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The aim of this project is to elucidate the mechanism of thermal runaway in VRLA batteries and especially the eventual role of the AGM separator. The oxygen evolution (OER) and oxygen recombination reactions (ORR) proceed in the cells, and the only work done by the input power is to transfer water from anode to cathode through these reactions, the remaining energy being dissipated as heat. The heat generators in the cell are the electrochemical reactions through their activation overpotentials, the exothermic chemical reactions and the exothermicity of ORR plus Joule Ohmic heating in electrolyte and solid phases. We have shown that, within the range of currents used (up to 1C), the major contribution comes from electrochemical reactions. The cell temperature is at most in a first order dependence of recombinant current. The OER at constant applied voltage and fixed ambient temperature is accelerated by the rise of internal cell temperature and overpotential, as the negative plate is depolarized by ORR due to enhanced separator permeability for oxygen flow by electrolyte displacement and/or drying and negative plate local desaturation. If the negative plate potential is stabilized at Pb/PbSO4 potential and cannot be shifted by any current applied, the cell behaviour is controlled solely by OER reaction. Thus, a lower applied voltage is enough to produce a current sufficient to generate conditions for thermal runaway. The results obtained in this study suggest strongly that by modification of separator properties it is possible to achieve better thermal control, in concert with other cell design improvements, plus the usual external measures as power input management and passive/active cooling., The original document contains color images.

The technical objective of the current project is to disclose the mechanism of the processes leading to Thermal Runaway (TRA) in VRLA batteries during overcharge. The TRA effect is related to an uncontrolled temperature rise reaching very high values of 80-100 degrees C on battery overcharge. An appropriate experimental set up was built for the purposes of the project work. It allowed us to monitor 5 cell parameters: current, positive and negative plate potentials, temperature and gassing rate of the cells. The experiments show that TRA may occur when the cell is exposed on continuous overcharge at a voltage higher than a certain critical value. It has been established that the reactions that proceed at the two types of electrodes are in selfaccelerating interrelation. At the positive plates, a reaction of water decomposition and evolution of oxygen proceeds. O2 is reduced at the negative plates whereby heat is released. Consequently, the temperature in the cell rises. The elevated temperature accelerates the reaction of water decomposition and oxygen evolution at the positive plates. Thus, the processes at the two types of plates accelerate each other. However, with increase of temperature, the heat exchange between the cell and the surrounding medium increases. When the exchanged heat becomes equal to the heat generated in the cell by the reactions at the negative plates, the cell temperature reaches a maximum. This phenomenon was called "self-accelerating interrelation between the reactions of the oxygen cycle (SAIR)", it refers to the spontaneous current increase. The thermal runaway (TRA), reflecting the temperature rise, and the SAIR are two sides of one and the same phenomenon designated by SAIR-TRA. Up to now the processes involved in the above phenomenon have been known as TRA effect. The increase in cell temperature and current during TRA depends strongly on the type of regulator used, the cell voltage, and the history of battery operation., The original document contains color images.