Electric vehicles (EVs) have grown in popularity in recent years due to their environmental friendliness and the potential scarcity of fossil fuels. Lithium-ion batteries (LIBs) are commonly utilized in EVs and hybrid electric vehicles (HEVs). They have a high specific charge, a high density of power, and a long life. A revolutionary design of a trapezoidal battery pack with a liquid cooling system based on composite phase change material (CPCM) is proposed in this research. The phase change material (PCM) is paraffin wax (PA), and the high thermal conductivity particles are graphite powder (GSP). CPCM is made in three different compositions and is filled in between cells with a 5 mm gap. Because PCM has a low thermal conductivity, it is filled with GSP, a high thermal conductive particle. The thermal conductivity is increased from 0.25 to 2.7 W/m K, which increases the heat transfer rate significantly. By adjusting different coolant flow velocities at varied discharge rates, the performance of the battery pack is examined. During the experiment, the discharge rates of 1 C, 2 C, and 3 C were used at a 28–30 °C ambient temperature. According to the findings, a trapezoidal battery pack based on CPCM exhibits a more efficient rate of heat transfer than a battery pack based on PCM. Moreover, BTMS with a liquid cooling system achieves consistent temperature distribution, with the maximum temperature remaining within the ideal range of below 45 °C under all test conditions.