12Passive thermal management systems using phase change materials (PCMs) 13 provides an effective solution to the overheating of lithium ion batteries. But this 14 study shows heat accumulation in PCMs caused by the inefficient cooling of air 15 natural convection leads to thermal management system failures: The temperature in a 16 battery pack operating continuously outranges the safety limit of 60 o C after two 17 cycles with discharge rate of 1.5C and 2 C. Here a hybrid system that integrates 18PCMs with forced air convection is presented. This combined system successfully 19 prevents heat accumulation and maintains the maximum temperature under 50 o C in 20 all cycles, even with 7 o C rise in the ambient temperature. Study on airspeed effects 21 reveals that thermo-physical properties of PCMs dictate the maximum temperature 22Recently, passive thermal management system using phase change materials 47

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