Automotive Electronics Practice Test 2026 - Free Automotive Electronics Practice Questions and Study Guide

When a battery is completely discharged, the chemical composition of the plates and the electrolyte undergo significant changes. In a lead-acid battery, the active material on the positive plate is lead dioxide (PbO2), and on the negative plate, it is sponge lead (Pb). The electrolyte is a diluted sulfuric acid solution (H2SO4).

During discharge, the battery converts the lead dioxide and sponge lead into lead sulfate (PbSO4) while simultaneously consuming sulfuric acid, leading to the dilution of the electrolyte as it gets converted into water (H2O).

When the battery reaches a fully discharged state, both the positive and negative plates have converted to lead sulfate (PbSO4). The remaining electrolyte has a lower concentration of sulfuric acid and is, in a sense, transformed into water as the discharge reaction progresses. Therefore, the state of the plates and the electrolyte at complete discharge is represented as lead sulfate (PbSO4) and a significant amount of water (H2O), making this option the correct representation of the process occurring in a lead-acid battery at full discharge.