Author M.Sc. Jonghyeon Kim has been a doctoral researcher for 4 years from 2017 to the present in the “Electrical Energy Storage Technology” laboratory of Technical University of Berlin, which is chaired by another author Prof. Dr. Ing. Julia Kowal. The author is currently studying the state monitoring method of Li-ion battery cells, and previously studied the state diagnosis of PEM fuel cell stacks. The authors' representative papers are; 'Kim, J.*; Kowal, J. A Method for Monitoring State-of-Charge of Lithium-Ion Cells Using Multi-Sine Signal Excitation. Batteries 2021, 7, 76, doi:10.3390/batteries7040076', 'Kim, J.*; Krüger, L.; Kowal, J. On-line state-of-health estimation of Lithium-ion battery cells using frequency excitation. Journal of Energy Storage 2020, 32, 101841, doi:10.1016/j.est.2020.101841.' and 'Lee, Y.-H.; Kim, J.*; Yoo, S. On-line and real-time diagnosis method for proton membrane fuel cell (PEMFC) stack by the superposition principle. Journal of Power Sources 2016, 326, 264–269, doi:10.1016/j.jpowsour.2016.06.113.'