This investigation into phase change material (PCM)-based passive thermal management systems was conducted via an experimental approach using 19.5 A h lithium iron phosphate cells with dimensions of (7.25 × 160 × 227) mm3. Trials were conducted at currents from 1 to 5C and environmental temperatures from 4 to 35 °C to simulate applications at which a Li-ion battery would be expected to perform. Based on comparisons, including an air-only control, the system consisting of PCM combined with five pores per inch (PPI) aluminum foam is the most effective at regulating average battery temperature and temperature gradient. During a 3C discharge trial at room temperature, the PCM-Al foam (5 PPI) system kept the average battery temperature and the maximum temperature difference below 28.1 and 5.2 °C, respectively, compared to the air-only control system which reached values of 48.0 and 17.2 °C, respectively. When analyzing data from trials at 4 and 35 °C, similar results are found with the PCM-Al foam systems being effective at thermal management. Thus, when compared to other systems, preliminary results show great promise in the future for the use of an PCM-Al foam passive thermal management system to effectively regulate the temperature of Li-ion batteries during use.