## 1 Introduction
Therefore, he performed a cyclic and an overcharge test. The latter is closest to the focused application of UPS systems. It was conducted on a 12 V AGM battery (95 Ah with 4 I\({}_{20}\) discharge current), which was charged with 2 A for 100 h and no voltage limitation. I\({}_{20}\) stands for the current value, which discharges the battery in 20 h. It is equivalent to a current rate of 0.05 C\({}_{20}\), which is referring to the nominal 20 h discharge capacity. The positive electrode saw overpotentials of 225 mV. Nevertheless, based on gas analysis the author expected that the overcharge has led mainly to water loss and that corrosion was negligible. The EEC parameters were extracted from impedance spectra performed with the Gamry impedance spectroscope and a superimposed 10 A DC discharge current. It was found that the water loss has led to an increase in the fitted parameter \(R_{\rm i}\). This parameter represents the internal resistance and was measured at high frequencies where the imaginary part is close to 0. \(R_{\rm i}\) had increased by about 27% at 50% SoC but showed only a slightly higher value at 100% SoC. Other aging mechanisms such as sulfation or the combination in cycling had no influence on the inner resistance. Consequently, the author states that the pure inner resistance measured at high frequencies and high SoC is not a good indicator for the capacity loss. For the cyclic test with sulfation as main aging mechanism, he proofs that the measurement of the charge transfer resistances can be correlated with the SoH. From his findings the development of impedances between 100 Hz and 1 Hz shall be monitored for changes on the negative electrode capacity, while impedances between 1 Hz and 0.01 Hz would give an indication for the positive electrode. According to [11] both values need to be measured for a correct indication, as the capacity limitation can be due to either one of the electrodes or the electrolyte. Such a separation of electrodes according to frequency range in the impedance spectrum have also been suggested by [8]. [PERSON] et al. [15] have studied the change of EEC elements for a 12 V flooded starting-lighting-ignition (SLI) battery tested with a driving profile. In this profile cycles with high currents (20 120) and different depth of discharges (up to 50%) were combined with pause phases. The expected aging mechanism was sulfation of the negative electrode. EIS measurements were regularly conducted with an EISmeter at 80% SoC and excitation frequencies between 5 kHz to 1 mHz and superimposed DC current. The spectra were fitted to a simple \(R_{\mathrm{i}}\) and RC circuit. The latter shall describe the process measured between 100 Hz up to 10 Hz. The change of these parameters with SoH was analyzed. An exponential increase of the internal resistance \(R_{\mathrm{i}}\) with increased capacity loss and a linear increase of \(R_{\mathrm{ct}}\) as well as a linear decrease of the corresponding capacitance of the single RC-element was found. In summary, literature and field experience agree that changes in impedance are a good indicator for a failing battery. However, a good correlation between measured impedance and available capacity or SoH still needs to be found. Furthermore, there are controversial statements about the visibility of different aging effects within the general EEC elements. However, most agree that the different aging mechanisms shall be detectable within the change of the EEC parameters. While other authors have analyzed the change of impedance measurements at one specific frequency during accelerated aging (e.g., [6]) or under standard UPS conditions (e.g., [3]), the authors of this work want to show the change of the full spectrum during standard float charge operation at room temperature. The experiment shows how the EIS and the extracted parameters of the EEC evolve over time for selected batteries of the two UPS strings under test. The analyses shall show the influence of the expected aging mechanisms on the impedance values.