Battery Failure Mode: Battery Dry Out and Thermal Run-away
When a battery is charged, evaporation occurs, which reduces the volume of electrolyte solution (Water + Sulphuric Acid) inside the battery. It is primarily the water volume that is lost in this process. A vicious cycle is created as lower volumes of electrolyte (now with higher acid-to-water ratios) increase internal resistance causing excessive heating during charge, and that causes a further increase in water loss through evaporation. At some point in this incremental process, the water volume depletes (battery dry-out) to the point where a battery’s growing internal resistance, combined with the corrosion processes described earlier, causes so much heat during charge that a thermal run-away event can occur such as battery fires or melting.
Battery dry-out and the chance for thermal run-away are accelerated by acid stratification. Moreover, modern vehicle batteries that operate in a Partial State of Charge (PSOC) condition, that seldom receive a full charge, and/or are constantly deeply cycled or micro-cycled combine with acid stratification to supercharge battery dry-out conditions and increase the likelihood of thermal run-away. For this reason and others, average battery life is declining for the first time since the beginning of the 20th century.