Abstract： To improve the performance of battery thermal management system, a battery thermal model was built based on the analysis of battery structure and the thermal conductive theory. With the help of this model, the internal and surface temperatures of the battery were predicted online. By comparison of predicted and measured temperatures, the thermal faults were diagnosed. The battery management system transmits diagnostic information to the whole car control unit to make the car react in time. The experimental results indicate that this system is able to detect the thermal faults such as fan failure and battery overcharging in time and exactly, which effectively avoids battery overheat under high temperature conditions.