Why Does the Positive Plate of a Lead-acid Battery Soften?

The positive plate of a lead-acid battery softens due to several interconnected chemical and physical processes that occur during the battery’s charge and discharge cycles.

  1. Formation and Growth of Lead Dioxide (PbO₂):
    • During the charging process, the lead sulfate (PbSO₄) on the positive plate is converted back to lead dioxide (PbO₂). Lead dioxide is less dense and more porous than lead sulfate, making the plate more prone to structural changes.
  2. Volume Changes:
    • The positive plate undergoes significant volume changes due to the repeated formation and dissolution of lead sulfate (PbSO₄). These volume changes create mechanical stress and strain on the plate’s material, leading to softening and degradation over time.
  3. Corrosion of the Grid:
    • The positive grid, usually made of lead or a lead alloy, undergoes corrosion in the acidic environment of the battery. This corrosion forms lead oxides that are not structurally supportive, weakening the grid and contributing to the softening of the plate.
  4. Shedding of Active Material:
    • As the battery cycles, some of the active material (PbO₂) may shed from the positive plate due to mechanical stress and the expansion and contraction of the material. The loss of active material reduces the structural integrity of the plate.
  5. Electrolyte Degradation:
    • The sulfuric acid electrolyte can degrade over time, leading to changes in its concentration and distribution. Variations in electrolyte concentration can cause uneven charging and discharging, exacerbating the stress on the positive plate and contributing to its softening.
  6. High Temperatures:
    • Elevated temperatures can accelerate the chemical reactions within the battery, increasing the rate of corrosion and the shedding of active material. High temperatures can also soften the lead grid directly, leading to further weakening of the positive plate.

These factors collectively lead to the softening of the positive plate in a lead-acid battery, which ultimately reduces the battery’s efficiency, capacity, and lifespan. Proper maintenance, including regular charging and ensuring the battery operates within recommended temperature ranges, can help mitigate some of these effects.


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