In this study, EIS measurements of commercially available Tekcell (Vitzrocell) Li/SOCl2 batteries and Li/SOCl2/SO2Cl2 mixture batteries were investigated. Equivalent circuit fits were done on the acquired spectra. Critical battery parameters were analyzed with respect to state-of-charge. A Kramers-Kronig (KK) analysis of the batteries was also performed, and residuals of the fits were evaluated. Thereafter, the non-linear harmonic analysis (NHA) of the batteries was studied and compared to the KK residuals. A correlation between the residuals and NHA of the batteries was presented. The terms KK-compatible and non-KK-compatible are explained in terms of residuals and NHA results. The results show a correlation between the KK residuals and non-linear harmonic analysis (NHA). Furthermore, we showed that the EIS measurement of the Li/SOCl2/SO2Cl2 system requires a few unique measurement conditions in order to perform linear, stable, and reproducible EIS measurements.