Battery management systems are among the key components in electric vehicles (EVs), which are increasingly replacing internal combustion engine (ICE) vehicles in the automotive industry. Battery management systems mainly focus on battery thermal management, efficiency, battery life and the safety conditions. Generally, lithium-ion batteries have been chosen in EV cars. Therefore, the internal resistance of Li-ion batteries plays a crucial role in the thermal behavior of the energy storage system. Most of the published studies rely on 0D-1D models to analyses single cell thermal behavior depending on the internal resistance at different ambient temperatures and charging/ discharging rates, and on the cooling system. However, these models, though fast, cannot provide detailed information about the temperature distribution within a cell or a module. Full 3D Computational Fluid Dynamics (CFD)- Conjugate Heat Transfer (CHT) simulations on the other hand, are very time consuming and require robust computational resources, but allow a deeper understanding of the cell/module thermal evolution with and without cooling. In this study, a method has been developed to reduce the time required for 3D simulations. In the 3D model of a battery module with 21700 Li-ion battery cells the liquid-cooled base plate of the battery module is replaced by a solid aluminum plate. The approach consists mainly in assigning constant temperature on the outer surface of the rectangular component during the simulations. In this way, it becomes possible to calculate the temperature evolution of the module’s cells without having to use the very time-consuming Conjugate Heat Transfer model, with only moderate penalties in terms of accuracy. The simulations were run at 1C and 1.5C discharge-charge rates at different ambient temperatures. In addition to the 3D simulations, the numerical results will be validated with some experimental measurements.