*Article* **A Method for Monitoring State-of-Charge of Lithium-Ion Cells Using Multi-Sine Signal Excitation**

**Jonghyeon Kim \* and Julia Kowal**

Department of Energy and Automation Technology, Electrical Energy Storage Technology, Technical University of Berlin, Einsteinufer 11, 10587 Berlin, Germany; julia.kowal@tu-berlin.de

**\*** Correspondence: jonghyeon.kim@eet.tu-berlin.de

**Abstract:** In this paper, a method for monitoring SoC of a lithium-ion battery cell through continuous impedance measurement during cell operation is introduced. A multi-sine signal is applied to the cell operating current, and the cell SoH and SoC can be simultaneously monitored via impedance at each frequency. Unlike existing studies in which cell impedance measurement is performed ex situ through EIS equipment, cell state estimation is performed in situ. The measured impedance takes into account cell temperature and cell SoH, enabling accurate SoC estimation. The measurement system configured for the experiment and considerations for the selection of measurement parameters are described, and the accuracy of cell SoC estimation is presented.

**Keywords:** lithium-ion battery; battery management system; state monitoring; state-of-charge
