Heat generated by gassing during float or equalization charging
can trigger thermal runaway in lead calcium batteries. All lead-acid batteries generate heat during normal operation. There is a small amount of joule heating simply from the currents flowing through the battery components. Also, lead-acid batteries are exothermic on charge, meaning the cell generates heat from the chemical recharge reactions. Under normal operating conditions, this generated heat is easily dissipated to the surroundings. However, after a battery is fully charged, the energy from the ongoing charge current breaks down the water in the electrolyte in a process known as electrolysis. The breaking down of the water into hydrogen and oxygen gasses is an endothermic process, which means that energy is required or absorbed during this reaction. However, if the oxygen migrates to the positive plate, it will immediately react and ultimately recombine back into water. This reaction is exothermic and will evolve heat. As with all normal chemical reactions, the resulting temperature rise will increase the rate of the reactions, thus causing the cell to accept an increased amount of charge current and that, in turn, causes more gassing and heat, thus precipitating the runaway condition.
What is therthermal runaway?What is therthermal runaway?
Thermal runaway occurs mostly – but not exclusively - in lead-acid cells or batteries. Within that family, thermal runaway occurs mostly in Valve-Regulated Lead-Acid (VRLA) cells.
What type of electrolyte is affected by thermal runaway?What type of electrolyte is affected by thermal runaway?
Within that family, thermal runaway occurs mostly in Valve-Regulated Lead-Acid (VRLA) cells. Vented Lead-Acid (VLA) cells are not immune to thermal runaway but, because of the large amount of liquid electrolyte they contain, and because water can be added to replace evaporated electrolyte, thermal runaway in VLA cells is rare.
What are the disadvantages of VRLA batteries?What are the disadvantages of VRLA batteries?
Plus, VRLA batteries have no mechanism for adding electrolyte to a dehydrating cell. Thermal runaway is the most catastrophic of all failure modes. If left unchecked, thermal runaway can lead to melting of the case, fire, or even (rarely) explosion.
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