This study investigates the thermal management in Lithium-Ion batteries (LIB) through a passive cooling mechanism utilizing a phase change material (PCM). This research aims to identify the critical factor in designing a PCM-based cooling system that effectively maintains the LIB operation temperature. This study examines three primary parameters: the PCM melting temperature, the shape of the PCM container, and the convective heat transfer coefficient, utilizing a numerical model implemented in Comsol Multiphysics. The findings indicate that by using 4.85 g of paraffin, the battery system can achieve a temperature reduction of about 22%. The parametric study identifies the PCM melting temperature as the most critical factor in developing an efficient thermal management system. Other aspects of the study reveal that variations in the thickness of the PCM layer and the convective coefficient at the PCM container's wall have minimal effect on the battery's operational temperature once an optimal PCM melting temperature has been chosen. This study will be the first approach to designing a PCM-based thermal management system, considering the effects of repeated charging and discharging cycles and environmental variables such as air moisture and solar exposure, which can affect the batteries in actual field conditions.