Analysis of the Causes of Thermal Runaway in Batteries and Leakage Failures

　　1. Causes of Thermal Runaway in Batteries
　　There are many factors that can trigger thermal runaway in batteries. High charging voltages and excessive gas evolution can both lead to thermal runaway. If a single cell within a battery pack experiences severe thermal runaway while the constant voltage setting for charging remains unchanged, the remaining cells will also experience relatively high charging voltages, potentially resulting in thermal runaway as well. The causes of battery thermal runaway include:
　　(l) The oxygen complexation reaction is an exothermic reaction:
　　2Pb + O₂ → 2PbO + Q₁
　　Q1 = 219.2 kJ/mol
　　PbO + H₂SO₄ → PbSO₄ + H₂O + Q₂
　　Q2 = 172.8 kJ/mol
　　It will cause the battery temperature to rise; if the float charge voltage is not reduced in a timely manner, the float charge current will increase, which in turn leads to greater oxygen evolution and an intensified composite reaction. As this cycle repeats and accumulates, thermal runaway will eventually occur in the battery. The heat generated in the battery originates from the oxygen cycle within the battery: once the charging voltage reaches 2.5 V per cell, the negative plates must absorb oxygen to prevent the battery from losing water. However, when the negative plates absorb oxygen, they generate a significant amount of heat—and if the battery uses a lead–antimony grid alloy with poor heat dissipation, thermal runaway becomes even more likely.
　　(2) In compactly assembled batteries, the electrolyte is stored within a porous separator, making heat dissipation relatively difficult. Unlike conventional lead-acid batteries, which can stir the electrolyte during charging to promote gas evolution and enhance heat dissipation, this type of battery lacks that advantage. When a localized short circuit occurs inside the battery, the battery temperature will rise even higher.
　　(3) In summer, temperatures are relatively high—sometimes even exceeding 35°C—causing the float charge current to increase accordingly. If the float charge voltage is not reduced in a timely manner, the battery temperature will rise rapidly. Excessively high charging voltage leads to excessive gas evolution and triggers an over‑oxygen cycle. If even a single battery within the battery pack has an internal micro‑short circuit, this effectively raises the charging voltage as well.
　　(4) The safety valve is not tight or the opening pressure is too low.
　　2. Analysis of Battery Leakage Faults
　　Battery leakage faults are primarily caused by manufacturing defects—such as excessive electrolyte injection, inadequate sealing, substandard sealing materials, or aging of sealing materials—rather than mechanical damage incurred during transportation or handling. During battery production, some manufacturers apply silicone oil around the terminal posts to enhance the sealing performance of the battery casing; consequently, non-acidic liquids may leak around the terminals during use. This is a normal phenomenon and does not constitute leakage—careful distinction is necessary.
　　For batteries that are leaking, first conduct a visual inspection to identify the locations of acid leakage. Remove the cover plate and check around the safety valve for any signs of acid seepage; then open the safety valve to observe whether there is any free-flowing electrolyte inside the battery. After completing these steps, if no abnormalities are still detected, perform an airtightness test (immerse the battery in water, inflate it under pressure, and observe whether bubbles form and rise to the surface—any bubbles indicate acid leakage). Finally, during the charging process, monitor for any flowing electrolyte; if present, it indicates that the issue stems from the manufacturing process. If flowing electrolyte is observed during charging, it should be completely drained out.
　　During the production process, many batteries remain in a flooded state after acid filling, with no oxygen recombination occurring. The excess electrolyte is expelled through a three‑charge, two‑discharge cycle while the battery is left open, allowing the sulfuric acid density to rise once again. When the safety valve is installed, the electrolyte has not been fully absorbed, and free acid still remains. Even if the free acid is promptly removed, the battery still exists in a “quasi‑lean” state. The amount of electrolyte within the separators is relatively higher. However, this slightly increased electrolyte content in the separators can hinder oxygen recombination. As a result, during the initial charging of new batteries, the gas evolution is quite substantial, leading to a significant loss of sulfuric acid—resulting in “acid leakage.” For gel batteries, the first 50–100 cycles represent a transition period from a flooded to a lean state, during which gas evolution is particularly severe. The gas released carries away gel particles, further contributing to “acid leakage.”
　　Batteries manufactured both domestically and abroad exhibit leakage problems to varying degrees, primarily in the form of electrolyte leakage from the terminal posts and poor sealing between the battery case and cover. There are two main methods for sealing the battery case and cover: adhesive sealing and heat sealing. Adhesive sealing involves using epoxy resin adhesive to seal the joint between the case and cover; the quality of the seal is heavily influenced by the performance of the epoxy resin. The aging and cracking of epoxy resin are major factors contributing to battery leakage.
　　Batteries sealed with epoxy resin adhesives often experience significant leakage. However, if the epoxy adhesive formulation and curing conditions are carefully controlled, effective sealing can be achieved. An autopsy of batteries that had leaked after being sealed with epoxy resin adhesive revealed that the sealant in these leaking batteries was bonded to the battery case at the interface, with only weak adhesion—making it prone to delamination. At the leakage points, there were either gaps where adhesive was missing or cracks in the seal. Because epoxy resin adhesives have relatively poor fluidity (especially when cured at low temperatures), certain localized areas of the sealed shell cover may not be fully filled with adhesive, thereby creating leakage pathways.
　　Heat sealing involves heating the ABS housing to a specific temperature—where it exhibits a certain degree of fluidity and adhesiveness—before filling the gap between the battery case and the cover. Because this method integrates the case and cover into a single unit, the entire bonded joint between the case and cover is made of ABS, resulting in highly reliable sealing and effectively eliminating leakage between the case and cover.