###### Abstract  
The demand for energy storage is increasing massively due to the electrification of transport and the expansion of renewable energies. Current battery technologies cannot satisfy this growing demand as they are difficult to recycle, as the necessary raw materials are mined under precarious conditions, and as the energy density is insufficient. Metal-air batteries offer a high energy density as there is only one active mass inside the cell and the cathodic reaction uses the ambient air. Various metals can be used, but zinc is very promising due to its disposability and non-toxic behavior, and as operation as a secondary cell is possible. Typical characteristics of zinc-air batteries are flat charge and discharge curves. On the one hand, this is an advantage for the subsequent power electronics, which can be optimized for smaller and constant voltage ranges. On the other hand, the state determination of the system becomes more complex, as the voltage level is not sufficient to determine the state of the battery. In this context, electrochemical impedance spectroscopy is a promising candidate as the resulting impedance spectra depend on the state of charge, working point, state of aging, and temperature. Previous approaches require a fixed operating state of the cell while impedance measurements are being performed. In this publication, electrochemical impedance spectroscopy is therefore combined with various machine learning techniques to also determine successfully the state of charge during charging of the cell at non-fixed charging currents. e 2021 2021 2022 2021 2022 2021 2022  
This trend is not expected to slow down, as topics such as the digitization of industry, ubiquitous computing, and the IoT are only on the rise. Industry 4.0 is the name given to a future project for the comprehensive digitization of industrial production [2]. Depending on the size, location, and type of production facility, the requirements may only be possible with the help of battery-powered sensors and devices, as well as with energy-autonomous systems that require a buffer battery. However, digitization is also taking place in areas outside industry. Battery electric vehicles (BEV) are the largest growth market for rechargeable batteries. Currently, the transportation sector within the EU is still largely based on fossil fuels and is responsible for more than 25% of Europe's greenhouse gas emissions. Excluding the COVID-19 crisis, this share is even growing [3]. In addition to global warming, vehicles with combustion engines also form a significant part contributing to air pollution. In particular, the proportion of particulate matter and nitrogen dioxide increases as a result of combustion engines [3]. Electrified or partially electrified vehicles can help here. The level of greenhouse gas emissions over the entire vehicle life cycle depends to a large extent on the CO\({}_{2}\) balance of the generated electricity [3]. Even in the case of combustion vehicles, partial electrification in the form of a PHEV in combination with a battery can lead to an improvement in the environmental balance. Together with a falling price, this is leading to an increase in BEVs around the world, as can be seen in Figure 2. Furthermore, the demand for battery capacity and the raw materials is also growing massively. As electric cars mainly use lithium-ion cells, the demand for cobalt in particular is considered critical. This is as, as with many other ones, mining is partly carried out under precarious conditions, so that alternative battery technologies have to be used. The environmental footprint of BEV is particularly good when renewable electricity is used to charge the electric cars. Renewable energy sources are energy sources that  
Figure 1: Total sold quantity of device batteries in Germany [1]. Figure 2: Worldwide new registrations of battery electric vehicles and plug-in hybrids (PHEV) [4]. are practically inexhaustible in the horizon of mankind for sustainable energy supply or are renewed relatively quickly [5; 6].