Predicting transient behavior of lead-acid batteries during charge and discharge processes is an important factor in many applications including hybrid electric vehicles (HEVs). The conventional mathematical models, which are used to predict the battery dynamics, are either inaccurate or time-consuming. In this study, an improved and efficient mathematical model for simulation of flooded lead-acid batteries based on Computational Fluid Dynamics (CFD) and Equivalent Circuit Model (ECM) has been introduced which inherits the accuracy of CFD model and the physical understanding of ECM. This approach makes the numerical procedure very efficient and easy to implement. Moreover, because of simplification of boundary conditions (BC's), it is very fast which makes it quite suitable for real-time simulations. The present approach is verified by previous CFD models and experimental data.