Preliminary Study on the Safety of a Lithium Ion Battery Pack Under Water Immersion

Abstract

Most battery packs are required to comply with IP67 standard to guarantee waterproof before commercialization. However, passing the IP67 standard cannot 100% guarantee that the battery pack do not leak when immersed in fluid like dirty water or sea water during the whole life cycle, especially under some extremely conditions. It is reported that, in 2012 several Fisker Karma electric vehicles were burned after sea water intrusion caused by Hurricane Sandy. In 2016, two electric buses were burned into fire after overnight water immersion due to a heavy rain in Nanjing, China. Obviously, it can be very dangerous when water seeps through the cover of battery packs and cause electrical short circuit inside the battery. Therefore, it is critical to study on the phenomena on and the mechanism of water immersion upon battery pack. It is known that the whole battery pack is a complex system. And it is hard to monitor all the phenomena and confirm the repeatability of the experiment if the whole battery pack is thrown into water directly during investigation. Moreover, ruining a whole battery pack cost so much. For these reasons, a substitute experiment was proposed. The two special batteries/modules, which at the highest potential or lowest potential, were considered as the most probably dangerous ones in a battery pack(Figure a). The voltage of the whole battery pack was simulated by a high voltage Digatron battery charger. A single cell was connected with the charger in series. When anode of the single cell is connected to the positive electrode of the charger, it represents that the cell is in the highest potential. Meanwhile the negative electrode of charger is connected with a copper sheet to simulate a full circuit of the battery pack, as shown in Figure b1. Similarly, to simulate the single cell being in the lowest potential, cathode of the single cell was connected to the negative electrode of charger, while the positive electrode of charger was connected with an aluminum sheet as shown in Figure b2. During the experiment, only the tested battery and metal sheet were soaked in salt water with a 3.5 wt% concentration of NaCl to simulate the environment of sea water. In addition, experiments with both electrodes of the charger connected with two metal sheets(Figure b3) or two single cells(Figure b4) were also conducted for references. Pouch cells and prismatic cells were used in the study. During the experiment, we found that when a single cell was immersed into water, electric arc can be observed near the poles of the cell. The poles of prismatic cell fused near the area where the electric arc was triggered. A ball of flames appeared obviously and some other small flames can be seen floated on the water surface. After experiment, the special odor of electrolyte can be smelt above the water, indicating that the electrolyte has leaked from the shell of prismatic battery(Figure c). The cell material and separator were found intact after disassembled. The test of the pouch cell showed similar phenomena of the electric arc, however, no obvious flame appeared on the water surface. For further analysis, dummy cells with only empty shells were used. Electrolyte was injected into the empty battery shells, then similar tests as Figure b2 were conducted. The same experimental phenomena for dummy cells were observed, and the electrolyte was considered the main cause for the flame on water. In conclusion, electric arc containing enormous energy will be released due to high voltage when the battery pack is immersed under water. And the arc hazard is only caused by high voltage, with limited relationship with the cell. The electric arc with large amount of energy will make the poles or the battery shell fuse then lead to battery failure, including electrolyte leakage. The leaked electrolyte can be ignited by electric arc which is one of the most dangerous factors in battery pack water immersion. Acknowledgement: This work was supported by Project U1564205 supported by National Natural Science Foundation of China. Figure 1