In a lead-acid battery, how does electrolyte specific gravity change with state of charge?

• A. It increases as the charge increases.
• B. It decreases as the charge increases.
• C. It remains constant with charge.
• D. It fluctuates with no relation to charge.

Answer: (A)

In a lead-acid battery, the electrolyte’s specific gravity reflects how much sulfuric acid is dissolved in the solution. When you charge the battery, the chemical reactions restore sulfuric acid in the electrolyte, increasing the acid concentration and making the solution denser, so the specific gravity rises. When you discharge, sulfuric acid is consumed to form lead sulfate on the plates, lowering the acid concentration and causing the electrolyte to become lighter, so the specific gravity falls. That’s why a fully charged cell shows a higher SG than a discharged one. Temperature matters for readings, so measurements are usually compensated to a standard temperature (about 25°C).