A two-dimensional time-dependent thermal model was developed to predict steady and unsteady behavior of valve-regulated lead-acid (VRLA) batteries under overcharge. The effects of various design and operating parameters on the thermal behavior of VRLA batteries are investigated. It is found that thermal runaway is a consequence of inappropriate combinations of these parameters : number of cells packed together, case conductivity, heat-transfer coefficient, water-loss fraction, float current, and the ambient temperature. The model could be used to help design more thermal-runaway-resistant VRLA batteries.