Lead Acid Battery | Construction, Working and Application

Lead Acid Battery

Lead acid battery is a type of rechargeable battery that works using lead plates and sulphuric acid. When the lead plates are placed in the acid, a chemical reaction takes place, which produces electricity. This process can be reversed to recharge the battery.

When several battery cells are joined together in series, parallel or a mix of both, they form a complete battery.

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Construction of Lead Acid Battery

A battery consists of a number of cells and each cell of a battery consists of following parts.

1. Plates: Plates of a lead acid cell are made of antimonial lead alloy grid. The grids used for both positive and negative plates have the same design.
2. Separator: The separators are thin sheets of a porous material which are place between +ve and -ve plates to prevent internal short circuit of the +ve and -ve plates.
3. Electrolyte: Electrolyte used in a lead-acid battery is a dilute sulphuric acid solution. It is usually a mix of three parts water and one part sulphuric acid.
4. Container: Plates and electrolyte are placed in a container which may be made of vulcanised rubber or moulded hard rubber, ceramic, glass or celluloid. Container is sealed at the top.
5. Cell cover: Each cell compartment has a cover, usually made of moulded hard rubber, opening are provided in these covers for the two terminal posts and vents. The joints between cover and container are sealed with an acid-resistance material.
6. Vent caps: Each cell cover has a hole for a vent cap, which helps release gas and prevents pressure build-up inside the battery.

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Working Principle of Lead Acid Battery

When a lead-acid cell is fully charged, the positive plate is made of lead peroxide (PbO₂) and has a chocolate brown color. The negative plate is made of spongy lead (Pb) and appears grey color. The electrolyte has a specific gravity of 1.25 and is a dilute solution of sulphuric acid (H₂SO₄).

Plates are dipped into this acid solution. When a load is connected to the cell , it starts delivering current. As this process continues, the cell discharges.

During discharging, the chemical energy stored in the battery is turned into electrical energy, which powers the connected load

1. Chemical Action During Discharging

During discharging process, the electrolyte breaks down into hydrogen ions and sulphate ions. The negatively charged hydrogen ions from the electrolyte (H₂SO₄) move toward the anode , while the positively charged sulphate ions move toward the cathode. At the cathode and anode, these ions react and a chemical reaction takes place as shown below:

AT Cathode:

Pb + SO₄ –> PbSO₄

At Anode:

PbO + 2H⁺ –> PbO + H₂O

PbO + H₂SO₄ –> PbSO₄ + H₂O

The movement of sulphate ions toward the cathode and hydrogen ions toward the anode takes place. Each hydrogen ion gains one electron from the sulphate ion , which forms hydrogen gas. This hydrogen gas then reacts with the anode. PbO (lead peroxide) reacts with sulphuric acid (H₂SO₄) to form PbSO₄ (lead sulphate).

It is seen that as the cell discharges, the lead in both electrodes turns into white lead sulphate (PbSO₄). Moreover, the SO₄ part of the sulphuric acid (H₂SO₄) gets used up and combines with oxygen to form water (H₂O). Because of this, the electrolyte becomes less dense and the specific gravity of the electrolyte goes down during discharging.

2. Chemical Action During Charging

Each hydrogen ion when reacting with the cathode, gains one electron given up by the sulphate ion. This forms hydrogen gas, which is liberated at the cathode. The hydrogen gas also reacts with the cathode material (PbSO₄) .

At Anode:

SO₄ + H₂O –> H₂SO₄ + O

The oxygen gas appears at the anode and react with the anode material (PbSO₄).

PbSO₄ + O + H₂O –> PbO₂ + H₂SO₄

AT Cathode:

PbSO₄ + 2H –> Pb + PbSO₄

Thus, it is seen that after the cell is completely charged, the anode is converted into lead peroxide (PbO₂) and the cathode becomes spongy lead (Pb). The specific gravity of the electrolyte increases due to the consumption of water. The electrical energy supplied by the mains is converted into chemical energy which is stored in the cell.

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Applications of Lead Acid Batteries

Lead Acid batteries are used for variety of application such as:

1. For petrol motor car starting and ignition.
2. As a source of power supply in telephone exchange, laboratories and broadcasting stations.
3. For local lighting of generating and substations during odd times and break down.
4. For starting rotary converters in substations.
5. For traffic control system.

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Frequently Asked Questions (FAQs)

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