## 1 Introduction  
Uninterruptible Power Supply Systems (UPS) are utilized as back-up source in electrical systems. They can be classified according to DIN 62040-3 [1] as a static, rotary or hybrid system. The static UPS always consists of batteries, which shall provide energy during grid disturbances or grid failures. In the event of a grid failure the critical load of the connected customer must be covered and the batteries are discharged. However, for most of the operation time the battery is kept at 100% state of charge (SoC) by a so-called float charge. Under these conditions and without regular check-ups or additional measurements the operator is usually unable to estimate the discharge behavior and thus the available capacity or state of health (SoH, which equals 1 in the new state and decreases to 0 at the defined end-of-life (EoL)). Due to calendric and cyclic aging the SoH decreases over the battery lifetime. Therefore, frequent maintenance is necessary to determine this value as an indicator for the available capacity under a specific load. One common but strongly discussed measure for the SoH of a battery are impedance or admittance readings as they are non-invasive and cause no down time compared to regular capacity tests. It is expected that the measured impedance increases over the lifetime of the battery and thus with decreasing SoH [2; 3; 4; 5; 6]. Likewise, the admittance decreases. It should be noted that due to run-in processes and an incomplete formation the measured impedance can decrease at the beginning of utilization. [PERSON] et al. have shown in [6] the exponential increase of the impedance measured at 69 Hz with the first LEM Sentinel sensor. In their experiment, 160 Ah batteries with an absorbent glass mat (AGM) separator were exposed to an accelerated aging test corresponding to a typical UPS application profile. The impedance values were measured at 100% SoC with superimposed charge currents (float charge). After a capacity decay of roughly 20% of the highest measurable capacity the impedance at this frequency has increased by over 45%. However, the way of measuring the battery impedance and thus the gained values vary in literature and the field. [PERSON] [7] has published a comprehensive overview on the different techniques and their implications. One variation, which makes the comparison between historic measurement results difficult, is the utilization of potentiostatic or galvanostatic devices. The latter method uses the current excitation signal and measures the voltage response. It is preferred over potentiostatic measurements as similar voltage levels can be reached at completely different states of the battery [8]. This can be due to overvoltages caused by a changed SoH or due to acid stratification. Therefore, impedance measurements which shall be used for state diagnostics should use a current excitation signal. In that case the voltage amplitude needs to be controlled during the measurement [7]. However, even with the same method the excitation signal can deviate. For example, monitoring systems such as the LEM Sentinel 2 sensor introduce a half wave signal at a frequency of about 70 Hz while the impedance measurement device by Hioki (here BT3554) measures with a 1 kHz sinusoidal signal. There are much more commercial solutions for impedance monitoring available. Here a handheld and an online monitoring system, for which the measurement technique is known, were chosen and compared. On the one hand, [PERSON] [6] has proven that the impedances gained by a half wave signal are close to the impedance values extracted from sinusoidal signals over the lifetime of a battery. On the other hand, the theory of electrochemical impedance spectroscopy (EIS) of battery systems claims that the impedances at specific frequencies describe different processes within the battery [9]. Furthermore, [PERSON] et al. [10] have presented that the impedances of lead-acid batteries at different frequencies have also different relative changing rates over the battery lifetime.