LEAD STORAGE BATTERY
A battery consists of a number of cells and each ell of the battery consists of the following components.
6.Cell covers and vent plugs
The container houses the plates and the electrolyte. It is made of acid resisting materials like glass or hard rubber depending upon service requirements.
2. Positive plate
Positive plate is made of lead-peroxide (PbO) deposited on a grid frame . The grid frame is made of antimony-lead alloy. The colour of the positive plate is dark brown.
3. Negative plate
Negative plate is made of spongy lead (Pb). It is also deposited on a grid frame for stiffness and strength. The colour of the negative plate is grey. The number of negative plates in a battery is always one more than the positive plates to make use of both the sides of the positive plate most effectively.
It is made of thin sheet of e Separators are placed betweenmaterials. Sersheet of porous insulatingplaced between positive and The positive and negative plates aree separators. The separators ee circulation of the electrolyte betweene made of specially treated wood,egative plates. The poNICted electrically by the separators. Thseparated elemust allow free circathe plates.glass, rubber etc.
The electrolyte is dilute sulphuric acid (H.SO). terv grade sulphuric acid is used for the prerrolyte. The specific gravity of the electrolyte is The recommended value of the specific gravity isof electrolyte. The spec1.21 for longer life of the batteries.
6. Cell covers and Vent plugs
Each cell has a cover made of moulded hard rubber. Openings are provided in these covers for two terminal posts and vent plug. Vent plug has a vent hole for easy escape of gas formed inside the cell during charging. Vent plugs can be easily removed for adding electrolyte.
7. Cell connectors
Cell connectors are used to connect the dividual cells in series to give the required voltage. Lead alloys are the material
8. Battery terminals
A battery has two terminals, the +ve and the ‑ve. polarities are marked on the terminals. The mals are generally made of lead alloys.terminals are gen
CHEMICAL REACTIONS DURING DISCHARGING IN LEAD STORAGE BATTERY
When the cell is fully charged its positive p or anode is PbO2, which is dark chocolate brown colour and the negative plate or cathode is Pb which grey in colour.
When the cell discharges, it sends current through the external load.During discharging the electrolyte H,50, is dissociated into positive H, and negative SO. ions. As the current within the cell is flowing from cathode to anode, H, ions move towards the anode and so, ions move towards the cathode.
At anode, H, combines with the oxygen of pbo: and H, SO, attacks lead to form PbSO4,. At cathode , S04 combines with Pb to form PbSO4,.
PHYSICAL CHANGES DURING DISCHARGING IN LEAD STORAGE BATTERY
Both the plates become Pbso4 which is whitish in colour.
Due to formation of water specific gravity of the electrolyte decreases
The voltage of the cell decreases
The cell gives out energy
CHEMICAL REACTIONS DURING CHARGING IN LEAD STORAGE BATTERY
To recharge a discharged cell, direct current is passed through the cell in the reverse direction by connecting the anode to the positive terminal of d.c. source and cathode to the negative terminal of the source
During charging the H, ions move towards cathode and so4, ions move towards the anode and the following chemical changes takes place.
PHYSICAL CHANGES DURING CHARGING IN LEAD STORAGE BATTERY
- Both the plates become Pbso4 which is whitish in colour
- Due to formation of water specific gravity of the electrolyte decreases
- The voltage of the cell decreases
- The cell gives out energy
ELECTRICAL CHARACTERISTICS OF LEAD ACID CELL
The three important characteristics of lead acid cells are
The open circuit voltage of a fully charged cell is approximately 2.2 volt. This value is not fixed but depends on
When the temperature increasethe voltage is not much increases.
It is measured in amp-hour (Ah). The capacity is always given at a specified rate of discharge. The capacity depends upon the following.
Rate of discharge
The capacity of a cell is measured in Ah depends on the discharge rate. Capacity decreases with increased rate of discharge. Rapid rate of discharge means, greater fall in voltage of the cell due to internal resistance of cell. Moreover, with rapid discharge, the weakening of acid in the pores of the plates is also greater
At high temperature
1.The chemical reactions within the cell take placemore vigrously.
2.The resistance of acid is decreased.
Hence high temperature, increases the capacity of the lead acid cell.
Hence it is better to operate the battery at a high temperature.
However at high temperature.
The acid attacks the antimony ‑lead alloy grid,terminal posts and wooden separators.
The paste is rapidly changed into lead sulphatesulphation is always accompanied by expansion of paste at the positive plates and resulto in buckling and cracking of the grid. Hence, it isnot advisable to work battery above 40°C.
Density of electrolyte
As the density of the electrolyte affects the internal resistance and the vigrous of chemical reaction, density of electrolyte has an important effect on the capacity capacity increases with the density.
Quantity of active materialsCapacity of the battery depends upon the amount of active material. For obtaining cells of greater capacity it is necessary to provide the plates with larger amount of active material.
1.The Ah efficiency decreases due to self dischargeof the plates caused due to local reactions.
2.Due to leakage of current because of faulty insulation between the cells of the battery the Wh efficiency decrecases when the increasing of charge volt or reducing of discharge volt.
- The Ah efficiency is of lead acid cell normally between 90–95%. and the Wh efficiency varies between 72–80%.