When a battery becomes completely discharged, both positive and negative plates become what, and the electrolyte becomes what?

When a battery becomes completely discharged, the chemical reaction that occurs results in the conversion of the active materials on the plates. The lead dioxide (PbO2) on the positive plates and the sponge lead (Pb) on the negative plates each change forms. In the discharged state, the positive plates are converted to lead sulfate (PbSO4), and the negative plates also become lead sulfate (PbSO4).

Simultaneously, the electrolyte, primarily sulfuric acid (H2SO4), undergoes a reaction that leads to a dilution effect, resulting in the concentration of water (H2O) increasing as the acid is consumed in the process. Thus, as the battery discharges, the electrolyte becomes less concentrated, effectively resembling water.

Therefore, the correct transformation when the battery is fully discharged is that both the positive and negative plates form lead sulfate (PbSO4), while the electrolyte is predominantly water (H2O).