Dynamic abuse testing of lithium-ion cells

Special test benches for dynamic impact loading of charged and discharged battery cells were erected at the Fraunhofer Ernst-Mach-Institute in order to investigate the influence of strain rate and state of charge on the mechanical behavior of cells. Both parameters are regarded as of major importance for a comprehensive safety evaluation of electric vehicles. The primary test bench is a special-purpose hydraulic machine which is set up in a safety environment so as to allow dynamic tests with fully charged real EV battery cells and modules. It provides a static force up to 500 kN and allows tests to be done in the velocity range between 0.5 mm/s and 5 m/s. A second complementary test stand is a commercial servo hydraulic machine, intended mainly for basic investigations on dynamic material properties and dynamic tests on small discharged cells. It offers a greater dynamic range, greater flexibility, and a higher force resolution compared with the bigger test stand. The combination of both test stands allows for detailed investigations of strain rate effects of battery cells. First tests with both machines reveal a significant influence of the test velocity on the results of destructive mechanical tests of battery cells. These findings are of major importance for a realistic evaluation of battery crash safety. They provide important input for EV crash simulations and stress the necessity for dynamic tests in order to access realistic failure limits for battery cells.