Replication of the thermal behaviour of lithium-ion cells with substitute cells

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The performance as well as the lifetime of lithium-ion battery cells are strongly temperature-dependent. To achieve optimal results in both aspects, efficient thermal management systems are needed in the automotive application. Along the development process of these systems, manufacturers face certain obstacles considering the validation of battery systems. An extensive experimental infrastructure is required and high safety regulations have to been met considering the investigation of lithium-ion battery cells. The thermal behaviour of the cells changes as a result of calendric and cyclic degradation, which leads to a reduced reproducibility considering the experimental validation of the thermal management systems. Extreme situation tests are impossible because of the danger of cell damage or thermal runaway. Finally, the availability of new cell generations is strongly limited in early development stages.
To overcome the mentioned obstacles this work focuses on the development of thermal substitute cells, which exactly replicate the dynamic thermal cell behaviour but no longer contain any electrochemical storage function. These substitute cells have no electrochemical components and therefore do not succumb to degradation. They require no complex equipment, have low safety limitations and enable the possibility of extreme situation investigations. Furthermore, they provide the possibility of flexible design adaptations and are highly available even in early stages. With these advantages they enable a reduction of development time and costs and improve the efficiency and quality of thermal management systems.
In this contribution, the development steps for thermal substitute cells are presented with the focus on the characterisation and investigation of the thermal behaviour of the chosen reference cell. This reference cell is a high-energy automotive prismatic hardcase cell. For investigation of the reference cell thermal behaviour a 3D simulation model is developed and parameterised by in-house characterisations. In simulation studies the interaction of the inner cell components with the outer thermal boundary condition are evaluated and the critical heat transfer paths are identified. Three different thermal management applications, namely the bottom, side and pole cooling, are considered as different thermal boundary conditions. The characteristic influence of each of these outer cooling applications on the inner temperature distribution of the cell is shown.
Additionally the thermal behaviour of a first prototype of the substitute cell, which was derived through a simulation-based development, will be compared with the thermal behaviour of the reference cell. The results of the substitute cell show a very good agreement with the reference cell and a successful first proof-of-concept can be demonstrated.

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