What is Lead Acid Battery? Construction, Working, Connection Diagram, Charging & Chemical Reaction

Figure 1: Lead Acid Battery.

The battery cells in which the chemical action taking place is reversible are known as the lead acid battery cells. So it is possible to recharge a lead acid battery cell if it is in the discharged state.

In the charging process we have to pass a charging current through the cell in the opposite direction to that of the discharging current. The electrical energy is stored in the form of chemical form, when the charging current is passed. lead acid battery cells are capable of producing a large amount of energy.

Construction of Lead Acid Battery

The construction of a lead acid battery cell is as shown in Fig. 1. It consists of the following parts :

1. Anode or positive terminal (or plate).
2. Cathode or negative terminal (or plate).
3. Electrolyte.
4. Separators.

Anode or positive terminal (or plate):

The positive plates are also called as anode. The material used for it is lead peroxide (PbO₂). It is a material of dark brown colour.

Cathode or negative terminal (or plate):

The negative plates are also called as cathode. The material used for the cathode is lead (Pb) and its colour is gray.

Electrolyte :

The electrolyte used is dilute sulphuric acid (H₂SO₄) with 3-parts of distilled water mixed with one part of H₂SO4. The specific gravity is 1.2. The anode and cathode both are immersed in the electrolyte.

Separators :

These are thin plates of porous insulated material like rubber. They are placed between main plates to avoid short circuit between the positive and negative plates.

The main plates are placed close to each other to reduce the internal resistance.

Container :

The container is made up of plastic or ceramic or rubber. All plates and electrolyte is placed in it. No chemical action should take place on the container.

Working of a Lead Acid Battery

Discharging action of the cell :

Figure 2: Discharging of Lead Acid Battery.

Assume that the cell is fully charged. When it starts discharging, the current starts flowing from the cell to the external load as shown in Fig. 2. Due to this current, the sulphuric acid H₂SO₄ is disassociated into positive H₂ and negative SO₄ Ions. The external load current flows from anode to cathode, but the internal current flows from cathode to anode through the electrolyte. Therefore the positive H₂ ions move to the anode and the negative SO₄ ions move to the cathode. The chemical action during Discharge is as follows :

At Anode :

\[\text{Pb}{{\text{O}}_{\text{2}}}\text{ + }{{\text{H}}_{\text{2}}}\text{ + }{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\to \text{ PbS}{{\text{O}}_{\text{4}}}\text{ + 2 }{{\text{H}}_{\text{2}}}\text{O}\]

At Cathode :

\[\text{ Pb + S}{{\text{O}}_{\text{4}}}\to \text{ PbS}{{\text{O}}_{\text{4}}}\]

Both the above expressions show that both the electrodes, anode and cathode become PbSO₄ (see RHS of Equations and water (H₂O) is also produced. Due to formation of PbSO₄, both the electrodes become whitish and due to the water formation, the specific gravity of the electrolyte reduces. Hence the output voltage of the cell decreases on discharging.

Charging action of the cell :

Figure 3: Charging of Lead Acid Battery.

As we have already explained, when the cell is completely discharged, the anode and cathode both transform into PbSO₄ (which is whitish in colour). During the charging process, a positive external voltage is applied to the anode of the battery and negative voltage is applied at the cathode as shown in Fig. 3. Due to the externally connected source, the current flows from anode to cathode inside the electrolyte. This current results in the following chemical action.

At Anode :

\[\text{ PbS}{{\text{O}}_{\text{4}}}+\text{S}{{\text{O}}_{\text{4}}}\text{ +2}{{\text{H}}_{\text{2}}}\text{O }\to \text{ Pb}{{\text{O}}_{2}}+2{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\]

At Cathode :

\[\text{ PbS}{{\text{O}}_{\text{4}}}+{{\text{H}}_{\text{2}}}\to {{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\]

These two expressions indicate (see RHS) that “PbO₂” is being formed at the anode and “Pb” at the cathode. Along with that the sulphuric acid (H₂SO₄) is being created newly. As the water is consumed and H₂SO₄ is created, the specific gravity of H₂SO₄ increases, energy is absorbed and the voltage on the cell increases, and the charging is said to have taken place. The single reversible equation representing the charging and discharging process is given by,

 

Applications of a Lead Acid Battery

Following are some of the important applications of lead – acid batteries :

1. As standby units in the distribution network.
2. In the Uninterrupted Power Supplies (UPS).
3. In the telephone system.
4. In the railway signaling.
5. In the battery operated vehicles.
6. In the automobiles for starting and lighting.