Battery systems in electric vehicles are key components deciding performance, electric driving range, and total cost of the vehicle. Recently, lithium-ion batteries are popularly adopted with their advantages of high energy density and performance. At the same time, protecting battery cells from crash-related damage is a serious concern for vehicle manufacturers. The Crashworthiness of battery systems according to the regulation-based crash mode has been widely studied in previous research. However, there is a lack of study of ground impact-based crash mode in real road conditions. This paper presents a comprehensive study of the ground impact on lithium-ion battery packs in electric vehicles. Realistic system-level test procedures and conditions are proposed and the battery case structures are optimized to achieve the test criteria. A scenario-based study was carried out to overcome the lack of regulation and field data on ground impact. Vehicle-level test modes were defined and the results are analyzed and compared with field cases. The test cases were classified based on the severity and failure modes of the battery pack and then two categories of pack-level test conditions were proposed. The reliability test represents normal use case and failure mode in lower speed impact with fixed obstacles. Safety test represents severe conditions in higher speed impact with unfixed random obstacles. To translate the crash energy from each test condition to system-level validation, free-fall drop tests were proposed. Four different types of pendulums with a weight of 10kg were used and the drop heights were determined 2m and 5m for reliability and safety, respectively. Battery case and protective structure are optimized based on the failure mode of each test condition and verified by pack-level tests. To strengthen the robustness of lower case, a sheet mold compound (SMC) based protective cover was adopted and the layer structure of the SMC sheet was optimized. The geometry of the protective cover was enhanced with topometry optimization technique, in considering the relation of connectivity and road pass of the lower case. By adopting the proposed design, the safety performance of the protective structure is improved. As a result of the drop test, the maximum deformation of the battery pack case was reduced by 28.1 %. At the same time, and weight of the safety structure can be reduced by 16.1% compared to the original design with the aluminum-based case. Comprehensive test standards for the ground impact scenarios and the optimized safety structure under the battery case were proposed. These study results are expected to contribute to the enhancement of safety for automotive battery systems and electric vehicles.
Mr. Yong-Hwan Choi, Part Leader / Senior Research Engineer, Hyundai Motor Company / Seoul National University