Correction of DC Bias in Embedded Impedance Spectroscopy Based on Excitation by DC/DC Converters for Li-Ion Batteries

In embedded impedance spectroscopy, when the available dc/dc converter in power systems is used to generate excitation signals, a bias current is typically present, leading to unavoidable charging and discharging processes. This violates a crucial condition for impedance spectroscopy, which is to maintain system stability during the measurement, and thus leads to systematic deviations. In this article, we investigate the stability and characterize the influence of charging/discharging during measurement on the impedance spectrum. Based on this investigation, we propose an excitation sequence followed by a two-step procedure to correct the influence of the unavoidable charging/discharging process and enable a correct impedance spectroscopy measurement. First, the influence of the bias current on the impedance in the diffusion process is identified and neutralized using the distribution of relaxation times (DRTs), followed by an averaging procedure to minimize the charge state discrepancy between measurement points. The proposed method is validated on a Samsung INR-18650-25R Li-ion battery cell, which was able to provide stable measurements for both the one-quadrant and two-quadrant settings, with rms magnitude and phase accuracy of 0.16% and 0.02 degrees, and 0.021% and 0.38 degrees, respectively.