Method for Dimensioning Battery and Thermal Management Systems for Heavy-Duty Vehicle Applications Using Aged Battery Experimental Data and Advanced Modelling Techniques

When a battery degrades, capacity and power capability fade while losses in battery cells increase. Despite from fading capacity, an electric vehicle battery needs to deliver designed performance until battery end-of-life. To dimension sufficient margin for battery power capability and thermal management, comprehension of battery performance degradation is required. This paper proposes battery cell and module level laboratory testing combined with advanced modelling techniques to obtain design specifications from aged batteries. Extensive laboratory cell aging and characterization tests are performed to parametrise customised Simulink based dynamic electrical battery models with aged battery data at different degradation levels. Furthermore, a finite element thermal model implemented with COMSOL Multiphysics is used to model heat transfer in battery pack structure and thermal management system. This model is validated using experimental data from a prototype battery module. The Simulink model is used to check and validate electrical performance using measured drive cycle data and to provide data on battery losses as an input for the COMSOL model. The latter model is then used to check the performance of a thermal management system. This paper provides an example of this design approach by dimensioning a battery for an opportunity charged electric city bus. The dimensioning is made by using real drive cycle and energy consumption data from an electric bus operating in the city of Espoo, Finland. The chosen cells are then characterized and aged to near end-of-life. Simulations on electrical and thermal performance are then conducted using Simulink and COMSOL with parametrisations representing fresh and aged cells at different levels of degradation. These results are then compared with each other. This gives an indication on how much margin should be designed for power capability and thermal management and tells the performance based end-of-life criteria for the battery system. The results are discussed and recommendations made on battery dimensioning using specifications of aged batteries.