Which of the processes listed occurs during the charging of a lead-acid storage battery?

Correct Answer: Option D — Lead sulfate is changed to lead peroxide in the positive plates and sponge lead in the negative plates respectively.

During the charging cycle, an external DC source reverses the chemical reaction. Lead sulfate on both plates is converted back into lead peroxide at the positive plate and sponge lead at the negative plate, while the electrolyte density increases as sulfate ions return to the sulfuric acid solution.