Evaluating the Thermal Runaway Propagation within a Battery Pack: The Unsolved Problems That Hinders the Consensus on the Test Profile in Electric-Vehicle-Safety Global-Technical-Regulation
Authors: Feng Xuning , Jin Changyong , Wang Fang , He Xiangming , Ouyang Minggao 
Published: February 2020

Electric is going to be a thing of reality rather than a thing of future, once its mileage extends to 300 miles or longer, and its cost can be reduced to $80 per kWh or lower. The lithium-ion batteries, of which the technology advancement is far beyond our imaginations, are quite near the target of cost and mileage. Even those previously “calm” manufacturers are now setting radical targets for producing electric cars.

Then we realize that there will be new problem for the international trade of electric vehicles. How to guarantee quality of the electric vehicles that are sold everywhere around the world? And how to avoid local protectionism from cross-border trade? The World Forum WP. 29 is establishing Global-Technical-Regulation (GTR) for Electric-Vehicle-Safety (EVS), to set same test standard for all the participant countries. Safety is the highest priority that the EVS-GTR want to guarantee. 5 years has passed since the origination of EVS-GTR, and the answers of phase 1 to guarantee safety were voted through. However, there are still disputes in the Thermal Propagation Test (TPT), the atmosphere of disagreements exists in every detail of the test profile.

As for the rationale of the TPT, most experts believe that it is unquestionable. Because it is the obligation of the EVS-GTR to guarantee product safety for the innovation of vehicle technologies. Moreover, if one battery pack can pass the TPT, it always means that the safety design is successful. However, different opinions also make sense, because they believe that there will be very low possibility for thermal runaway triggering. Despite the TPT, the other test procedures, such as overcharge protection, have already eliminate the possibility of thermal runaway triggering in previous accidents. In this talk, we will give comments on the rationale of the TPT. Some strong evidences will support our arguments.

As for the triggering of the TPT, there are still several problems for the participants to reach an agreement. The requirement of smallest modification of the battery pack under TPT seems to be most difficult technical problem for the engineers. Talents are called for this annoying point. Spontaneous internal-short-circuit might be the cause of field failure of lithium-ion batteries. However, due to lack of details for the accidents, sometimes we suspect the existence of the spontaneous internal-short-circuit. The judgement of thermal runaway forms another problem, because with small modification of the battery pack, we have to judge the successful triggering of thermal runaway by the measured voltage and temperature. Unfortunately, the features of battery thermal runaway are quite different for cells with variant chemistries. In this talk, we will give solutions to the problems on the triggering approach of the TPT. Open questions will also be discussed.

As for the propagation judgment of the TPT, we need a fair criterion to judge whether a battery pack can pass the test or not. The current pass flow looks good, but not perfect. Questions goes to that shall we conduct the test at vehicle level or at pack level. Vehicle level is more reasonable, but cost too much; while pack level deviates from reality, and smoke is also hazardous to the passengers, but some believe that smoke can be allowed during TPT. Reasons for the current Chinese proposal, including details in the test parameters, will be provided in the talk.

The last step for the battery industry seems to increase the energy density from 250Wh/kg to 300Wh/kg. Li-ion batteries with nickel-rich cathode can fill the gap of cost and mileage. Although academic knows that the safety of the lithium-ion batteries at this step may have worrying deficiency in thermal stability. The industry has charged too far into the battlefield. We will also show the thermal runaway propagation test results for cells with different energy densities, providing references for the audience to stay calm with the utilization of high-energy batteries. Furthermore, we will show our concern that if the TPT does not stand, there might be severe hazards for the cells with higher energy density.

Keywords: Battery Safety; Thermal Runaway; Thermal Runaway Propagation; Standard

Acknowledgement: This work is supported the Ministry of Science and Technology of China [Grant No. 2018YFB0104404 and No. 2016YFE0102200]; by the National Natural Science Foundation of China [Grant No. 51706117] and [Grant No. U1564205]; the China Postdoctoral Science Foundation [Grant No. 2017M610086]. The first author thanks the support of “Young Elite Scientist Sponsorship Program” from China Association for Science and Technology.