The secondary cells are repeated action cells. The secondary cells can be recharged by Passing electricity through them after every use. As a result, the secondary cells can be used again and again over much longer periods of time. In these cells, the electrical energy is stored in the form of chemical energy. Therefore, these cells are also called storage cells or, accumulators. Lead-acid cell, (or lead storage cell), nickel-cadmium cell etc., are typical secondary cells.

Lead storage battery.

The batteries used in automobiles, e.g., in cars, buses, trucks etc., are Lead storage batteries. The commercial forms of lead storage batteries consists of six or twelve lead Storage cells connected together.

In each cell, the anode and cathode plates are arranged alternately, and are separated from each other by separators. Separators are the sheets of insulating material placed in between the two plates. The anode and cathode plates are separately connected to each other. Thus, each electrode in a battery consists of many plates connected in parallels. This increases the surface area of each Electrode and hence increases the current producing capacity of the battery.

The plates in each electrode are called grids. The grids are made of lead or an alloy of lead and antimony. All cathode plates (grids) (marked as + ve) are coated with red-brown lead dioxide (PbO2).

Anode plates (marked as negative) are coated with spongy lead. The plates are immersed in dilute H2SO4 (l40%) (sp. gravity : 1.15).

A lead storage cell may be represented as:

Pb | PbSO4(s) | H2SO4 (40%) | PbO2(s) | Pb
(Anode)                                              (Cathode)

Lead in the cathode (right hand electrode) serves the purpose of an electrical contact, and is not involved in the cell reaction. The reactions which take place in the cell during discharge (when current is drawn from the cell) are,

At anode: (oxidation) Pb + SO42–→PbSO4(s) + 2e–

At cathode: (reduction) PbO2(s) + 4H++ SO42–+ 2e– → PbSO4(s) + 2H2O

Net cell reaction: Pb + PbO(s) + 4H+ + 2SO42– → 2PbSO4(s) + 2H2O

The lead-acid storage cell gives an emf of 2 volt. So, a lead-acid battery consisting of six cells will give 12 V, and that containing twelve cells will give 24 V. During use(discharge step), both the electrodes of the lead storage cell get coated with white precipitate of lead sulphate (PbSO4), and the sulphuric acid gets diluted(relative density decreases to about 1.1).

When both the electrodes get covered with lead sulphate, reaction stops, and the cell is said to be dead or discharged.

Recharging of a discharged lead storage cell/battery. In the discharged state, the lead (Pb) of the anode, and lead dioxide (PbO2) of the cathode are completely converted to lead sulphate (PbSO4). The water produced in the reaction dilutes the sulphuric acid. Under these conditions, the cell cannot supply any electricity. A discharged lead storage cell however can be recharged by passing a direct current (dc) through the storage cell. This direct current reverses the electrode reactions, and converts the lead sulphate back to lead and lead dioxide on the respective electrodes. During charging, the negative electrode of the storage cell is connected to the negative of the dc source, and the positive electrode is connected to the positive of the dc source.

The reactions taking place on the two electrodes during recharging are,

Negative electrode: PbSO4(s) + 2e– electric current ( ) dc→ Pb(s) +SO42– (aq) (reduction)
Electrode material in solution recovered to once again act as anode

Positive electrode: PbSO4(s) + 2H2O (electric current) → PbO2(s) + 4H+ +SO42–+ 2e–(oxidation)
Electrode material recovered to once again act as cathode

Net charging reaction:
2PbSO4(s) + 2H2O (charging) → Pb(s) + PbO2(s) + 4H++ 2SO42–

Thus, during charging, the electrode materials are converted back to their original forms, and

The cell once again starts generating electricity.

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