Abstract: Battery safety problem, which is resulted by thermal runaway, still hinders the wider market penetration of electric vehicles. Thermal runaway will be initiate when battery exposed in abuse conditions, then the amounts of heat are released, resulting the thermal runaway propagation. Although the mechanism of thermal runaway and thermal runaway propagation is well documented, most of researches base on cell and module. For exploring the thermal runaway propagation mechanism of pack, a thermal runaway propagation test is conducted for pack. According to the level of energy release, the thermal runaway propagation of system is divided into three phases:the propagation within module, the propagation between modules and the deflagration. Additionally, modals have been concluded from test data that include the ordered propagation, the synchronous propagation and the reversal propagation. The mechanism of thermal runaway propagation is revealed considering the propagation results and pack construction. The liquid cooling plate have a guiding effect on the thermal runaway propagation path. The loss of pack integration is dangerous, because it leads to high-potential combustion. The air gap can mitigate thermal runaway propagation. Overall, the characterization and mechanism of thermal runaway propagation of system is revealed through above analyzes, and the results provides a basis for the safety design of pack.

Key words: lithium-ion batteries, battery pack, thermal runaway, battery safety, energy storage