## 3 Results  
### Measurements on Test Cell  
Before the impedance spectra, performed during _Ref. X_ and _Strat. X_, were fitted, the single values of the spectra were checked with the _zHit_ method [21]. In all spectra with superimposed discharging DC-currents the impedance values passed the _zHit_ check, which correspond to the high-frequency tail and the first semi-circle. These spectra could be used for fitting without any restrictions. The spectra with superimposed charging DC-currents had less valid impedance values. None of the spectra with \(+4.5\) I\({}_{20}\), \(+6\) I\({}_{20}\) and \(+8\) I\({}_{20}\) had enough valid data points in the first semi-circle. With progressing test procedure more and more spectra with superimposed charging DC-currents became useless for fitting. In Figure 5 impedance spectra with superimposed DC-currents of \(+1\) I\({}_{20}\), \(+3\) I\({}_{20}\) and \(-8\) I\({}_{20}\) are presented, recorded during the test procedure. The squares mark the impedance values, which passed the check with the _zHit_ method. While during _Ref. 1_ all of the presented spectra have enough valid points for fitting, the number of valid points decreased with the following EIS sets. The \(+3\) I\({}_{20}\) spectra in _Ref. 3_ and _Strat. 2_ had almost no valid points. After the full-charge of the test cell (_Ref. 4_) the validity of the spectra improved and is comparable with the validity of the spectra performed in _Ref. 1_. The \(-8\) I\({}_{20}\) spectra are presented here for comparison and as example for all spectra with superimposed discharge currents, as these spectra provided always an adequate number of valid data points during the whole test procedure. In the following the fitting results are presented. Figure 6 shows the EEC element values together with the residual from fitting of the spectra from EIS sets _Ref. 1_, _Strat. 1_, _Ref. 2_ and _Ref. 4_. The EIS set _Ref. 4_ was performed after 24 h full-charge of the test cell and are presented in the figure for a direct comparison with _Ref. 1_. In Figure 7 the fitting results from _Ref. 2_, _Strat. 2_, _Ref. 3_ and \(4\) are shown. Only the results are presented, which come from the impedance spectra with sufficiently enough data points for fitting. For the \(\pm 4.5\) I\({}_{20}\) spectra two data points per EIS set are presented in the figures, as these EIS were performed at the beginning and at the end of the EIS set. Values for \(R_{ohmic}\), \(R_{ct,neg}\) and \(A_{dl,neg}\) determined from the last \(\pm 4.5\) I\({}_{20}\) spectra are all larger than from the first \(\pm 4.5\) I\({}_{20}\) spectra. The inductance \(L\), which is generated by the connections of the test cell, varied by 10 % between the EIS sets. This comes from unplugging the measurement equipment and demounting the housing cover during the test procedure. This is not an effect of stratification. The residual for all EIS sets is low with \(0.4\times 10^{-4}\) Ohm on average, which indicates successful fittings. Almost all values of \(R_{ohmic}\) during _Strat. 1_ are smaller by \(0.03\) m\(\Omega\) than during the reference measurements presented in Figure 7. \(R_{ohmic}\) varies between the reference measurements by \(0.03\) m\(\Omega\). Furthermore, a light current dependency of the ohmic resistance is visible. The resistance values increase by 6% from large to small discharging currents. The charge-transfer resistance \(R_{ct,neg}\) as well as the double-layer capacitance \(A_{dl,neg}\) in Figures 6 and 7 increase continuously from _Ref. 1_ to _Ref. 3_. The increasing rate from one EIS set to the next EIS set of \(R_{ct,neg}\) is larger towards charging DC-currents. The values corresponding to _Ref. 4_ decreased again. Over the superimposed current rates \(R_{ct,neg}\) describes the well known current-dependency. Figure 5: Impedance spectra with superimposed DC-currents of \(+1\) I\({}_{20}\), \(+3\) I\({}_{20}\) and \(-8\) I\({}_{20}\) performed during the reference measurements and measurements with stratified acid presented as Nyquist-diagrams.