A Lead Acid Battery when ready for use contains two plates in dilute H2SO4 acting as the electrolyte. The positive plate (anode) is lead peroxide (PbO2) which has chocolate be own co lour and negative spongy lead (Pb) which is grey in color.
The following chemical changes take place in the cell during discharging and charging :
When the cell discharges, sulfuric acid (H2 SO4) is dissociated into halogen (H+H+) and sulfate (SO4″) ions. The SO4″ ions move towards the cathode and hydrogen (H+H+) ions towards the anode.
Every SO4 ” ion on reaching the cathode gives up two electrons to it and becomes SO4 radical. These given up electrons continue their journey towards the anode in the external resistance R. The SO4radical cannot exist and, therefore, enters into chemical, reaction with cathode material (Pb) to form lead sulfate according to the following equation :
Pb +SO4 → PbSO4
(Grey) (whitish in color)
On charging the cell (see fig. 57 ), headstrong ions (H+H+) move towards the cathode and sulfate ions (SO4″) towards the anode. following chemical reactions are taking place :
At cathode : PbSO2 2H-> Pb +H2SO4
At anode: SO4+H2SO4 -> H2SO4+O
PbSO4+O+H2O -> PbO2+H2SO4
PbSO4+SO4+2H2O -> PbO2+ 2H2SO4
Thus, the anode and cathode rad converted into PbO2 and Pb respectively. Moreover, sulphuric acid is fumed at the expense of water.
The following points may be noted dyeing charging :
(i) The anode is covered into peroxide (PbO2) and cathode into Pb.
(ii) Due to the consumption of water in the reaction, a specific gravity of H2SO4is increased. The specific gravity of a fully charged cell is about 1.25.
Care and maintenance of lead-acid batteries
The widespread use of lead-acid cells necessitates their proper care and maintenance. In order to obtain longer life and efficient service, the following points should be kept in view :
Level of electrolyte
The level of the electrolyte should always be about 1 cm above the plates. Water is added to maintain liquid level. Pure distilled water is generally preferred but any water suitable for drinking is considered. Sulfuric acid is never added unless it is known to have been lost by spilling.
Lead acid cells should be charged with d.c. supply. During charging, the following points should be kept in view :
(i) The +be of d.c. supply should be connected to the terminal of the cell and negative of supply should be connected to the -be terminal of the cell.
(ii) The charging current should be that recommended by the manufacturer.
(iii) The specific gravity of the electrolyte (H2SO4) should be about 1.25.when the cell is taken off charge.
(iv) Overcharging is harmful and should be avoided. Due to overcharging, high internal heat is generated which may buckle the separators. Buckling of separators can cause internal short circuit, lowering the output voltage. Also excessive charging results in excessive gassing which may wash active materials off the plates.
When the cell is delivering current i.e., discharging, the following, points should be kept in vying:
(i) The e.m.f.of the cell should not be allowed to fall below 1.8 volts.
(ii) The cell shod not be draw a current more than that recommended by the manufacturer.
In addition, the following miscellaneous points should also be kept view :
(i) Distilled water should be added occasionally to the acid to compensate for the evaporation of water. It should be noted that acid does not evaporate and unless there is excessive spraying due to the gases are given off, the quantity of the acid is the cell will not change.
(ii) Nader short circuit the terminals of the cell. This may generate excessive heat, resulting from the buckling of plates.
(iii) The terminals of the cell should be kept clean and periodically greased with Vaseline to prevent any corrosion.
(iv) Lead acid cells should not be stored whit electrolyte in the cells, especially in the discharged condition. It is because lead sulfate changes gradually to an insoluble form which may make recharging impossible.
Applications of lead-acid batteries
Lead acid cells have huge commercial applications. They are used for :
(a) automobile lighting and starting
(b) linting on steam-railway trains
(c) emergency lighting
(d) telephone battery system
(e) operating control equipment in powerhouses and substations
(f) railway signaling.