What is the working Principle of Conventional Battery?
One of the primary requirements of any telephone system is that service shall be available to the subscribers at all times. The electrical energy required for signaling, switching, speech transmission etc. in telephone exchanges is derived either directly or indirectly from the public electricity system. In order to provide uninterrupted service, the exchange power supply system is designed to give continuous energy to the system. So provision is also made for alternate source of supply in the event of mains failure. This emergency energy is derived from
- Batteries of secondary cells.
- A combination of battery and prime mover generator sets.
The secondary cells in general use in BSNL are of lead acid type. Secondary cells are electrolytic cells for generation of electric energy. These cells can be restored to its original condition after they are discharged. This restoration is done by passing a current in a direction opposite to the flow of current in the cell during the discharge.
General requirements of a good cell
- There must be no local action, i.e., little or no wastage of the materials when the cell is not delivering current.
- The e.m.f of the cell must be of such magnitude so as to enable the cell to deliver a reasonable amount of energy with moderate current.
- Frequent replacements of materials must not be necessary and such materials must not be expensive.
- The internal resistance must be small.
There are three types of storage (secondary) cells in use. They are (1) lead-lead-acid type (2) Nickel-iron-alkaline and (3) Nickel-Cadmium alkaline type. They are commonly known as lead acid type cell. These cells have electrodes of lead immersed in an electrolyte of dilute sulphuric acid in a suitable container.
Chemical action during charge and discharge of secondary cells:
The chemical action in every lead sulphuric acid cell is same. Under fully charged condition, the active material on positive plate is peroxide of lead, whilst that on negative plate is spongy lead. On discharge an increasing proportion of the active material of both positive and negative plate is converted to lead sulphate. On charge the positive plate assumes a rich chocolate colour and the colour of negative plate is light gray.
When the cells are discharged, the true shade of colour of positive plate grows somewhat lighter, due to sulphation.
The theory of secondary cell is based on the double sulphation theory which is accepted as a close approximation to the probable facts.
When the lead acid cell is in the charged state, the active material of the positive plate is lead peroxide PBO2 and that of the negative plate is metallic lead (Pb), in a spongy state. As the cell discharges, the positive hydrogen ions of sulphuric acid (H2SO4)move through the electrolyte to the positive plate, where they give up their charges, the free hydrogen reacts to reduce the lead peroxide to lead monoxide (PbO) which then combines with the sulphuric acid to form lead sulphate(Pb SO4)
PbO 2 + H2 –> Pbo + H2O
Pbo + H2SO4 –> PbSO4 + H2O
The negative sulphate ions (SO4) travel to the negative plate and combine with the lead to form lead sulphate.
Pb + SO4 –> Pb SO4
Thus the lead sulphate is formed on both positive and negative plates.
When the cell is being recharged, the hydrogen ions move to the negative plate and sulphate ions to the positive plate. The chemical action is now the reverse of those given above (during discharge).
at Positive plate:
Pb SO4 + SO4 +2 H2O –> PbO2 + 2 H2SO4
at Negative plate:
Pb SO4 + H2 –> Pb + H2SO4
In practice the chemical changes indicated above do not extend to the limit of the available material, the charge and discharge each being stopped before the whole of the active material has undergone chemical change. Lead sulphate to a slight extent is therefore always present in both plates when charged.
As the cell approaches full charge, the amount of lead sulphate present on the plates is insufficient to combine with all the ions reaching the plate, as a result some of the water of the electrolyte is decomposed; hydrogen gas is liberated at the negative plate, whilst oxygen is given off at the positive plate from the combination of the sulphions with water forming sulphuric acid.
SO4 +H2O –> H2SO4 + O
The stage at which the evolution of oxygen and hydrogen takes place is known as ‘Gassing’ and indicates that the cell is nearing fully charged condition.
Apart from the chemical changes and changes in the colour of plates, it is important to note that the terminal voltage as well as specific gravity of the cell vary during the charge and discharge process.
The voltage and S.G go down during discharge and increases during charge.
Capacity of a cell
Capacity of a battery is specified in Ampere Hour (A.H). If the capacity of a battery is 500 AH, it can be discharged at the rate of 50 Amps for 10 hours. The same battery can be discharged for 50 hours at the rate of 10 Amps. This can also be discharged at 100 Amps for 5 hours.
The capacity of the battery is decided on the following considerations.
- How long it is required to supply the power to maintain service in the event of mains failure?
- The busy hour load of the exchange/exchanges getting supply from the batteries.
Broadly speaking, the batteries for small exchanges are designed to provide a 24 hour reserve of energy. For large exchanges the capacity of the battery is designed to provide 6 to 8 hours of reserve energy.
Depending on the capacity of the battery, different types of containers are used.
The characteristics of the containers should be as noted below,
- They should be strong enough to with stand the load imposed by the weight of the plates and electrolyte.
- They should not react with the electrolyte.
- They should not conduct electricity
Following are the types of containers used in secondary cell presently.
- . Hard rubber containers– For tubular positive cells
- . Fiber glass
Construction of Cells
To obtain a useful capacity it is usually necessary to employ several plates in a cell. The plates of like polarity are burned to a lead bar, the group of plates being referred to as a ‘Section’. When placed in the cell, the positive and negative plates interleave, the number of plates in the negative section being one more than the number of positive plates in order to make use of all the positive surfaces and to avoid buckling.