Lead Acid battery. Charging and discharging mechanism

Lead acid battery is a way of storing electrical energy with the help of chemical compounds. It is also called secondary cell because of its rechargeable feature. Its construction is very simple but can store huge amount of energy. So, lead acid battery is very popular all over the world. But the problem is its bigger size.

Construction

            Lead and Sulfuric acid are the main components of lead acid battery. There are also some sub-ordinate components in its construction. Important components to building it are given below:

          1. Lead metal (Pb)

          2. Lead oxide (PbO₂)

          3. Sulfuric acid

          4. Glass fiber (to isolate anode and cathode)

          5. External casing

The external plastic casing has six chamber to build six different cell. Every cell can store 2.1 V electromotive force. All the cells remain connected in series. So, the total storage capability become 12.6 V. In every plastic chamber, there are lead (Pb) powder grid as anode, lead oxide (PbO₂) powder grid as cathode and sulfuric acid as electrolyte. Both the electrodes being separated with glass fiber insulator. Here those components powder is used in order to increasing the surface area of the reactants. All the components remain drown in liquid H₂SO₄. Two marginal electrodes of whole battery are used as positive and negative terminal.

Mechanism

The mechanism of a lead acid battery can be divided into two part. Those are:

                        1. Conducting mechanism or discharging mechanism

                        2. Recharging mechanism

Discharging mechanism:

              In order to create electrical conduction reduction and oxidation are occur here. Lead metal grid become oxidized and lead oxide grid become redacted. The cell diagram is

Pb,PbSO₄/H₂SO₄(aq)/PbO₂,Pb

Both the half-cell reactions are given bellow

(-) Anode oxidation :  Pb(s) + HSO₄^- (aq) → PbSO­₄ (s)+ H⁺ (aq)+ 2e^-     E^o = 0.3 V

(+) Cathode reduction:  PbO₂(s) + 3H⁺ (aq)+ HSO₄^- (aq) + 2e^-  → PbSO­₄ (s) + 2H₂O(l)                 E^o = 1.7 V

Total cell reaction: Pb(s) + PbO₂(s) + 2H⁺ (aq)+ 2HSO₄^- (aq) → 2PbSO­₄ (s) + 2H₂O(l)                  E^o = 2 V

Recharging mechanism:

            After using if the electromotive force become les than 1.17V ,the battery must be recharged. In Order to recharging we have to flow current from outer side through the electrodes.  At that time here occurs electrolysis of PbSO­₄. It is also called induced reaction. Discharging is very important for recharging because the main product of discharging period which is lead sulfate is used here as electrolyte. The half-cell reactions of recharging period are given bellow

(+) Cathode reduction:  PbSO­₄ (s)+ H⁺ (aq)+ 2e^-  → Pb(s) + HSO₄^- (aq)

(-) Anode oxidation     :  PbSO­₄ (s) + 2H₂O(l) → PbO₂(s) + 3H⁺ (aq)+ HSO₄^- (aq) + 2e^-      

Total cell reaction       :  2PbSO­₄ (s) + 2H₂O(l)  →  Pb(s) + PbO₂(s) + 2H⁺ (aq)+ 2HSO₄^- (aq)

Those are the reverse reactions of discharging period’s reactions.