## 4 Conclusions  
An analysis of different impedance measurement devices and their correlation has been conducted. For this experiment two strings of each 24 VRLA batteries with 12 V rating have been operated under float charge at room temperature for over 560 days. The blocks were monitored continuously and were put into regular check-ups. The commercial devices Hioki BT3554 and LEM Sentinel 2 were utilized for single-frequency measurements. Additionally, regular electrochemical impedance spectroscopy has been performed with a uEIS. The Hioki handheld has a good correlation with 1 kHz impedance readings of the uEIS. Higher failure possibilities at lower frequencies lead to higher deviation between 78 Hz EIS and the LEM Sentinel 2, which should collect impedances at about 70 Hz. However, the gained impedances are more likely representing values at even lower frequencies (here \(<\)32 Hz). All in all, the implications drawn from 1 kHz and 78 Hz EIS measurements are very similar. The fitted \(R_{\rm i}\) taken from EIS does show comparable behavior. Consequently, it can be said that current measurement techniques do cover the change in high frequency impedance very well. One issue is that the high frequency values will not foresee the aging mechanisms also influencing the lower frequency behavior, such as water loss and sulfation. These lower frequency values are more important for the estimation of available capacity and thus SoH. While the high frequency impedances of the tested batteries showed no to marginal increases in impedance, lower frequencies already show a significant change. Thus, the age of the batteries would be underestimated at high frequencies. Therefore, the impedance spectra were taken and fitted under utilization of a DRT analysis. The fitting of EIS parameters for the utilized EEC as well as the DRT show an increase of parameters describing the charge transfer processes. From the DRT it can also be concluded that the frequency of the processes changes over time, while the range for the first process is 100 Hz and 1 Hz and the second 1 Hz and 0.1 Hz. This corresponds well with the assumed time constants for the negative and positive electrode, respectively. As field measurements typically do not allow for highly complex measurements and analysis such as recording complete impedance spectra and fitting, another methodology is suggested. Comparable results to the fitted EEC parameters were gained from the analysis of two single-frequency measurements. Namely, the real part of the 100 Hz and 1 Hz impedance were analyzed to calculate \(R_{\rm i}\) and \(R_{\rm ct,NAM}\). With a device monitoring the development of these two impedances much more information can be gained from the batteries in operation. The stability of low frequency measurements in the field must be further investigated. Additionally, it must be controlled how much relative change in impedance at low frequencies can occur before the battery reaches its EoL. Further analysis at the EoL will be performed with post mortem analyses to check on the intensity of each aging mechanism. In another step, the monitoring device will be coupled with further models (e.g., diffusion) to optimize the online diagnostics. [PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the experiments; [PERSON] analyzed the data; [PERSON] wrote the paper; [PERSON], [PERSON] and [PERSON] peer reviewed the paper prior to submission. The underlying work has been conducted within the project BSMS (EU-1-1-081) funded through the European EREE program. The publication as open source document will be sponsored by this publicly funded project. The authors want to thank [PERSON], [PERSON] for their support in conducting the experiments as well as [PERSON] and [PERSON] for their valuable feedback. We want to thank the EFEE program for its financial support. **Conflicts of Interest:** The authors declare no conflict of interest.