State of Charge (SOC) and State of Health (SOH) are important parameters for the users and manufacturers of home storage systems. The state diagnostics of LFP/graphite cells is challenging due to the inherent thermodynamic hysteresis and the flat, but unsymmetrical discharge/charge characteristics. Battery models allow a knowledge-based and therefore more reliable state diagnostics.
We present the development and parameterization of an equivalent circuit model of a 180 Ah prismatic LFP/graphite cell. The model consists of an R(RC)(RC) series of circuit elements representing electrolyte and both electrodes, respectively. Open-circuit voltage is calculated as function of Li stoichiometry in the active materials rather than as function of global charge throughput. This allows a physically-based description of electrode resistances as function of stoichiometry. The necessary electrode balancing parameters are obtained from experimental quasi-OCV curves through mathematical optimization . Asymmetric overpotentials of LFP are included in the model by using a current-dependent R element.
Charge/discharge experiments at different C-rates and temperatures  as well as electrochemical impedance spectroscopy (EIS) and internal resistance measurements are carried out as basis for model parameterization. Equivalent circuit fits are used for parameters identification.
The parameterized model shows a good agreement with the original experimental data in the time domain. For validation we use measured and simulated charge and discharge curves of different C-Rates and temperatures. With help of the parameterized and validated equivalent circuit model we operate a model-based state estimator to estimate the State of Charge and State of Health of LFP/graphite home storage cells. The Results of the state estimation aren’t part of the poster presentation and will publish in another way.
 M. Mayur, M. C. Yagci, S. Carelli, P. Margulies, D. Velten, and W. G. Bessler, “Identification of stoichiometric and microstructural parameters of a lithium-ion cell with blend electrode”, Phys. Chem. Chem. Phys. 21, 23672-23684 (2019), DOI: 10.1039/c9cp04262h.
 M. C. Yagci, R. Behmann, V. Daubert, J. A. Braun, D. Velten, and W. G. Bessler, “Electrical and Structural Characterization of Large‐Format Lithium Iron Phosphate Cells used in Home‐Storage Systems”, Energy Technology (2021), DOI: 10.1002/ente.202000911.
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