Valve-regulated lead-acid (VRLA) batteries that have aged on a float charge at constant voltage occasionally suffer from thermal runaway. Operating conditions for a VRLA battery have been simulated by changing the electrolyte saturation level in the separator and the ambient temperature. The charge current, battery temperature and cell overpressure were measured during current-limited constant-voltage charging. The experiments show that applied voltage, saturation level and ambient temperature are significant variables in the oxygen cycle. However, the saturation level of the electrolyte in the separator pore volume is critical. When it is lower than 80%, thermal runaway occurs readily. Significant corrosion of the positive grid and poor conductivity between the grid and the active mass (AM) is also found in aged VRLA batteries, and many inactive PbSO4 crystals appear on the negative plates. As a result, both positive and negative plates have a very high resistance, which can accelerate thermal runaway.