3D electro-thermal characterization and modelling of lithium ion cells as well as batteries in order to assess cooling designs and strategies

The thesis presents a comprehensive 3D electro-thermal characterization of lithium ion cells in traction application up to 300 A current load. The 3D electro-thermal behavior of lithium ion cells is investigated applying novel measurement techniques as well as a novel 3D electro-thermal model approach, coupling a 3D thermal FEM model with a 3D electrical equivalent circuit model. The study focuses on inhomogeneities of internal state values like temperature, current, SOC, voltage, power loss and SOH. These inhomogeneities occur during application especially in case of active cooling. Inhomogeneities of internal state values have major impact on lifetime, performance and range of lithium ion batteries in HEV and EV application. Therefore, the thermal management and the cooling strategy are an integral part of the development of lithium ion batteries for traction application. Coupling this 3D electro-thermal model with 3D ageing models provides the possibility to assess cooling designs and strategies with respect to their impact on the occurrence of inhomogeneities as well as their impact on the battery lifetime. From the results in this thesis design rules for active cooling systems are deduced as well as improved cooling strategies are developed in order to increase lifetime and performance of lithium ion cells. Moreover, in this thesis novel control algorithms are presented, which can be applied on a BMS, in order to control battery current with respect to local current differences. This technique prevents local cell degradation due to lithium plating effects, which is identified a key issue especially for upcoming vehicle application, due to higher power and range requirements on future batteries, induced by the electrification of heavier vehicles.