This thesis studied thermal behaviour of a phase change material (PCM) as a battery thermal management system. The objective was to develop a computational fluid dynamics (CFD) model and capture the effects of PCM and possible natural convection. The module consisted of 12 cylindrical Li-ion cells, and the studied PCM was Rubitherm’s RT35HC. Other objectives included to study the components, configuration, and other basics of Li-ion batteries and battery thermal management systems (BTMS). It was found that the performance and safety of Li-ion batteries are highly affected by internal and ambient temperatures. In worst scenario, increased temperature may lead to thermal runaway and even to an explosion. Thus, it is important to pay attention to the thermal behaviour of batteries. The temperatures can be controlled via BTMS. PCM has raised interest due to its low costs and simplicity. The working principle of PCM is based on its latent heat, the energy needed for the material to undergo a phase change. The CFD model was developed successfully. PCM was found to delay the temperature rise of the module significantly. Simplified no-PCM case showed temperature increase over 28 ℃ compared to the module with PCM. Natural convection was captured successfully and was found to expedite the melting from the top. Thus, the PCM didn’t maintain uniform melting in vertical direction. Results showed that only up to 50% of melting range, the PCM was effective for reducing the temperature rise. These results guide future studies on optimizing PCM based battery thermal management systems.