An air-cooling battery thermal management system is a reliable and cost-effective system to control the operating temperatures of the electric vehicle battery pack within an ideal range. Different from most designs of the rectangular battery pack in previous research, this one proposed a novel isosceles trapezoid layout to improve system heat dissipations. The simulation results showed that the trapezoid design delivered better cooling performances than the rectangular one with a maximum temperature reduction of 0.9 °C and maximum temperature difference reduction of 1.17 °C at the inlet air flow rate of 60 L/s. Moreover, the cooling performance was further boosted by an aluminum heat spreader. The boosted design delivers an average Max T (32.95 °C) and an average ΔT (3.10 °C) at five different flow rates, which are 8.8% and 66.1% lower the one without the spreader (35.85 °C and 5.15 °C). Compared with the rectangular design without the spreader, the average Max T and ΔT of the boosted trapezoid design are reduced by 10.4% and 91.9% in addition to a space-saving of about 5.26%.