In a discharged state, the active mass on the positive and negative plates is lead sulfate (PbSO4). The lowest possible density is approx. 1.10 kg/l. The battery fluid (electrolyte) is a mixture of sulfuric acid (H2SO4) and distilled water (H20).
The battery is connected to a DC voltage source during charging. The flow of electrons runs from the negative to the positive plate. Electrons are therefore supplied to the negative plate while electrons are taken away from the positive one.
The flow of electrons causes lead sulfate to be reduced to metallic lead at the negative plate, while the lead sulfate at the positive plate is oxidized to lead dioxide by losing electrons and gaining two oxygen atoms.
Lead dioxide (PbSO2) is formed from lead sulfate (PbSO2) and water by loss of sulfate ions (SO42-) and hydrogen ions (H+). Sulfuric acid (H2SO4) is produced in the fluid and water (H2O) is consumed.
If the lead battery is charged the active mass of the negative plate becomes porous lead (Pb) and the active mass of the positive plate becomes lead dioxide (PbO2). The concentration of the electrolyte (battery acid) - diluted sulfuric acid (H2SO4) - rises to 1.28 kg/l compared to the discharged state.
A current begins to flow if the battery is connected to a consumer, which in turn discharges the battery.
The electrons move via the external circuit and to the device from the positive to the negative plate.
To balance the circuit, sulfate ions (SO42-) travel from the electrolyte to the negative plate and combine with lead (Pb) to form lead sulfate (PbSO4). Lead sulfate (PbSO4) and water (H2O) are formed from lead dioxide (PbO2) at the positive plate while sulfate ions (SO42-) and hydrogen ions (H+) are consumed.