An accurate lumped electro-thermal coupled (ETC) model for a full battery pack is proposed and validated using the 3D computational fluid dynamic (CFD) approach. The electrical performance is modeled using equivalent circuit model (ECM). The thermal part is modeled using the reduced-order method (ROM) for each of the individual modules. The ECM and ROM are two-way coupled. The module ROM cold plate connection is through heat transfer coefficient with varying flow rate conditions. Finally, the accuracy of lumped ETC model is validated and compared with 3D CFD simulations. Using this approach, the comparison shows that the results from such a lumped ETC model with varying flow rates are within 5% of the CFD results.