Valve-regulated lead-acid battery continues to be studied in some cost-sensitive applications, such as microhybrid electric vehicle and scooters. A dynamic battery I-V model is needed for system design, simulation, and real-time control purposes. Battery discharging behavior is one of the most important parts of the model. By reviewing previous studies, the authors find that there is a contradiction between the linear and nonlinear I-V behaviors, and this serves as a motivation for the current experimental study. Through regression analysis of experimental data, the nonlinear behavior is verified and a nonlinear dynamic Thevenin model is developed. The model is validated by voltage response tests with multistep current profiles as model input. The calculated voltage response results from the model fit the test results well.