Lithium-Ion Battery Relaxation Effects

The accurate estimation of the state-of-charge (SoC) of lithium-ion batteries is crucial for safely operating electric vehicles. One way to obtain information about SoC is to utilize battery impedance profiles. Effects of temperature, SoC, and state-of-health (SoH) on impedance have been studied using Electrochemical impedance spectroscopy (EIS) but the effect of relaxation period following a charge or discharge cycle requires more attention. In this study EIS results are obtained with respect to relaxation period at different SoCs and temperatures. An impedance model is fit to the data and the change of model parameters with relaxation is analyzed. The results show that the behaviour of the model parameters is in good agreement with electrochemical theory. Furthermore, it is found that changes in some model parameters are significant when compared to changes in SoC. This highlights the need to account for the relaxation effect when measuring battery impedance.