In a lead-acid battery, how does electrolyte density relate to the charge state?

• 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)

Lead-acid electrolyte density reflects how much sulfuric acid is dissolved in the water. When the battery is more charged, the chemical reactions dissolve more sulfate back into the solution, increasing the acid concentration. Higher acid concentration makes the electrolyte denser, so the density rises with state of charge. As the battery discharges, sulfate is pulled into solid PbSO4 on the plates, lowering the acid concentration in the liquid and decreasing the density. Temperature can shift exact numbers a bit, but the overall trend is clear: electrolyte density increases as the charge increases.