Lead Acid Storage Cell

Lead Acid Storage Cell - SECONDARY BATTERY

Storage cell

A lead acid storage cell is a secondary battery, which can operate both as a voltaic cell and as an electrolytic cell. When it acts as a voltaic cell, it supplies electrical energy and becomes "run down". When it is recharged, the cell operates as an electrolytic cell.

Description

Anode: Lead

Cathode: Lead dioxide

Electrolyte: H₂SO₄

Emf: 12 V

Cell diagram: Pb/PbSO₄ // H₂SO₄ (aq) / PbO₂ / Pb

Construction

A lead‒acid storage battery consists of a number of (3 to 6) voltaic cells connected in series to get get 6 to 12 V battery. In each cell, the anode is made of lead. The cathode is made of lead dioxide PbO₂ (or) a grid made of lead, packed with PbO₂. A number of lead plates (anodes) are connected in parallel and a number of PbO₂ plates (cathodes) are also connected in parallel.

Various plates are separated from the adjacent one by insulators like rubber (or) glass fibre. The entire combinations is then immersed in dil. H₂SO₄ (38% by mass) having a density of 1.30 gm/ml.

Working (Discharging)

When the lead‒acid storage battery operates, the following reaction occurs.

At anode: Lead is oxidized to Pb²⁺ ions, which further combines with SO₄^2‒ forms insoluble PbSO₄.

At cathode: PbO₂ is reduced to Pb²⁺ ions, which further combines with SO₄^2‒ forms insoluble PbSO₄.

Overall cell reaction during use (discharging):

From the above cell reactions it is clear that, PbSO₄ is precipitated at both the electrodes and H₂SO₄ is used up. As a result, the concentration of H₂SO₄ decreases and hence the density of H₂SO₄ falls below 1.2 gm/ml. So the battery needs recharging.

Recharging the Battery

The cell can be charged by passing electric current in the opposite direction. The electrode reaction gets reversed. As a result, Pb is deposited on anode and PbO₂ on the cathode. The density of H₂SO₄ also increases.

The net reaction during charging is

Advantages of lead‒acid batteries

(i) It is made easily.

(ii) It produces very high current.

(iii) The self‒discharging rate is low when compared to other rechargeable batteries.

(iv) It also acts effectively at low temperature.

Disadvantages of lead‒acid batteries

(i) Recycling of this battery causes environmental hazards.

(ii) Mechanical strain and normal bumping reduces battery capacity.

Uses

1. Lead storage cell is used to supply current mainly in automobiles such as cars, buses, trucks, etc.,

2. It is also used in gas engine ignition, telephone exchanges, hospitals, power stations, etc.,