## 1 Introduction  
Increasing power and energy densities in modern Li-ion battery cells allows for the specification of energy-intensive power tools, such as chainsaws, drilling hammers, and angle grinders. However, these new application fields lead to new challenges in terms of cell pack design. In particular, adequate cooling is of high importance during intensive use [1, 2, 3], which involves high-energy consumption in very short time spans and (fast) charging, as cells can get damaged if a certain temperature is exceeded [4, 5, 6]. For commercially available round cells such as 18650 or 21700 format cells, which are used in most power tool battery packs, cell manufacturers provide the maximum currents as well as the temperature ranges for charging and discharging to prevent short- and long-term damage to the cells [7]. Power tool manufactures have to ensure, through pack design and management software, that these limits are not exceeded to guarantee the safety and long service life of their power tool battery packs [8]. Battery packs are controlled by the battery management system (BMS) [9, 10]. Simple systems control the current flowing to the cells and monitor cell and pack voltage, thus ensuring that safety limits and cell balancing are maintained. More complex systems are also able to estimate the current battery state, such as the current state of charge and state of health, and predict battery behavior [11]. This allows for the battery to be controlled much more finely and thus also makes it possible, for example, to use improved (fast) charging protocols, which specifically accelerate the charging process and improve the aging behavior [12]. The BMS can be implemented as an intelligent overarching central system or as an individual system in each cell [13]. Depending on the architecture, there are also limitations with regard to the performance of the systems, especially in terms of computing power and the number and type of sensors. The battery management system works closely with the battery cooling system or they are combined into one system. This guarantees that both systems function optimally, that safety limits are complied with, and that the system is always in a safe state for cell aging [14]. It is essential to know the characteristics of the cooling system as precioly as possible, as this is the only way to ensure good control and to get the best possible performance and user-friendliness from the use of the battery pack. A great variety of different Li ion battery cooling systems have been proposed and investigated so far [15; 16; 17; 18]. In scientific studies dealing with battery cooling systems, experimental approaches are often linked with simulations to be able to predict their performance in various usage scenarios [19]. In most studies, active cooling systems are investigated, pumping air or cold liquids around the cells or through cooling plates adjacent to the cells [20; 21; 22], as these systems are mainly used in huge battery packs, e.g., in electric vehicles [23; 24]. However, the effort in terms of the weight and complexity of implementing such systems is high. Passive cooling systems by contrast are lightweight, small, and independent of external power sources, and therefore easily implemented in small battery packs [25]. They are widely used, e.g., in e-bikes or power tool batteries [26]. In passive cooling systems, cells are embedded in materials with high heat conduction to ensure the fast transport of the produced heat to the pack's surface. Latent heat storage materials make additional use of the energy used up by the phase transition of the material and are often combined with heat-conductive materials [27; 28; 29; 30]. The performance of passive battery cooling systems using phase change materials has been confirmed in many studies [19; 31; 32; 33]. [PERSON] et al. [34] provide an overview of recent developments. They compare the performance of composite phase change materials, phase change fluids, flexible phase change materials, and hybrid cooling systems.