What is the correct specific gravity reading for an electrolyte in a fully charged leadacid cell? 

Specific gravity is the ratio of the density of a substance to the density of a reference substance, typically water for liquids. In the context of a lead-acid cell, it measures the density of the electrolyte (sulfuric acid solution) relative to water. The specific gravity indicates the state of charge of the battery. A fully charged lead-acid battery generally has a specific gravity reading of approximately 1.27 at 25°C (77°F).

Working Principle:

The lead-acid battery operates based on the chemical reaction between lead dioxide (PbO₂), sponge lead (Pb), and sulfuric acid (H₂SO₄). During discharge, the active materials react with the sulfuric acid, forming lead sulfate (PbSO₄) and water (H₂O), which lowers the concentration of sulfuric acid in the electrolyte. During charging, the reaction reverses, and the sulfuric acid concentration increases, raising the specific gravity of the electrolyte.

The specific gravity of the electrolyte serves as an indicator of the battery's state of charge:

• Fully Charged: Specific gravity around 1.27 (varies slightly depending on the battery design).
• Half Charged: Specific gravity around 1.20.
• Discharged: Specific gravity around 1.15 or lower.