Lithium-ion batteries for long-range electric vehicles entail a higher cell packing density. External influences or faults in individual cells in the battery module can lead to a thermal runaway. When this happens the cell is destroyed by internal exothermic reactions, and large amounts of energy are released. This can lead to chain reactions (propagation) in the storage system that affect the entire battery pack – usually extremely large amounts of energy are released and the system catches fire. As part of a project with the Joint Research Centre, Directorate for Energy, Transport and Climate of the European Commission, experimental studies are being carried out to determine how test procedures can be designed to evaluate the propagation properties of batteries. The results will later be incorporated into legislation on battery testing and qualification.
Excitation methods have been investigated to reproduce the thermal runaway in lithium-ion cells of different types. Stimulation by micro-heating elements or by nail penetration (see fig. above) proved to be suitable. The inductive heat input method (contactless) proved promising but still requires some further development. Further research involved the applicability of the excitation methods for battery module tests, error propagation, emission analysis and quantification. 5s1p and 10s2p modules from a replacement battery of an original electric vehicle were examined (see fig.). The cell is a LiC-NMC cell with a capacity of 40Ah and an energy density of about 180Wh/kg. Investigations were carried out to determine the stability and repeatability of the tests, to assess the variability of the results under the same test conditions, the effect of thermal barriers between the cells and the delay or prevention of propagation.