A transient nonisothermal model for valve-regulated lead-acid (VRLA) batteries under float was developed. The model includes the mass balances of all reactive species in the electrolyte and all possible reactions occurring during float charge. The overall energy balance and the function of the Bunsen valve are incorporated so that the temperature and pressure responses of the cell can be simulated. Effects of parameters such as the heat-transfer coefficients, effective diffusivities of gases, exchange current densities of reactions, gas volume, and the ambient temperature on the transient behavior of VRLA cells are studied using this model. It was found that thermal runaway could occur due to the combined effects of poor heat transfer, high float current density, high oxygen cycle efficiency (high gas diffusivities), and high ambient operation temperature. High gas diffusivities can improve the oxygen cycle efficiency and reduce water loss for VRLA cells under normal operation; however, they could also exacerbate the situation for cells with poor heat dissipation and high exchange current densities, or operations under prolonged high ambient temperature condition.