Working of Lead Acid Battery

Contents

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Key learnings:

Lead Acid Battery Defined: A lead acid battery is defined as a rechargeable storage device where electrical energy is transformed into chemical energy during charging, and vice versa during discharging.
Materials and Composition: Essential materials include lead peroxide and sponge lead, used in the positive and negative plates respectively, submerged in dilute sulfuric acid.
Working of Lead Acid Battery: The battery operates by converting stored chemical energy into electrical energy through a series of electron exchanges between its lead plates during discharge.
Chemical Changes: Key reactions involve hydrogen and sulfate ions interacting with lead plates to form lead sulfate, dictating the flow of electrons and hence current through the battery.
Charging Process: Recharging the battery reverses the chemical reactions, converting lead sulfate back into lead peroxide and pure lead, thus restoring and enhancing battery capacity.

A lead-acid storage battery stores electrical energy as chemical energy and returns it as electricity. Charge uses an external source to drive the plate chemistry one way. Discharge runs that chemistry the other way and supplies a load.
During charging of battery, current through the cell changes the plates and stores energy in those new compounds.

On a load those compounds reverse and give the stored energy back as current.
That cycle is the working of lead acid battery. The usual secondary cell that does it is the lead acid battery.

Materials used for Lead Acid Storage Battery Cells

The main active materials in a lead acid battery are

Lead peroxide (PbO₂).
Sponge lead (Pb)
Dilute sulfuric acid (H₂SO₄).

Lead Peroxide (PbO₂)

The positive plate is lead peroxide. That material is dark brown, hard and brittle.

Sponge Lead (Pb)

The negative plate is soft sponge lead.

Dilute Sulfuric Acid (H₂SO₄)

The dilute sulfuric acid used for a lead acid battery is mixed water : acid = 3:1.

The lead acid storage battery is a lead peroxide plate and a sponge lead plate standing in dilute sulfuric acid. A load is connected between the plates. In the acid the molecules split into hydrogen ions (H⁺) and sulfate ions (SO₄^{− −}). Hydrogen ions at the PbO₂ plate take electrons and become a hydrogen atom which then attacks PbO₂ and form PbO and H₂O (water). That PbO reacts with H₂ SO₄ and forms PbSO₄ and H₂O (water).

SO₄^{− −} ions move freely, so some reach the pure Pb plate, give up electrons and become radical SO₄. The radical SO₄ cannot stand alone, so it attacks Pb and forms PbSO₄.
H+ ions take electrons from the PbO₂ plate and SO₄^{− −} ions give electrons to the Pb plate, so the plates become unbalanced. Current then flows in the external load to restore the electron count. That path is discharge of the lead acid battery.
Lead sulfate (PbSO₄) is whitish. During discharging,

Both of the plates are covered with PbSO₄.
Specific gravity of sulfuric acid solution falls due to formation of water during reaction at PbO₂ plate.
As a result, the rate of reaction falls which implies the potential difference between the plates decreases during discharging process.

Disconnect the load. Connect the PbSO₄ covered former PbO₂ plate to the positive terminal of a DC source and the PbO₂ covered former Pb plate to the negative terminal. Acid density has fallen, but sulfuric acid is still present as H⁺ and SO₄^{− −} ions. Positive hydrogen ions move to the electrode (cathode) on the negative DC terminal. Each H⁺ ion takes one electron and becomes a hydrogen atom. Those atoms then attack PbSO₄ and form lead and sulfuric acid.

SO₄^{− −} ions (anions) move to the electrode (anode) on the positive DC terminal, give up electrons and become radical SO₄. This radical SO₄ cannot stand alone, so it reacts with PbSO₄ on the anode and forms lead peroxide (PbO₂) and sulfuric acid (H₂SO₄).

Charging the lead acid storage battery cell therefore gives