In this work, an improved design of phase change material (PCM) packed in a mini-chambers cooling system was introduced and integrated with the lithium-ion battery to enhance heat transfer. A three-dimensional numerical analysis was conducted to investigate the impact of geometry on the use of phase change material (PCM) in battery cooling systems. The heat transfer evaluation was performed using the Newmann, Tiedemann, Gu, and Kim (NTGK) sub-model within the Multi-Scale Multi-Domain battery model of ANSYS Fluent. The study examined the temperature rise in the battery and the liquid fraction of the PCM with PCM packed in the mini-chambers and compared it with the PCM in a simple block geometry for different discharging rates of 2C, 3C, and 4C. The results revealed that when an equivalent amount of PCM was used in both the mini-chamber and block geometries, the minichamber geometry effectively reduced the system's temperature at a discharge rate of 4C. However, the block geometry experienced a temperature increase of 10-12K due to complete melting and sensible heating of the PCM. The results also reveal that the block geometry can be suitable for discharge rates up to 2C and 3C.