The thermal management systems using EG-based phase change materials (PCMs) can provide power batteries with a proper operating temperature, slow temperature rise rate and uniform temperature distribution. In this study, a systematical investigation on the effects of thermo-physical properties of the used PCMs on the performance of the systems has been conducted. A series of paraffin/expanded graphite (EG) composites have been applied to a simulative battery thermal management system and to find out the PCM with the best thermal properties. The performances of PCMs varying with the kind of paraffin used, the paraffin mass fraction in composites and the packing density of the composites have been compared. It is found that the paraffin with the melting point of 44°C offers batteries the best operating temperature. Furthermore, the synergetic effect of the mass fraction of paraffin in the composite PCM and the packing density of the composite in the thermal management system has been studied. The temperature rise can be slowed down by increasing the composites density and the temperature uniformity can be improved by the increase in EG mass fraction and composite density. After cycle tests, the paraffin/EG composite with paraffin mass fraction of 75% and density of 890kgm−3 shows the best thermal management performance. In addition, numerical research with the computational fluid dynamics (CFD) software, FLUENT was also carried out. The numerical results are in a good agreement with the experiment data.