## 2 Materials and Methods
### PCM Enhanced Battery Packs
#### 2.1.1 Phase Change Material  
Heat is stored in sensitive heat storage materials by the temperature difference between the storage medium before and after the charging process. Temperature difference, heat capacity, and storage medium mass all play a role in the amount of energy stored. Sensitive heat storage systems, which can be found in almost every household as a warmwater tank and a buffer storage require good heat insulation. PCM--heat storage systems are also known as latent heat storage because the heat is stored more _latently_ than _sensible_ in a PCM. The charging and discharging are accomplished by changing the aggregate state of the medium based on its respective enthalpy. These systems can store heat relatively lossless over long periods of time [4]. Figure 4 depicts the distinction between sensible and latent heat storage. The benefit of larger energy storage amount is limited to a certain temperature range. The solid-liquid phase transition and vice versa is the most widely employed principle. Special salts or paraffin are frequently melted as a storage medium for charging the storage, as they absorb a lot of thermal energy. The discharge happens as the storage medium solidifies, releasing the previously absorbed massive amount of heat back into the environment as solidification heat. The amount of heat depends not only on the system's temperature difference but also on the material's mass and specific heat capacity. Table 1 shows the specific heat  
Figure 3: Passive cooling with liquid circulation [2]. Figure 2: Passive heating and cooling with cabin air [2]. capacities of some substances. Since mass is not the only determinant of energy storage, consistency, and volumetric heat capacity are often also specified. The heat capacity of a material indicates how much heat it can store in 1 m\({}^{3}\) when the temperature is increased by 1 K [4]. Different material classes are used as latent heat storage, depending on the temperature range. Solid-liquid transition is mainly used because the volume change is easier to handle than solid-liquid transition and the enthalpy of the transition is sufficiently high compared to solid-liquid transition. The longest and most commonly used PCM is water or ice. It meets all the required standards and is very cheap and available everywhere. In addition to water, the substance classes salt hydrates and paraffins are most frequently used [5]. Therefore, the application determines the storage medium with the optimum phase transition temperature. Water and salt solutions are primarily used for refrigerated storage. There are various materials that are suitable for phase change heat storage. In practice, solid-to-liquid transitions are mostly used. A phase change from liquid to gas is avoided despite the high phase transition enthalpy due to the large change in volume [4]. Salts such as nitrates, chlorides, carbonates, and fluorides are mainly used for heat storage above 130 \({}^{\circ}\)C, but mixtures of these substances are also used at these temperatures. The use of salt hydrates as PCMs results in so-called discordant melting [6], which affects the reversibility of the phase change process and thus reduces the heat storage capacity of the medium. By adding suitable additives to salt hydrates in pure form, it is possible to