## 3 Results and Discussion
### Alternative Detection Method for the Field Application  
Impedance measurements with a spectroscope are inapplicable in the field. The measurement technique is too expensive for most applications. Furthermore, the fitting process which leads to the parameters (\(R_{\mathrm{i}}\), \(R_{\mathrm{ct,NAM}}\) etc.) would have to be automated and would need additional computational power. Therefore, based on the former results an alternative detection method for the field application is suggested. As the main aging effects could be detected with the change of \(R_{\mathrm{i}}\) and \(R_{\mathrm{ct,NAM}}\) it is argued to use a correlation between two basic impedance measurements. The first should be taken around 100 Hz, while the second should come from a 1 Hz excitation signal. Selecting these two  
Figure 8: Change of fitted EIS parameters over time for the three batteries of string 2. (**a**) normalized change of the internal resistance \(R_{\mathrm{i}}\); (**b**) normalized change of the charge transfer resistance of the ZARC element; c: normalized change of the time constant \(\tau_{\mathrm{ct,NAM}}\) of the ZARC element; d: normalized change of the capacitance of the CPE. Figure 7: DRT analysis of the impedance spectra of battery 2-014 of string 2 for all check-up measurements. frequencies is a compromise between the number of necessary measurement points as well as necessary registration rate and thus equipment cost and the accuracy of the measurement. Of course, a 1 kHz measurement would be more accurate for the measurement of the ohmic resistance as described above. Measurements have however shown that the relative change, which shall be evaluated, is deviating only marginally. It can be argued that the frequency should be adapted in a way that it is not a multiple of the electrical grid frequency, which leads to a practical frequency of 100 Hz + x in the field application. For the validation of this assumption the impedances measured at these two frequencies with the \(\mu\)EIS have been utilized. It would have been possible to use the measurements of the two commercial products instead. However, the results of the LEM Sentinel 2 were discussable and thus not further evaluated. As real impedance values are hard to collect in the field, the implication from evaluating only the real part has been investigated and compared to the impedance values. Figure 9 illustrates the normalized development of the internal resistance gained from the measurements at 100 Hz (\(R_{i,\text{calc}}\)) as well as the normalized resistance calculated by a subtraction of the full impedance or the real part measured at 100 Hz and 1 Hz. A similar result was gained for the batteries of string 3 (see Figure in Appendix A). In comparison to the implications of the impedance fit (see Figure 8), this simple implementation delivers results with similar indications for the resistance developments. Table 6 summarizes the values of the different resistances at the last check-up as well as their relative increase based on the initial measurement. It is worth noticing that the analysis of the EIS measurements as well as the suggested evaluation method also indicate battery 2-024 and 3-008 as the batteries with noticeable changes. However, the recognized higher proportional weight loss of the batteries in string 3 is neither influencing the low frequency process nor is it detectable in a higher change of internal resistance. Further, investigations in this prolonged test as well as final post mortem analysis will be performed to validate