Chemical Action of Lead Acid Battery

Chemical Action of Lead Acid Battery Points : Chemical Action of Lead Acid Battery, The Changes During Discharging When battery is connected to the load, it supplies electric energy Thus during discharging chemical energy Is converted to electric energy. When the cell Is fully charged Its positive plate is of lead peroxide (Pb0₂), which Is dark brown In colour. The negative plate is spongy lead (Pb) of slate grey colour. The electrolyte is dilute sulphuric acid, When the battery supplies current to the external load then the sulphuric acid is split up into hydrogen ions (H) and sulphate ions (SO4) move toward cathode (negative plate) and H₂ ions towards positive plate. The (SO₄) ions react with the active material of negative plate and from lead sulphate (PbSO₄). The chemical reaction is given below: At Negative plate: Pb + SO₄ — PbSO₄ At Positive plate The hydrogen ions (H₊) moves towards the anode and there they react the active material to form lead sulphate PbSO₄ and water H₂0 as under,

PbO₂ + 2H + H₂SO₄ — PbSO₄ + 2H₂

As discharging goes on, a layer of PbSO₄ is formed on both the plates, which gets thicker and gradually the voltage of the cell fall down. The specific gravity of electrolyte also decreases. The cell is discharged till the voltage falls to 1.8 volt at no load and specific gravity of electrolyte decreases to 1.180. If the cell is further discharged beyond this limit then the layer of PbSO₄ become so thick and hard that it cannot be reconverted to Pb or PbO₂. The Changes During Discharging (1) The positive plate is covered with a layer of PbSO₄ which is white in colour.
(ii) The negative plate is also covered with a layer of PbSO4 and changes the plate from slate grey to white colour.
(iii) Due to formation of water specific gravity of electrolyte decreases.
(iv) The voltage of the pell falls from 2.1 volt to 1.8 volts (at no load).
(v) Chemical energy stored in the cell change into electrical energy.