Does the specific gravity of the electrolyte in a lead-acid battery increase or decrease as the battery becomes discharged?

• A. Increases
• B. Decreases
• C. Remains the same
• D. Varies with temperature

Answer: (B)

When a lead-acid battery discharges, the chemical reactions on the plates consume sulfuric acid and produce water, forming lead sulfate on both electrodes. This uses up the acid in the electrolyte and leaves it more diluted. Since concentrated sulfuric acid is denser than water, a diluted electrolyte is lighter, so its specific gravity decreases. That’s why the specific gravity reads lower as the battery becomes discharged. When you recharge, the reactions reverse and sulfuric acid is rebuilt in the electrolyte, raising the specific gravity back toward its fully charged value. Temperature does affect density readings, so readings should be temperature-compensated, but the overall trend with discharge is a decrease in specific gravity.