## 5 Conclusions  
The results presented here indicate a correlation between the measured impedance and the residual capacity. It is recommended that impedance spectra be recorded with small superimposed discharge currents in the range of \(-1\)I\({}_{20}\). These spectra were valid over the whole measured frequency range, independently from the pre-history and during the complete aging tests. In contrast, impedance spectra with charging DC-currents are mostly useless due to permanent change of the state during one full _EIS_ measurement.  
When pre-history, frequency, and DC-current were equal, single impedance values of different test cells increased congeners with similar impedance values during aging. Thereby, the pre-history and frequency were found to have an additional influence on the correlation between capacity and impedance. If these parameters are kept constant during a check-up, single impedance values are sufficient for _SoH_ estimation.  
With the use of the whole spectrum, a slower decrease of the double-layer capacitance and therefore a slower sulfation during _soft AK3.4_ aging was observed, caused by regular 24 h re-charging. As a characteristic change of the spectrum during aging, it can be outlined, that \(\tau_{neg}\) measured after the cycling phase dropped below 0.01 s for all test cells, when the aging test finished. At the same time, \(R_{olmic}\) was twice as high as at the beginning.  
The following are available online at [[http://www.mdpi.com/2076-3417/8/6/873/s1](http://www.mdpi.com/2076-3417/8/6/873/s1)]([http://www.mdpi.com/2076-3417/8/6/873/s1](http://www.mdpi.com/2076-3417/8/6/873/s1)): All performed impedance spectra together with the current and voltage profiles of the aging tests as Matlab files; Figures S1 and S2: Impedance spectra measured on _TC1_ of _soft AK3.4_ with superimposed discharging and charging current after 15 FCE at the end of cycling phase and at 80%; Figures S3 and 4: Impedance spectra measured on _TC1_ of _soH AK3.4_ with superimposed discharging and charging current after 90 FCE at the end of cycling phase and at 80%; Table S1: The average slopes of impedance amplitude increase over capacity loss \(C_{\rm loss}\) for different frequencies and states from \(-3\)I\({}_{20}\) impedance spectra; Figure S5: Single impedance values of all test cells in _hard AK3.4_ and _soft AK3.4_ test recorded with \(-3\)I\({}_{20}\) after cycling phase (left) and after charging (right). In the upper graph, the frequencies, at which the imaginary part of impedance is near zero, are plotted over capacity loss \(C_{loss}\) and in the middle graph the corresponding impedance amplitudes are given. In the lower graph, impedance amplitudes measured at 0.1 Hz, 10 Hz, and 1 kHz are presented. The data of different test cells are distinguishable by the marks and the different frequencies in the lower graph are distinguishable by the line color.  
[PERSON], [PERSON], [PERSON], and [PERSON] conceived and designed the experiments. [PERSON], [PERSON], and [PERSON] performed the experiments. [PERSON] analyzed the data and wrote the paper.  
**Acknowledgments:** The underlying work was conducted within the project BSMS (EU-1-1-081) funded through the European EFRE program. Funds were received from the same program for covering the costs to publish in open access.  
**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.