In this study, we analyzed the thermal performance of electric vehicle lithium batteries with phase-change materials (PCMs) at 26 °C. PCMs and temperature sensors are the major factors affecting the thermal performance. To improve the heat transfer rate of PCMs in a lithium polymer pouch battery with 16 Ah capacity, a battery thermal management system with different PCMs, namely, RT15, RT31, EG5, and EG26, is proposed and experimentally investigated. The results show that the temperature of the battery surface without PCMs increased to 35.9 °C at different discharge rates at a room temperature of 25 °C. In the case of the battery with EG26, its surface temperature was 30.5 °C, which shows a temperature difference of 5.4 °C from the battery without a PCM. A low temperature-rise rate and a uniform temperature distribution were observed in the batteries with PCMs. In addition, a numerical simulation using the computational fluid dynamics software FLUENT was also carried out. The numerical simulation results are in good agreement with experimental data, such as the temperatures of batteries monitored with sensors.