Electrical Engineering ⇒ Topic : Lead-Acid Cell

Lead-Acid Cell

The most inexpensive secondary cell is the leadacid cell and is widely used for commercial purposes. A lead-acid cell when ready for *use contains two plates immersed in a dilute sulphuric acid (H₂SO₄) of specific gravity about 1.28. The positive plate (anode) is of lead-peroxide (Pb0₂) which has chocolate brown colour and the negative plate (cathode) is lead (Pb) which is of grey colour.

(1) When the cell supplies current to a load (i.e. discharging), the chemical action that takes place forms lead sulphate (PbSO₄) on both the plates with water being formed in the electrolyte.After a certain amount of energy has been withdrawn from the cell, both plates are transformed into the same material (i.e. PbSO₄) and the specific gravity of the electrolyte (H₂SO₄) is lowered. The cell is then said to be discharged. There are several methods (See Art. 10.19) to ascertain whether the cell is discharged or not.

(2) To charge the cell, direct current is passed through the cell in the reverse direction to that inwhich the cell provided current. This reverses the chemical process and again forms a lead peroxide (Pb0₂) positive plate and a pure lead (Pb) negative plate. At the same time, H₂SO₄ is formed at the expense of water, restoring the electrolyte (H₂SO₄) to its original condition.

The chemical changes that occur during discharging and recharging of a lead-acid cell are discussed in the articles that follow.