Thermal Runaway

Abstract

Heat may be generated in batteries by chemical and electrochemical reactions and by physical processes. If the rate of heat generation exceeds the rate of heat removal, the temperature of the battery will increase. As the temperature increases, so do the rates of any exothermic reactions and so does the rate of heat transfer. Chemical reaction rates tend to increase exponentially with temperature, and heat transfer rates increase linearly with temperature at moderate temperatures. At higher temperatures where radiation contributes to heat transfer, the rate of heat transfer has a component that increases with the fourth power of the temperature. Two thermal time patterns may be considered (Fig. 6.1A and B). Actual histories fall somewhere between the two extreme patterns shown in the figure. During the initial phase of pattern A, more heat is generated than can be removed by heat transfer, and the battery temperature increases. Then at time t ₁, the rate of increase of heat generation reaches a maximum value, and the rate of increase then decreases until at time t ₂ the heat generation attains its maximum value, beyond which it decreases. It may decrease because of the exhaustion of the reactive materials or because some external driver of heat generation has ceased to function. At time t ₃, the rate of heat generation becomes equal to the rate of heat transfer, and the battery temperature decreases beyond that time. If the maximum temperature reached by the battery is less than that required to induce venting, rupture, or explosions, the battery has experienced what we call a benign, temporary thermal runaway.