A three-phase, electrochemical and thermal coupled model is developed for valve-regulated lead-acid (VRLA) batteries. Physical phenomena important to the VRLA battery overcharge process, such as gas generation, transport, and recombination, electrolyte displacement and capillary flow, and the venting event during discharge/rest/charge, are incorporated in the model. The effects of important parameters, including the electrolyte saturation level, interfacial mass transfer coefficient of oxygen, and electrode morphology factor (MF), are extensively studied. An overview of the simulation capabilities of the present comprehensive model is provided.