Abstract: In order to explore the law of influence of battery number, state of charge and battery spacing on thermal runaway spread of 18650 ternary lithium battery pack, the experiment platform of measuring characteristic parameters of battery overcharge initiation and out- of- control process was designed and established. The results show that single master battery shall experience about 13 s from incipient detonation to flame extinction on condition of the current experiment and will not initiate thermal runaway to the other batteries. However, runaway of two master batteries can initiate thermal runaway of single receiving battery with above 60% SOC or 2 mm battery spacing, but not for single receiving battery with 40% SOC or above 3 mm battery spacing. Accordingly, thermal runaway will happen to receiving battery with sufficient electric quantity on condition of receiving sufficient heat and heat flow intensity during runaway of master battery. It can be inferred that if the air in the battery box contains sufficient amount of effective explosive inhibitor, it can avoid the burning of open flame, and thermal runaway spread of battery pack can also be avoided if thermal resistance between the master and receiving batteries increases. 

Key words: ternary lithium battery pack, thermal runaway, state of charge, space of batteries