A Test Approach for Evaluating the Safety Considering Thermal Runaway Propagation within the Battery Pack
Authors: Gao Shang , Ouyang Minggao , Lu Languang , Ren Dongsheng , Feng Xuning 
Published: February 2020

With increased energy density and extended cycle life, lithium ion battery is preferred to be the rechargeable energy storage system (REESS) for electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, accidents with thermal runaway (TR) of lithium ion batteries occurred sporadically. The TR of battery can be triggered by various abuse conditions, i.e. overheat, overcharge, penetration, spontaneously triggered internal short circuit, etc. In order to diminish the TR hazard from abuse conditions, battery must pass specific test standards, i.e. UN 38.3, UN R100, SAE-J2464, IEC-62133, GB/T-31485, and others. However, the mechanisms of the spontaneously triggered internal short circuit are still unclear and unpredictable. Moreover, the abuse conditions may vary in practical conditions. The TR triggering at single cells cannot be totally eliminated although passing the test standards.

Once the TR of a single cell occurs, it will dissipate heat to the surroundings and trigger TR of the adjacent cells, resulting in TR propagation. The TR energy released by a single cell is limited, whereas the total energy of the battery pack during TR propagation can be huge and catastrophic. Therefore the prevention of the TR propagation must be considered in safety design of the REESS. When TR of a single cell in the REESS occurs, the propagation speed should be regulated and set aside enough time for the passenger to escape from the EV. Previous works have been proposed to study the characterization of the TR propagation in the battery pack. However, there are still few literatures that have discussed the test approach for evaluating the hazard level of the TR propagation. A standardized test approach to evaluate the hazard potential of TR propagation in the battery pack is proposed in this paper.

Fig.1(a) shows the flow chart of the test approach. First, a heater is selected to heat a specific battery in the battery pack. The heater is inserted in the battery pack, and the thermal couples and the voltage sensors are set to monitor the temperature and voltage of the specific battery. Next, the specific battery is heated by the heater with a preset power, followed by the judgement of whether the battery undergoes TR. The TR judgement is conducted by the characteristic of temperature and voltage. The battery is continuously heated until the preset temperature limit is reached. If TR still does not occur at the temperature limit, stop the test and the battery pack is regarded as safe. If external fire or explosion can be observed by visual inspection within a certain period time after TR is triggered at the specific battery, the battery pack is regarded as poor in the prevention of TR propagation. The escape time for the passenger would be insufficient once a TR is triggered at single cell.

The judgement of TR is determined by three criterions: 1) Maximum temperature exceeds temperature limit Tx. 2) Voltage drops to an unacceptable level Vx. 3) Temperature rise rate exceed a threshold of Rx.

Experiments are conducted to validate the proposed approach to evaluate safety of the battery pack under TR propagation. Fig.1(a) shows that the heater is inserted between two specific batteries within the battery pack. The appearance of the whole battery pack during test is also illustrated in Fig.1(a)(b). The result of one trial test is selected. The maximum temperature of the specific battery exceeds 300℃ (Tx), and the voltage drop of the battery is beyond 1V (Vx), and the temperature rate also transcends 10℃/s (Rx). After the TR is triggered in the specific battery, no external fire was observed by visual inspection within the following 5 minutes. Therefore, the battery pack is judged to be safe and the TR propagation hazard is regulated at low level.

In conclusion, a test approach to evaluate the TR propagation hazard in the battery pack is proposed and the judgement of TR is determined by three criterions. A battery pack is tested utilizing the proposed approach. It is verified that the battery pack have qualified safety in TR propagation hazard.

Figure 1