## 2 Materials and Methods
### Check-Up Measurements  
Check-up measurements have been conducted after 184, 324, 456, 514 and 568 days under float charge conditions. An initial measurement was performed after a few days under float charge and is marked with 0. The check-ups were first conducted after longer periods under float charge as no changes were expected and the influence of the check-ups should be kept as small as possible. Within the process of writing this paper the check-ups were performed more frequently. For the check-up the strings were disconnected from the power supply. In a first step, the impedance of each mono-block was measured with a Hioki BT3554 within the first 12 h after disconnection. The Hioki was connected through the pin type leads 9465-10 to perform a 4-terminal measurement. For the chosen measurement range of 30 m\(\Omega\) or 300 m\(\Omega\) a sinusoidal signal with excitation currents of 160 mA or 16 mA, respectively, is introduced into the battery at a frequency of 1 kHz \(\pm\) 30 Hz [22]. The accuracy of the impedance reading is given with 3% (with \(\pm\)0.8% on the reading (rdg.) \(\pm\) 6 digits (dgt.) absolute error due to the resolution). The influence of superimposed charge currents on the impedance reading was measured simultaneously to the LEM Sentinel 2 test described above. Table 11 in Appendix A summarizes the results. It can be concluded that the small charge current does not influence the measured resistance above the range of accuracy. However, for the analysis over time only the measurements in disconnected state without current flow and after some relaxation will be compared. In a second step, the EIS measurements were conducted on five selected batteries with an impedance spectroscope developed by ISEA. It is described in more detail in [8; 23]. The device is called \(\mu\)EIS and is a pre-version of the now commercially available EISmeter from Digatron [24]. The frequency of the excitation signal has been varied between 6.25 kHz and 10 mHz with 8 measurement points per decade and a minimum of 3 periods per frequency. The \(\mu\)EIS is a galvanostatic impedance spectroscope, which imposes a sinusoidal current on the battery and the voltage response is measured. The amplitude of this AC signal was set to a maximum of 0.5 A. Based on the amplitude of the voltage signal the current is then automatically decreased for decreasing frequency or increasing impedance respectively. The AC voltage response is controlled and kept in a limit of \(\pm\)18 mV for the 12 V battery. Each battery was at full state of charge before the EIS measurement, which was first conducted with no overlaying DC current (0 DC) and afterwards with a discharge current of 0.35 A (\(\sim\)I\({}_{20}\) DC). The resulting voltage drift during measurement is automatically compensated. The discharge was performed according to the description of mini-cycling during impedance measurements as suggested by [PERSON] et al. [8] and refined by [PERSON] et al. [25]. In this process the EIS measurement starts after a discharge of 1% C\({}_{\mathrm{n}}\) (the nominal capacity) and is conducted during the discharge of another 4% C\({}_{\mathrm{n}}\). Here the discharge is steered through a time limit of 1 h and thus a maximal discharge of 0.35 Ah. Furthermore, the batteries were kept at room temperature within a water bath. The cable connection as well as the measurement channel was kept equal for all check-ups and each battery.