Lead–acid batteries — chemistry at full discharge In a fully discharged automotive lead–acid battery, which compound predominantly forms on both the positive and negative plates?

Introduction / Context:
Understanding the chemical state of a lead–acid battery at various charge levels is essential for diagnostics and maintenance. Plate chemistry directly influences terminal voltage, internal resistance, and recoverability on recharge.

Given Data / Assumptions:

• Conventional flooded lead–acid starting battery.
• Battery is at or near full discharge (no catastrophic damage assumed).
• Electrolyte is aqueous sulphuric acid solution.

Concept / Approach:
In a charged state, the positive plate is primarily lead dioxide (PbO2) and the negative plate is spongy lead (Pb). During discharge, both plates react with sulphate ions, forming lead sulphate (PbSO4) while the electrolyte becomes more water-rich (specific gravity drops). Thus, at full discharge both plates are covered largely with PbSO4.

Step-by-Step Solution:

Verification / Alternative check:
Hydrometer readings show reduced specific gravity; visual inspection after teardown reveals sulphation on both plates in a deeply discharged battery.

Why Other Options Are Wrong:

• PbO2 and Pb are the charged-state materials, not discharged.
• H2SO4 is the electrolyte, not a solid plate deposit.
• PbO alone is not the dominant discharge product.

Common Pitfalls:
Allowing prolonged deep discharge causes hard sulphation that resists normal charging, permanently reducing capacity.

Final Answer: