Lead-acid batteries, despite their long history and widespread use, are susceptible to several failure modes that can significantly reduce their performance and lifespan. Understanding these failure mechanisms is critical for improving battery design, operation, and diagnostics. The most common failure modes include positive grid corrosion, negative plate sulfation, active material shedding, electrolyte stratification, and internal short circuits. Each of these failures has distinct root causes, often linked to operational conditions such as overcharging, deep discharging, and exposure to temperature extremes.

Positive grid corrosion is one of the primary failure mechanisms in lead-acid batteries, particularly in flooded and valve-regulated lead-acid (VRLA) types. The positive grid, typically made of lead or lead alloys, gradually degrades due to electrochemical oxidation during charging. Overcharging accelerates this process by increasing the potential at which the grid corrodes. High temperatures further exacerbate corrosion rates, as chemical reactions proceed more rapidly under elevated thermal conditions. The corrosion layer that forms is less conductive than the original lead grid, increasing internal resistance and reducing battery capacity. In severe cases, grid corrosion can lead to mechanical weakening and eventual grid disintegration, causing catastrophic failure.

Negative plate sulfation occurs when lead sulfate crystals, which form during discharge, fail to fully convert back to active material during recharging. This issue is particularly prevalent in batteries subjected to prolonged periods of partial state-of-charge or deep discharging. Large, stable sulfate crystals accumulate on the negative plates, reducing the available surface area for electrochemical reactions. Sulfation increases internal resistance and diminishes capacity, eventually rendering the battery unusable. Temperature extremes worsen sulfation; cold conditions slow the dissolution of sulfate crystals, while high temperatures accelerate secondary reactions that promote irreversible sulfation.

Active material shedding is another critical failure mode, primarily affecting the positive plates. The lead dioxide active material gradually detaches from the grid due to repeated expansion and contraction during charge-discharge cycles. Overcharging intensifies this problem by generating excessive oxygen evolution, which disrupts the structural integrity of the active material. Vibration and mechanical shock can also contribute to shedding, particularly in automotive applications. The lost active material accumulates at the bottom of the battery case, potentially causing short circuits if it bridges the plates. Shedding directly reduces the battery's capacity and cycle life.

Electrolyte stratification is a failure mode specific to flooded lead-acid batteries. During operation, the sulfuric acid electrolyte becomes denser at the bottom of the cell due to gravity-induced separation. This non-uniform acid concentration creates localized differences in electrochemical potential, leading to uneven current distribution and accelerated plate degradation. Stratification is more pronounced in batteries that operate at partial state-of-charge or experience infrequent full charging. Over time, stratification reduces efficiency and contributes to premature failure.

Internal short circuits arise from several mechanisms, including dendrite growth, separator failure, and debris accumulation. Dendrites, or conductive lead filaments, can form between plates due to improper charging or plate deformation. Separator degradation, caused by chemical attack or mechanical stress, may allow direct plate-to-plate contact. In flooded batteries, shed active material or loose grid fragments can create conductive paths. Short circuits lead to rapid self-discharge, localized heating, and in extreme cases, thermal runaway.

Failure analysis methodologies for lead-acid batteries involve a combination of electrical testing, physical inspection, and chemical analysis. Capacity testing under controlled conditions helps identify performance degradation trends. Electrochemical impedance spectroscopy (EIS) is useful for detecting increased internal resistance caused by sulfation or corrosion. Post-mortem analysis includes visual inspection of plates, separators, and sediment buildup. Scanning electron microscopy (SEM) can reveal microstructural changes in active materials, while energy-dispersive X-ray spectroscopy (EDS) identifies elemental composition shifts in corroded grids.

Overcharging is a major root cause of multiple failure modes. Excessive charging voltages accelerate grid corrosion, promote active material shedding, and increase water loss in VRLA batteries. Charge controllers must maintain voltages within manufacturer-specified limits to avoid these issues. Deep discharging, on the other hand, drives sulfation and can cause irreversible damage if the battery remains discharged for extended periods. Temperature management is equally critical, as both high and low extremes negatively impact battery chemistry and material stability.

Preventive measures focus on optimizing charging protocols, maintaining appropriate state-of-charge levels, and ensuring proper temperature conditions. Battery management systems (BMS) play a crucial role in monitoring and controlling these parameters. Regular equalization charging can mitigate stratification in flooded batteries, while pulse charging techniques may help reduce sulfation in certain applications.

Understanding these failure modes and their root causes enables better battery design, more accurate diagnostics, and improved operational practices. While lead-acid technology remains robust for many applications, addressing these failure mechanisms is essential for maximizing performance and longevity. Future advancements in materials science and battery management may further mitigate these issues, but the fundamental electrochemical challenges will persist without proper operational safeguards.