The applications of Li-ion batteries used in energy storage system (ESS) in building environment is drawing more and more public attention and it is predicted to hold the most share of battery market in the year of 2020. In this paper, an equivalent circuit battery model for the ESSs applications in building environment is proposed, and the parameters are extracted experimentally. A new methodology to separate the fast and slow dynamics of the model is presented, and the parameter extraction results show a high-performance accuracy. The State of Energy instead of the State of Charge of the battery is used as model output in order for accurate battery energy state prediction during discharging. The discharging power is taken as the direct input of the proposed model, instead of the discharging current. The model is set up in Matlab environment, and the testing results of a commercial Li-ion battery are presented.