On-line state-of-health estimation of Lithium-ion battery cells using frequency excitation

•Li-ion cell SoH estimation during cell CC charging and discharging.•Estimate cell SoH without knowing cell temperature and cell SoC.•Battery cells do not need to be removed from the circuit for SoH estimation. With the rapid growth of the battery market, improved cell state estimation techniques are required as well. The cell state-of-health (SoH) estimation can be applied to battery management systems (BMS) to extend the usable cycle life and to prevent critical safety issues caused by ignition or explosion. Electrochemical impedance spectroscopy (EIS) is one of the non-destructive battery cell analysis methods that allows estimation of electrochemical parameters. However, in order to use the EIS, it is not only time-consuming to measure the impedance in a wide frequency range, but also the cells must be separated from the circuit for measurement. The purpose of this paper is to estimate the SoH of the Lithium-ion battery cell in operation. For this purpose, cell impedances at different cell SoC, cell SoH and cell temperature are compared. The experimental results are shown using a measurement system designed to measure the cell impedance during cell charging and discharging. Through this, it is shown that SoH estimation is possible even when the cell SoC and the cell temperature are unknown.