Complexity in battery systems: Thermal runaway in VRLA batteries

During battery discharge, the heat generated is the sum of the Joule (resistive) and enthalpic (chemical) heating effects. Conversely, during battery charging, the heat generated is the Joule minus the enthalpic heating. If the conditions are carefully selected, one can observe a net battery cooling during charging. However, an interesting phenomenon takes place during overcharge. Those cells designed as sealed recombinant systems develop significant heating. Flooded designs do not exhibit this effect. The applied electric power generates energetic reaction products as a consequence of the electrochemical reactions. This is an energy absorbing process. The gasses are then vented into the environment. Since the sealed cells undergo a closed recombination cycle, i.e., no material is exchanged with the environment, the rate of heat generated is proportional to the power input to the cell. Essentially, the cell is behaving in the manner of a resistor. In this connection, the thermal runaway phenomenon that has been often observed in starved electrolyte cell designs raises a potential problem in battery applications. It is not efficient to design around the worst case scenario, i.e., anticipating the thermal runaway effect. It is wiser to detect its onset and shut down the charging process. An alternative approach is to develop an understanding of the thermal runaway process and, perhaps, develop a method for eliminating or effectively controlling it. A study was performed in an attempt to model the thermal runaway effect. In short, the effect appears to be related to the electrolyte distribution in the separator. This suggests that modification of the AGM separator properties could provide a means for better controlling the thermal runaway failure mode.