The safety of lithium ion battery power system should be enhanced before the massive application of electric vehicles. The safety problem needs to be fully combed in order to improve the safety of the vehicle power battery system. This paper attempts to interpret the safety problem of lithium ion power battery system and to find further solutions. The safety of power battery system is divided into three levels in this paper, namely, "evolution", "trigger" and "propagation". The "evolution" refers to that the failure may be experienced a long evolutionary process before the battery safety accident occurs. The "trigger" is the turning point of the "evolution", and can also be unexpected events that destroy the power battery system upon vehicle accidents. The thermal runaway mechanism of lithium ion power battery is expounded, and different triggering ways of thermal runaway are analyzed. On the issue of the "trigger" of the safety accident of the power battery, the most critical point is the thermal runaway. Its "propagation" should be prevented when thermal runaway "trigger" occurs. Understanding more about the thermal runaway "propagation" mechanism can help designers to optimize safety design, prevent thermal runaway "propagation", and reduce the degree of damage caused by safety incidents. Based on the discussion on the "evolution", "trigger" and "propagation", this paper puts forward a number of measures for accident prevention and safety monitoring.

lithium-ion battery power system  /  thermal runaway  /  battery management system  /  safety monitoring