Saturday, 19 January 2013


Storage Batteries


                Storage Batteries are a group of cells which converts the electrical energy in to chemical energy and stores in it while current from an external source is implied to it and reversed while discharged.
                During charging the cell, when current is passed through it certain chemical changes takes place in the active materials of the cell. Such chemical reactions absorb energy during their formation. The chemical reactions are completed the cell is said to be charged. No further charging current is passing the cell.
                When the cell is connected to external load circuit, the active materials of the cell reverse the changes which occurred during charging. The absorbed energy is released in the form of electric current. This process is known as discharging.
                More than one cell is connected in series or parallel then the set is called as battery.

Constructional details of Lead acid cell

                A cell consists of
1.       Positive and negative plates
2.       Separators and
3.       Electrolyte and all contained in a container.

Plates

               The plate consists of a lattice type of grid. It is made up of antimony lead alloy and covered with active material. The active material for positive plate is lead peroxide (PbO2) and negative plate is sponge lead (Pb). It gives mechanical support as well as act as a conducting material. The positive and negative plates are in same design and placed alternatively. The number of –ve plate is one more than number of the +ve plate and two extreme plates.

Separators

                                It will be placed in between the +ve and –ve plates and prevent contact between them. It avoids internal short circuiting of plates. It is made of wood or glass wool mat or micro porous rubber or perforated p.v.c.

Electrolyte

                                Dilute sulphuric acid is used as electrolyte. It fills the cell to immerse the plates completely.

Container

It is normally rectangular in shape. It is made up of rubber or molded plastics or ceramics or glass or Celluloid.

Bottom grooved support Blocks


There are raised ribs fitted with bottom of the container. It will give support for die plate and hold them in position to avoid internal short circuiting.

Connecting bar

It is made of lead alloy. All the positive plates are connected into one bar and similarly all die negative plates are connected into another bar. These two bars are act as a +ve and -ve terminals.

Indications of a fully charged lead acid cell

The indications of fully charged lead acid cell are:
1.       Gassing
2.       Voltage
3.       specific gravity and
4.       color of plates

Gassing

Fully charged cell freely give hydrogen at cathode and oxygen at anode. This process is called as gassing. When gassing occur, it means that the cell is fully charged. Fully charged cell electrolyte is a milky appearance.

Voltage

When the cell is charged, the voltage will increase from 1.8 V to 2.1 volts. The fully charged cell voltage is 2.1 volts.

Specific gravity

The density of electrolyte increases during charging due to absorp­tion of water. The specific gravity of electrolyte at fully charged condition is 1.21.

Colour

The colour of plates, on full charge, a deep chocolate brown for the positive plate and clear slate gray for negative plate. The cell looks quite brisk and alive.

Chemical action and physical changes during charging and discharging in lead acid cell                  

The active material of lead acid cell are :
Lead peroxide (Pb02) for +ve plate is dark chocolate brown in color and is quite hard but brittle substance.
Sponge lead (Pb) for -ve plate is pure lead in soft sponge or porous condition and slate brown in color.
Dilute sulphuric acid (H2SO4) as electrolyte is approximately 3 parts of water and one part of sulphuric acid. The +ve and -ve plates are immersed in electrolyte.

Discharging

               When the cell discharges, ie., it sends current through the external load, then H2SO4, is dissociated into positive H2 and negative SO4 ions. As the current with in the cell flowing from cathode (-ve plate) to anode (+ve plate), H2 ions move to cathode. At anode H2 combines with the oxygen of PbO2 and H2SO4 attacks lead to form PbSO4
               PbO2+H2+H2SO4 →PbSO4+2H2O
At cathode, SO4 combines with it to form PbSO4
               Pb+SO4→PbSO4

Points will be noted during discharging

1.       Both the +ve and –ve plate become PbSO4 and is whitish in color
2.       Specific gravity of the acid decreases due to formation of water.
3.       Voltage of the cell decreases.
4.       The cell gives out energy.

Charging

                When the cell is charged, the H2 ions move to cathode and SO4 ions go to anode.
                                At cathode          PbSO4 + H2 → Pb + H2SO4
                                At anode             PbSO4 +SO4+2H2O →PbO2 + H2SO4

Points will be noted during charging

1.       The anode becomes dark chocolate brown in color and cathode become slate gray.
2.       Specific gravity of the acid is increased.
3.       Voltage of the cell increase.
4.       Energy is absorbed by the cell.

Factors deciding the capacity

                The capacity of the battery depends upon the following factors.
1.       Numbers of plates.
2.       Area of plates.
3.       Charge and discharge voltage.
4.       Discharging rate.
5.       Specific gravity of electrolyte.
6.       Quantity of electrolyte.
7.       Design of separators.
8.       Temperature and
9.       Age and life chart of battery.

Troubles occurring in lead acid cell

1.       Buckling of plates.
2.       Sulphation
3.       Short circuit
4.       Low specific gravity
5.       Corrosion on the terminals
6.       Hole or crack on the partition wall
7.       Low capacity.

Ampere hour efficiency

                The ratio between ampere hour discharges to ampere hour charge. Normally the A.H efficiency of lead acid cell is 90 to 95%.
   A.H. efficiency =   amp. hour discharge/amp. hour charge

Watt hour efficiency


          The ratio between watt hour discharge to watt hour charge.
            Normally the W.H. efficiency of lead acid cell is 70 to 80%

            W.H. efficiency =  Watt hour discharge/Watt hour charge

            W.H. efficiency = A.H. efficiency ×  Average Volt on discharge/ Average Volt on charge                           

Applications

1.       Used at generating station during the period of plant breakdown.
2.       As a power source for industrial and mining, locomotives and for road vehicles like trucks and cars.
3.       As a power source for submarines when submerged.
4.       In automobiles for starting and ignition, etc.
5.       In backup units for both domestic and industrial purpose.
6.       In electronic items such as rechargeable lights, mobile phones, laptop computers, etc.

Precautions for maintaining the cell

The following points should be important to maintain the Lead acid cell in good condition:
1. Discharging should not be prolonged after the minimum value of die voltage for the particular rate of discharge is reached.
2. It should not be left-in discharged condition for long period.
3. The level of the electrolyte should always be 10 to 15 mm above the top of the plates which must not be left exposed to air. Evaporation of electrolyte should be compensated adding distilled water occasionally.
4. The battery should never be discharged beyond 1.8V.
5. The specific gravity of the electrolyte should be checked frequently.

Methods of charging the battery:

The following two methods may be used to charge the battery:
1. The constant current system
2. The constant voltage system

Constant current system

In this type of charging a motor-generator set is required.
In this method the charging current is constant. At uncharged condi­tion the applied voltage is low and when the battery voltage increases, the back e.m.f also increase. Hence the applied voltage may be increased to maintain constant current.
The charging current is so chosen that there would be no excessive gassing during final stage of charging and the temperature does not ex­ceed 45°C.
In this method the charging time is comparatively longer.

Constant voltage method

In this method, the voltage is kept constant. In the initial stage of charging the charging current is large. Due to quicker charging, the back e.m.f is increased. Hence in the later stage of the charging current is low.
In this method the time of charging is reduced. It will increase the capacity by 20% but reduces the efficiency by 10% approximately.

Constructional details of Nickel Iron Cell

The active materials in a nickel iron cell are
1.       Nickel hydroxide Ni(OH)4 or apple green nickel peroxide NiO2 for the positive plate. To increase the Conductivity, 17 percent of graphite and 2 percent barium hydroxide are added as additive to increase the life.
2.       Powdered iron and its oxides for the negative plate. To improve the performance small amount of nickel sulphate and ferrous sulphate are added.
3.       21 percent solution of Caustic potash (KOH) is used, as an electrolyte in which lithium hydrate LioH is added in small quantity to increase the capacity of cell.
Positive and negative plates are placed alternatively and placed in a steel container. The body and the cover are nickel plated. The number of negative plate is one more than the positive plates. The two extreme plates are negative plates.

Chemical changes taking place during charging & discharging in nickel iron cell

First, let us assume that at positive plate, nickel oxide is in its hydrated form Ni(OH)4. During discharge, electrolyte KO4 splits up into +ve K ions and -ve OH ions. The K ions go to anode and reduce Ni(OH)4. The OH ions travel towards the cathode and oxidized iron. During charging just opposite reaction taking place, i.e., K ions go to cathode and OH ions go to anode.
Hence                   KOH → K + OH

Discharge

Positive plate:  Ni(OH)4 + 2K       → Ni(OH)2 + 2KOH
Negative plate: Fe + 20H              → Fe(OH)2

Charge

Positive plate:  Ni(OH)2+ 20H  →  Ni(OH)4
Negative plate: Fe(OH)2 + 2K→ F2 + 2K0H
In nickel iron cell the specify gravity of the electrolyte remains practically constant both during charging and discharging.

The active materials used in Nickel Cadmium Batteries

The active materials used in a nickel cadmium cell are,
1.       Ni(OH)4 for the positive plate exactly as in the nickel iron cell.
2.       A mixture of cadmium or cadmium oxide and iron mass to which is added about 3 percent of solar oil for stabilizing the electrode capacity.. The use of cadmium results in reduced internal resistance of the cell.
3.       The electrolyte is the same as in the nickel iron cell.
The cell grouping and plate arrangement is identical with nickel iron batteries except that the number of positive plates is more than die nega­tive plates. Such batteries are more suitable than nickel iron batteries for floating duties in conjunction with a charging dynamo because, in their case, the difference between charging and discharging emf is not as great as in nickel iron batteries.

The Charging and Discharging of Nickel Cadmium Batteries                

The chemical changes are more or less similar to nickel iron cell. As before the electrolyte is split up in to positive K ion and negative OH ions. The chemical reaction at the two plates are as below :

During discharge

Positive plate: Ni(OH)4 + 2K → Ni(OH)2 + 2KOH
Negative plate: Cd+20H                →Cd(OH)2

During charge

Positive plate:  Ni(0H)2 +20H → Ni(OH)4
Negative plate: Cd(OH)4 + 2K → Cd+2KOH



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