion Cells Under Laboratory and Microgrid Operation
Authors: Yagci Mehmet C. , Feldmann Thomas , Bollin Elmar , Bessler Wolfgang G. 
Published: February 2020

Batteries are commonly used for energy storage in renewable energy systems. A microgrid is a small, independent or grid-connected energy system that enables the coupling and management of energy sources (photovoltaic, wind, grid), energy storage systems (batteries, supercapacitors) and different loads (water heating, electric car, etc.). Lithium-ion technology is today the most widely distributed battery technology for stationary energy storage. Unfortunately, the lifetime of state-of-the-art battery systems is considered still too low to make energy storage commercially viable. Aging on the cell level is a complex process depending strongly on operating conditions [1]. On the battery level, additional aging drivers can result from unbalanced cells or cells with different capacity.
In the present study we investigate the aging behavior of a stationary storage system consisting of two lithium-ion battery systems embedded in a microgrid consisting of renewable energy sources (photovoltaic panels and wind turbine), real electrical loads (electric car, offices) and load simulators [2]. 28 commercial 180 Ah high-energy LFP/graphite prismatic cells were assembled into two 8 kWh stacks with different architectures, that is, serial and parallel connection of the cells. The individual cells were characterized under laboratory conditions before system assembly, and selection of the cells was carried out, providing cells of similar capacity to the serial configuration and cells with a large capacity spread to the parallel system. Thus, the influence of the stack architecture both on performance and on aging could be investigated. The stacks were operated in the microgrid, providing both stacks strictly with the same power demand. In regular intervals the individual cells were disassembled from the system to perform laboratory checkups that consisted of capacity, internal resistance and electrochemical impedance measurements. The capacity loss of individual cells and stacks was quantified by laboratory checkups and microgrid operation data, respectively.
In parallel, two nominally identical cells were aged under laboratory conditions by continuous full cycling (100 % DOD) for a total of 1,200 cycles. These cells were investigated by the same checkups as the microgrid cells, performed at the beginning of life and at certain cycle times. By combining the laboratory and microgrid operation data, this study allows the comparison of stack-level aging as function of stack architecture and the comparison of cell-level aging under laboratory cyclic aging.

Acknowledgements: The authors acknowledge the Federal Ministry of Education and Research (BMBF) for funding this work as part of the STABIL project (13FH004PX5).

Reference
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[1] B. Weißhar and W. G. Bessler, “Model-based lifetime prediction of an LFP/graphite lithium-ion battery in a stationary photovoltaic battery system”, J. Energy Storage 14, 179-191 (2017).
[2] D. Dongol, T. Feldmann, and E. Bollin, “An overview to the concept of smart coupling and battery management for grid connected photovoltaic system”, J. Electronic Science and Technology 13, 367-372 (2015).

Figure 1