## 2 Materials and Methods
### Test Setup of UPS Systems  
For this experiment two serial battery strings (in the following named as string 2 and string 3) consisting each of 24 battery mono-blocks with a nominal voltage of 12 V were installed in October 2016. Before the installation the battery voltage, weight and inner resistance were measured during inventory. The latter was taken with the same Hioki BT3554 as in the following regular check-ups described in Section 2.2. The key parameters of the utilized batteries are listed in Table 1. The Panasonic batteries (string 3) showed a slightly higher deviation in inner resistance while the Yuasa batteries (string 2) had a much lower weight as stated by the data sheet of the manufacturer. However, overall, the batteries in one string are very similar to each other, which is important for the aging test. Both strings are connected in parallel with a third string to a power supply system by converter (M5000-12). This rectifier charges the batteries with a constant-current constant-voltage (CC-CV) charging program. The maximum current can be 1.6 A and the nominal voltage per string is set to 328.80 V at 20 \({}^{\circ}\)C. This results in theoretical 13.70 V per mono-block, which was the recommended floating voltage for the sealed lead-acid batteries. Between \(-\)20 \({}^{\circ}\)C and +60 \({}^{\circ}\)C the voltage is linearly compensated with 26.4 mV per 1 \({}^{\circ}\)C and block, subsequently a lower voltage is set at higher temperatures then 20 \({}^{\circ}\)C. During further float charge (CV-phase) the rectifier performs pulse tests every 300 s with forced discharges over a 30 kf resistor for 2 s (\(\sim\)0.011 A). This is part of an internal functionality test. Its status is \"ok\" as long as the string voltage stays above 270 V.  
In the field, different float voltages will be utilized in accordance to manufacturer recommendations. The influence of different float voltages on the measurable inner resistance is not part of this analysis. Instead, the current work is focusing on the change of impedance readings with the operation lifetime of the batteries. Nevertheless, a further evaluation of the impedance dependency on the registered voltages within this test is included in Appendix A. It shows no clear correlation of these two values for the devices under test. The average voltage per mono-block for string 2 (Yuasa) and string 3 (Panasonic) is 13.53 V during the full test period (including pause phases). String 2 has a higher deviation on average within the string of 0.115 V compared to 0.061 V for string 3. This is counterintuitive if the initial resistances and the measured deviation shown in Table 1 are considered. However, over the course of the float charge operation the difference between the minimum and maximum weekly average voltage (\(\overline{V}_{i}\), with \(i\) = [1; 24]) of string 2 stays longer at higher values compared to string 3. Consequently, for the following analysis of the mean voltages the measurement was divided into two phases. The first phase ends as soon as the voltage deviation of the string 2, presented in Figure 1, sinks below 0.5 V. This was the case on the 21 th of May 2017. This measurement indicates that string 3 has a faster equalization among the batteries e.g., through a better activation of the oxygen recombination cycle. At the beginning of the VRLA battery life, the process of oxygen recombination at the negative electrode is typically hindered due to a lack of open channels in the saturated AGM fleece [11]. Gas channels emerge with water loss and allow for a faster diffusion of oxygen and thus higher recombination efficiency. With this comes a depolarization of the negative electrode (see also [11] page 56) and as a result, the deviation in voltage among batteries with limited charging voltages decrease. The whole test bench was set up in an air-conditioned room.