## 7 Conclusions  
In this publication, it has first been shown why a massive expansion of battery technology can be expected in the future. One important point here is certainly the electrification of vehicles. However, electric cars are particularly climate-friendly only if the electricity they use is also generated ecologically. To counteract global warming, the share of renewable energies must therefore be massively expanded in the future. However, electricity from PV systems and wind turbines, which account for a large proportion of renewable energies, is only available intermittently or irregularly. At the moment, the stabilization of the transmission grid can still be maintained with power plants and their rotating masses. If the expansion of renewable energies leads to the shutdown of the majority of power plants, system stabilization must be made possible in an alternative way. Therefore, a massive expansion of battery technology is to be expected in this point as well. This massive expansion of battery technology cannot be based on existing battery technologies alone for several reasons. First, the raw materials of existing technologies are limited and are mainly found in countries that do not allow safe working conditions. Zinc, on the other hand, is more often found in countries that do not exploit their employees, such as Australia and North America. Another point concerning raw materials is the recyclability cycle. The simple construction of zinc-air batteries helps here. They can be opened without danger and the components can be separated without great effort. Secondly, the theoretical energy density of metal-air batteries is much greater than existing battery technologies. For example, the theoretical energy density of zinc-air cells is three times greater than that of lithium-ion cells. However, the operation of zinc-air cells is more complex, as existing BMS cannot be used. The reason for this is the small voltage differences that occur during charging and discharging. State of the art battery technologies typically use the cell voltage to detect the state of charge. When a zinc-air cell is charged, the cell voltage increases by only 40 mV during the charging process, i.e., by an order of magnitude less than in other battery technologies where the cell voltage can be used to detect the state of charge. State of charge detection based on cell voltage is not possible, in particular as the influence of temperature and electrolyte concentration on cell voltage is greater than the state of charge itself. The charging process is further complicated by the fact that an accompanying electrolysis process starts at about the same voltage level as the battery charge voltage when the cell is overcharged. This means that overcharging cannot be detected from the cell voltage. Both overcharging and deep discharging damage the cell. However, when discharging, a large voltage drop occurs towards the end of the discharge process, which can be used to detect the end of discharge. Previously, it was therefore not possible to use zinc-air batteries without coulomb counting. The typical application scenarios are excluded by this obstacle, as the cell must be operated continuously in order not to lose the known SoC due to self-discharge. In order to take advantage of the benefits of zinc-air batteries, a method has therefore been developed that enables the detection of the state of charge. A very promising method to generate measurement data that depend on the state of charge of the cell is EIS. Here, the impedance of the cell is measured for a whole set of different frequencies. However, the unique electrode arrangement of zinc-air secondary cells means that conventional EIS measurement systems are not suitable for use with zinc-air batteries. Therefore, the development of an adapter board was demonstrated that allows existing EIS measurement systems to be used with zinc-air batteries.