The Vanadium reduction oxidation (redox) flow battery is one type of rechargeable batteries. The battery has abilities of high-speed response and overload operation. Characteristics of the redox flow battery have been experimentally and numerically discussed for periodical operation and transient states, at many institutes and companies. This paper focuses on the modeling of redox flow battery near practical operating setups. In particular, the equivalent series resistance and the overpotential at cell are estimated based on the experimental results.