## 2 Materials and Methods
### Design, Modeling and Simulation  
To assess the feasibility and functionality of BTMS, the development process is supported by simulations. In today's industry, simulation is an everyday tool, especially in companies with strong engineering ties, such as the automotive industry or mechanical and plant engineering. Therefore, in science as well as in business, there is a strong demand for ways to utilize computers to provide competitive and functional products and to manufacture and sell them efficiently, with high quality and safety [15]. Computational fluid dynamics (CFD) is a computational mathematical modelling tool that can be viewed as a fusion of theory and experiment in the fields of fluid flow and heat transfer. CFD simulations provide information about flow and fluid properties that can be difficult or expensive to obtain by measurement, providing insight and understanding of product performance or flow behaviour in specific situations. CFD is now used in many industries, starting from aerospace and power generation to consumer goods, biomedical, pharmaceutical, built environment, and many more. Where CFD has historically been used to predict or understand the flow performance of existing designs or conditions. The most effective and productive usage is achieved when applied early in the design process to drive the design towards better or more consistent performance [16]. Even the simplest heat exchange system is actually quite complex when viewed through rigorous analysis. Therefore, we decided to perform flow analysis in BTMS using a computational fluid dynamics software tool that offers 3D flow analysis. CFD is a software tool for simulating the behaviour of systems involving fluid flow, heat transfer, and other similar physical processes. It solves the equations of fluid flow over a defined region. The specified conditions on the boundary of that region must be known. Creating such CFD models and analysing the results is quick and easy thanks to advanced computing power, powerful graphics and interactive 3D manipulation of the models. Advanced solvers contain algorithms that provide robust solutions to flow fields in a reasonable amount of time. For these reasons, CFD is now an established industrial design tool that offers a cost-effective and accurate alternative to scale model testing. Various solutions are used in CFD codes, the most common is known as the finite volume technique. In this technique, the region of interest is divided into smaller sub-regions called control volumes. The equations are discretized and solved iteratively for each control volume. As a result, an approximation of the value of each variable can be obtained at specific points throughout the domain and we get a full picture of flow behaviour. The estimation of the heat dissipation of solids by convection can be made only by an abstraction of the considered body to very simple geometries. However, these empirical approximations lose their meaning when the geometry of the object and the abstraction diverge significantly. Heat sources with very complex shapes cannot be meaningfully represented by abstractions and the Heat output can only be calculated under very simple conditions. In order to make statements about the thermal behaviour of battery systems with passive PCM cooling, CFD models of batteries, and surrounding thermal mass have been created. The CFD tool Autodesk CFD 2023 was used for this. In the passive BTMS, the heat generated within the battery cells of a battery pack should be transferred to the PCM. The generated heat is stored in the PCM and provides an ideal temperature range for the battery cells. In Section 2.3, we clarify how we will construct a structure so that we do not require a support structure given by the slicer. The other two basic focuses are cooling or heating and PCM space. We design channels inside the structure for cooling and heating.