Investigating electrochemical impedance spectroscopy for state of health estimation of LiFePO4 batteries in idle state

Online methods for determining the State of Health (SOH) and predicting the Remaining Useful Lifetime of lithium iron phosphate (LiFePO4) cells are becoming increasingly relevant. Due to the requirements for safety and availability of all-electric subsea applications, measurement methods for an accurate non-destructive battery cell diagnosis are especially important. Currently used methods are not applicable for power to safety applications, because they change the battery State of Charge (SOC) during measurement. This study aims to determine the extent to which the application of EIS provides a battery state estimation in idle state. A Design of Experiments (DoE) has been developed and realized. The EIS measurement results are analyzed in the Nyquist plot to identify curve characteristic points that refer to the SOH. The DVA is used to provide a capacity reference for the EIS evaluation. The local maximum point of the EIS is identified as a possible analysis point as it has an almost linear correlation with the DVA. It is discussed whether the gained results on cell level can be applied in further research to develop a diagnosis method that can be used on pack level in field applications.