## 3 Results and Discussion
### Comparison of Commercial Measurement Devices and EIS  
The marked check-up values within the continuous measurements of the LEM Sentinel 2 were compared to the check-up measurements of the \(\upmu\)EIS at 78 Hz with superimposed discharge currents (\(-1\) I\({}_{20}\)). For the comparison, the offset between the first valid measurement of the LEM Sentinel 2 in January 2017 and the initial EIS measurement in the winter of 2016 was subtracted from all following measurements. The absolute impedance values are plotted in Figure 4. Additionally, the Hioki BT3554 as well as the corresponding 1 kHz \(\upmu\)EIS measurements with no superimposed DC current are plotted. A good accordance between the Hioki measurement and the 1 kHz \(\upmu\)EIS can be seen. The relative deviation depicted in Figure 5 is within the given range of accuracy for most of the measurements. The deviation between the LEM Sentinel 2 and the 78 Hz measurement is higher and outside of the accuracy range. The 5 th check-up shows the strongest deviation with high impedance measured by the LEM Sentinel 2. This could be explained, as previously stated and visualized in Figure 3, by a change in the contact resistance. The comparison of the relative change shows that the tendency of the compared values is similar (see Figure 4). At first, the impedance decreases for both frequency ranges. This is probably due to a run-in effect of the new batteries, which can mean a further formation of the active mass reducing the inner resistance as well as a decrease in AGM saturation and thus a better recombination cycle, which changes the parameters of the ZARC element. Similar developments were seen for the other batteries (see Table 4).  
Figure 4: **(a) Absolute impedance and (b) relative impedance values of the two measurement devices LEM Sentinel 2 (\(\cdot\)) and Hioki BT3554 (\(\circ\)) with the corresponding EIS values at 78 Hz with superimposed current of \(-1\) I\({}_{20}\) (\(\vartriangle\)) and at 1 kHz with no superimposed current (0 DC) (\(\%\)).**  
Figure 5: Relative deviation between commercial measurement devices and the corresponding EIS measurements with the commercial measurement as base. Left axis: 1 kHz impedance comparison (\(\circ\)). Right axis: \(\sim\)70 Hz impedance comparison (\(\square\)).  
Up to now the 1 kHz measurement with no superimposed DC current indicates no aging for most of the batteries as the values are below the initially measured impedances. Only battery 3-008 shows an increase for high frequency impedance. For the lower frequency values an increase in impedance was detected for all batteries and is in the range of 5 to 28% for the LEM Sentinel and up to 15% for the \(\mu\)EIS measurement. It becomes clear that especially the lower frequency measurements, which are more prone to inaccuracies, deviate from each other and give contradictive indications (e.g., for battery 3-004). The continuous measurements of the LEM Sentinel 2 with overcharge currents at 100% SoC will also be affected by the gassing reaction and the resulting gas bubbles within the porous electrode structure. The coverage of the active surface with gas bubbles will influence the impedance but should be of less relevance at the given frequency. An analysis of the absolute value of the LEM Sentinel 2 in comparison to impedances collected through EIS showed a better correlation to values gained at lower frequencies <32 Hz. The high impedance of battery 3-004 can only be seen at less than 13 Hz in the EIS measurements.  
In total, based on these relative changes, all batteries would still be described as healthy and would stay in operation. The typically stated EoL criteria would be a 100% increase of the initial internal resistance. For the increase of resistances at lower frequency no rule of thumb is known; however former analyses such as the one of [6] have shown increase of over 45%. Based on these measurements, battery 3-008 and 2-024 should be in the scope of further observation.