Equivalent circuit model of a 180 Ah LFP/graphite home storage cell for model-based state diagnostics

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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 [1]. 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 [2] 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.

[1] 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.
[2] 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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