## 3 Results and Discussion
### Conditions for the Check-Up Batteries  
The voltage and temperature measurements were evaluated for two different periods, phase 1 and phase 2 as described in Section 2.1. The average values measured for each cell are listed in Table 3. For string 2 the voltage readings deviate between 48 to 180 mV and for string 3 between 12 to 120 mV per mono-block. As these voltage readings as well as the deviation between temperatures is within the accuracy range of the measurement device the conditions for all cells are expected to be similar and comparable. However, differences in aging can arise from deviations within the battery (e.g., saturation of the fleece, grid casts etc.).  
Weight measurements are utilized as an indicator for water loss in flooded batteries. However, the actual water loss in a VRLA battery can be higher than the measured weight loss. This is due to corrosion reactions at the positive electrode, which bind the heavier oxygen as the counterpart to hydrogen evolution on the negative electrode, which is in total accounting for most of the water loss [31]. The batteries investigated show a weight decrease by less than 0.3% based on the total weight. If a proportion for the electrolyte of about 60% is assumed for AGM batteries, the measured weight loss corresponds to a water loss of less than 0.5%. Additionally, the OCV measurements before EIS are not yet showing a reproducible increase due to concentration changes. Based on these values, water loss is expected to be marginal at this point.  
An exemplary result of the continuous measurement for one battery of string 2 is depicted in Figure 3. The check-up measurements are marked with red circles. Each time step of a resistance measurement is marked with \(t_{\rm R,j}\) [\(i\) = 0,..., 5]. Temperature and voltage measurements were taken immediately before the impedance measurement. This time step is described by \(t_{\rm R,j}\)-\(\delta t\). During the check-ups the temperature was 21.6 (\(\pm\)1.8) \({}^{\circ}\)C and the voltages was 13.59 (\(\pm\)0.08) V. Within the profile  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Battery No.**} & \multirow{2}{*}{**String No.**} & \multicolumn{2}{c}{**Ø Voltage (in V)**} & \multicolumn{2}{c}{**Ø Temperature (\(\text{\textasciitiit can be seen that the detected impedance rises steeply above a ratio of 1.8 in the spring of 2018. After the 5 th check-up the impedance shows a steep decline. A similar behavior with less intensity can be seen before and after the previous check-ups. It is expected that the readings are influenced by changes at the contact of the battery poles. This is an important implication for field applications as the measurement would have indicated a necessary exchange of the battery too early. Thus, the connection of the measurement devices should be checked if implausible steep rises or declines can be seen within the data. The registered lower temperatures in the winter of 2017 were explained in Section 2.1. Two other disruptions in the acclimatization of the test field can be seen. Aside from these events the temperature profile is between +20 \({}^{\circ}\)C and +25 \({}^{\circ}\)C. The high voltage deviation at the beginning of the measurement is within phase 1, which is defined in Section 2.1. Here the deviating voltages within one string seem to steer this higher fluctuation. Additionally, it should be noted that the accuracy of the device at 13.75 V allows for a deviation of \(\pm\)0.069 V.  
Figure 3: Example of continuous measurements for battery 2-014 of string 2. Red circles indicate the event of a check-up at the time step \(t_{\text{R},i}\) [\(i\) = 0,..., 5]. (**a**) Resistance by the LEM Sentinel 2 as an absolute value; (**b**) Resistance as a relative value based on the first 5 resistance measurements; (**c**) Temperature development of the battery with measurement before each check-up; (**d**) Voltage development of the battery with voltage measurements one time step before the resistance measurement in \(t_{\text{R},i}\).