Abstract

The use of Li-ion pouch cells in electric vehicles has increased significantly over the past few years. To ensure smooth operation and long life of the battery pack, it is essential to provide optimal conditions for the cells. Appropriate thermal management system has to be designed to maintain the temperature inside the battery pack at optimal condition. A finite element model that can estimate the temperature in a cell is crucial for the design of battery packs and thermal management systems. EDAG has developed a methodology using a fully coupled finite element macro-model that can describe the electrical, thermal and mechanical behaviour of a pouch cell. This methodology is also transferred to cylindrical cells. Internal resistance is an important electrical parameter of a cell. The macro model is built in such a way that its resistance in between two tabs corresponds to the internal resistance of the cell. The internal resistance in the model is also made dependent on State of charge (SOC) and temperature. Subroutines are used to calculate the SOC as a function of current, time and cell capacity. The approach can be extended to ageing dependency if the necessary data is given. The parameters important for the electrical and thermal behaviour are derived from tests. Main parameters are internal resistance with respect to SOC and temperature, cell capacity and specific heat capacity. Other Parameters to describe convection, radiation and conduction are taken from literature study and verified with cooling curves of cells. Multiple charging and discharging tests have been conducted with different current profiles and the temperature measurements from the tests have been used to validate the macro models of pouch cell and cylindrical cell. This method can now be applied to design module layouts with respect to optimal temperature distributions and is the basis -together with a CFD simulation coupling- to design complete cooling systems of batteries in electric vehicles.