CELL AND BATTERY INTRODUCTION A cell is a device which converts chemical energy into electrical energy. ~ INFOTECHNO

5.1. INTRODUCTION

A cell is a device which converts chemical energy into electrical energy. There are two main types of cells — (1) primary (2) secondary.

1. Primary Cell. A cell which produces electrical energy and after few hours of working becomes inactive, is called a primary cell. It requires a fresh electrolyte for becoming active. Its main types are —Voltaic cell, Daniel cell, Lechlanche cell, Dry cell.

2. Secondary Cell. A cell which is used repeatedly for producing electrical energy after charging it, is called a secondary cell. Actually speaking it does not produce electrical energy but it merely accumulates the energy and hence it is also, known as accumulator. Its main types are — Lead Acid cell, Edison cell, Nickel-Cadmium cell.

5.2. VOLTAIC CELL

The very first electric producing device made by famous scientist Volta in the year 1800 is called a Voltaic cell.

(a) Construction. It consists of a glass container filled with dilute sulphuric acid (H₂SO₄) which is called electrolyte. It has two electrodes, one of Zinc (Zn) and other of Copper (Cu). Copper rod works as anode (+ plate) and Zinc rod works as cathode (– plate).

Fig 5.1 Voltaic Cell

(b) Reactions. H₂SO₄solution contains H+ and SO₄-- ions.

H₂SO₄ ---> 2H⁺ + SO₄ --

Chemical reactions start in the cell on connecting a load between anode and cathode (a resistor or bulb etc.) by means of conducting wires and an e.m.f. is generated.

At cathode : Zn → Zn⁺⁺ + 2e

Zn⁺⁺ + SO₄ --- → ZnSO₄

At anode : 2H⁺ + 2e — H₂

In this way, a flow of electrons is set up from copper rod to zinc rod inside the cell and from zinc rod to copper rod in the external circuit. The flow of current (i.e., conventional flow of current) is considered from anode to cathode in the external circuit. The e.m.f. of the cell is 1.08 volts.

(c) Defects and their Remedy. The simple cell has following two main defects:

(i) Local Action. Generally a zinc rod has impurities of lead, iron, carbon and arsenic etc. A tiny particle of the said elements present at the surface of zinc rod, makes a tiny cell at that point. In this way, a number of tiny cells are formed at the zinc rod. The flow of current set up by these tiny cells causes an unwanted consumption of chemical energy; the action is termed as local action.

In order to eliminate the defect, the zinc rod is plated with mercury. The mercury forms an amalgum at the surface of zinc rod, where as it does not form such amalgum with other elements present in the rod. As a result, only zinc can establish contact with electrolyte and not the other elements and the local action defect is eliminated.

(ii) Polarisation. Hydrogen ions reach at the anode, give up their charge and get deposited around copper rod in the form of bubbles. The action is termed as polarisation. After a Few hours of working the cell becomes inactive because the anode remains no longer in contact with the electrolyte.

Manganese dioxide granules (MnO₂) are filled around the copper rod in a porous pot, MnO₂ converts hydrogen atoms into water and it gets converted into Mn₂O₃. Mn₂O₃is an unstable compound, it again converts into MnO₂ with the help of oxygen present in the air. .In this way, the polarisation defect is eliminated.

5.3. DANIEL CELL

The first useful cell was made by Prof. Daniel in the year 1836.

(a) Construction. It consists of a cylindrical copper pot which acts as anode. It is filled with copper sulphate (CuSO₄) solution. A balcony is made at the upper end of the pot with a copper mesh. CuSO₄ crystals are kept in the balcony. A porous pot is placed in the solution which contains dilute sulphuric acid (H₂SO₄) and a zinc rod to act as cathode.

Fig 5.2 Daniel Cell

(b) Reactions. Dilute H₂SO₄has H⁺ and SO₄- ions.

H₂SO₄ → 2H⁺+ SO₄—

On connecting a load between anode and cathode

At cathode : Zn → Zn⁺⁺ + 2e

Zn⁺⁺ + SO₄- - - →ZnSO4

In the electrolyte CuSO₄ - →Cu⁺⁺ + SO₄-

2H⁺ + SO₄- ---> H₂SO₄

At anode : Cu⁺⁺ + 2e → Cu

The e.m.f. of a Daniel cell is 1.1 volts.

(c) Defects and their remedy

(i) Copper gets deposited continuously on the copper pot and a shortage of CuSO₄ is created in the electrolyte. Crystals of copper sulphate kept in the balcony supply required amount of CuSO₄ to the electrolyte.

(ii) The cell has no local action and polarisation.

5.4. LECHLANCHE CELL

(a) Construction. It consists of a glass pot filled with a solution of ammonium chloride (NH₄C1). A porcelain porous pot is placed in the solution. The porous pot is filled with manganese dioxide granules and a carbon rod is erected, in the MnO₂. A zinc rod is erected in the NH₄C1 solution. Zinc rod acts as cathode and the carbon rod acts as anode.

Fig 5.3 Lechlance Cell

(b) Reactions. Aqueous solution of NH₄C1 has NH₄⁺ and

Cl^- ions.

NH₄C1→ NH₄⁺+ Cl^-

On connecting a load between anode and cathode —

At cathode : Zn --> Zn⁺⁺+ 2e

Zn⁺⁺ + 2CI^-→ ZnCI₂

At anode : 2NH₄ + 2e ---> 2NH₃ + H₂

In MnO₂ : H₂ + 2MnO₂ ----> Mn₂O₃ + H₂O

2Mn₂O₃ + 0₂→ 4MnO₂

(from air)

The e.m.f of a Lechlanche cell is 1.5 volts.

(c) Defects and their remedy

(i)Local Action. Zinc rod is plated with mercury to eliminate local action.

(ii) Polarisation. Manganese dioxide (MnO₂) is used as depolariser.

Note. Both the above stated remedies have already been explained in detail in the Article 5.2.

5.5. DRY CELL

(a) Introduction. A dry cell is the modified form of a Lechlanche cell. It contains a paste in place of liquid electrolyte. Its construction allows it to be placed erect, inverted or lying. Its size can he reduced upto 6 mm x 2.5 mm which is smaller than a shirt-button.

Fig 5.4 Dry Cell

(b) Construction. It consists of a cylindrical pot of zinc. A carbon rod with a brass cap acts as anode and it is placed in a bag of coarse cloth filled with a mixture of Manganese dioxide. Carbon, Ammonium Chloride and Zinc Chloride in the ratio of 10: 10: 2: 1. The bag is placed in the pot and a paste of Ammonium Chloride, Zinc Chloride and Plaster of Paris is filled in the space between the pot and the bag. The mixture acts as depolariser and the paste acts as electrolyte. Plaster of Paris provides toughness to the paste. The cell is then sealed and a small hole is made in the top cover to provide an escape to gases evolved. The e.m.f. of a dry cell is 1.5 volts.

(c) Reactions. Reactions of a dry cell are quite similar to those of Lechlanche cell. See Article 5.4.

(d) Uses. Dry cells are used in torches, transistorised receivers, telephones, electric bells, clocks and wrist-watches, tape recorders, cameras, toys etc.

5.6. LEAD ACID CELL

(a) Construction. It consists of a hard rubber container which is generally rectangular. It has two groups of plates; one is positive group and the other negative. The number of plates of negative group is one more than the number of plates of positive pot group. In this way, each positive plate remains active from both the sides. Lead mesh is used for the construction of plates. Red lead (Pb₃O₄) paste is deposited in slots of the lead mesh. Hard rubber resters are used for supporting the plates. Hard rubber wooden separators are used between the positive and negative plates. Dilute H₂SO₄ is used as electrolyte. The plate groups are connected to the terminals projected outside the top cover of the cell marked as P and N indicating positive and negative respectively. A vent hole is made in the top cover.

Fig 5.5 Lead Acid Cell

(b) Reactions. Reactions of a lead acid cell can be classified into following three groups:

(i) Forming. It is usually performed in the industry which manufactures the cell. On filling dilute H₂SO₄ in the cell, the following reaction is started:

Pb₃O₄ + 2H₂SO₄ → PbO₂ + 2PbSO₄ + 2H₂O

After a few hours (approximately 12 hours) the cell is connected to the D.C. supply with its positive terminal to the positive and negative terminal to the negative of the source. Electrolysis of water is started

H₂O → 2H⁺ + O^{- -}

At anode: PbSO₄ + H₂O + O^{- -} → PbO₂ + H₂SO₄

At cathode: PbSO₄ + 2H⁺→ Pb + H₂SO₄

Thus in forming, red lead is converted into lead dioxide and spongy lead and the formation of sulphuric acid increases the specific gravity of the electrolyte.

(ii) Discharging. On connecting a load to the cell, the charge of the cell begins to reduce. The action is called discharging. It involves following reactions :

H2O ----> 2H⁺ + O^--

At anode : PbO₂ + 2H⁺ + H₂SO₄ → PbSO₄ + 2H₂O

At cathode : Pb + O^-- + H₂SO₄ → PbSO₄ + H₂O

Thus in discharging, both the plate groups are converted into lead-sulphate (PbSO₄) and the formation of water reduces the specific gravity of the electrolyte.

(iii) Recharging. If the e.m.f. of a cell has reduced upto 1.8 volts due to its regular use or in other words, its charge has sufficiently reduced, it requires recharging. For this, its positive terminal is connected to the positive and negative terminal to the negative of the D.C. supply. It involves following reactions:

H₂O ---> 2H⁺ + O^—

At anode: PbSO₄ + O^--+ H₂O ----> PbO₂ + H₂SO₄

At cathode: PbSO₄ + 2H⁺→ Pb + H₂SO₄

Thus after recharging, the condition of a cell becomes the same as it was after forming. The formation of sulphuric acid during recharging increases the specific gravity of the electrolyte. The e.m.f. of a fully charged lead-acid cell is 2.2 volts.

(c) Defects and their remedy

(i) Corrosion. Terminals of a lead-acid cell are made of copper coated with lead. An oxide layer is formed on the terminals due to acid (H₂SO₄) and humidity. The oxide layer acts as an insulator between the terminal and the connecting wire-lead clamped on it. The defect is known as corrosion. In order to remove this defect, the cell terminals should be periodically cleaned with a piece of cloth and warm water and coated with a thin layer of grease.

(ii) Sedimentation. Sediments are deposited at the bottom of a cell due to decay of plates during repeated charging and discharging. Besides it, electrolysis of water present in the electrolyte also causes the impurities to be disassociated and get deposited at the bottom of the cell. In this way, sufficient amount of sediments get deposited at the bottom of the cell within a few months of working. The defect is known as sedimentation. The defect is reduced by using distilled water as and when required.

(iii) Sulphation. All the positive and negative plates of a cell get changed into lead sulphate (PbSO₄) during discharging. If the cell is left discharged for a long time (more than 2-3 weeks), the lead sulphate becomes hard enough and cell remains no longer chargeable. The defect is known as sulphation. In order to avoid the cell to be sulphated, it should be charged and discharged atleast once a week. A new life may be given to a sulphated cell by means of trickle charging.

(iv) Buckling. If a cell is charged or discharged above its rated ampere (usually 25 amperes) value, its plates can buckle and the cell may become useless due to short-circuit between the plates. The defect is known as buckling. For the elimination of this defect, a cell should not be charged or discharged above 25 amperes in normal case.

5.7. EDISON CELL OR NICKEL-IRON CELL

(a) Construction. In this cell, positive plates are made of nickel hydrate [Ni(OH₂)] and negative plates are made of iron hydrate[Fe (OH)₂]. A mixture of potassium hydroxide (KOH) and lithium hydrate (LiOH) is used as electrolyte. Positive plates are constructed with nickel plated steel ribbon wound in tubular form. The ribbon is perforated and its holes or slots are filled with the paste of Ni (OH)₂. For negative plates, there are pockets in the positive plates which are filled with Fe(OH)₂ powder. All the plates are arranged in a suitable container. The cell is also known as Alkaline cell.

(b) Reactions

(i) During discharging:

KOH → K⁺+ OH^-

At anode: Ni(OH)₂+ 2K⁺→ Ni (OH)₂ + 2KOH

At cathode: Fe⁺+ 2OH^-→ Fe (OH)₂

(ii) Recharging:

KOH → K⁺+ OH^-

At anode: Ni (OH)₂ + 20H^- → Ni (OH)₄

At cathode: Fe (OH)₂ + 2K⁺ → Fe + 2KOH

The specific gravity of electrolyte of the cell remains stable. Its resistance is greater than that of a lead-acid cell.

(c) Characteristics

(i) Steel plates being tough in structure last longer and the sedimentation defect is negligible.

(ii) It may be kept in discharged state for a long time because there is no sulphation type defect in it.

(d) Defects

(i) Its charging and discharging rate is lower in comparison to a lead-acid cell

(ii) Its internal resistance is quite high.

(iii) Its e.m.f. is reduced with a rise in surrounding's temperature.

(iv) Its ampere-hour capacity is quite low.

5.8. NICKEL CADMIUM CELL

In this cell, positive plates are made of nickel and negative plates are made of cadmium. The construction of the cell is similar to that of a nickel-iron cell. The use of cadmium reduces its internal resistance to a great extent in comparison to nickel-iron cell.

5.9. SOURCE AND LOAD E.M.F.

No load e.m.f. of a cell is always greater than the e.m.f. with a load connected across the positive and negative terminals of the cell. The reason for the above effect is the voltage drop across the cell itself. Since V= I . R, thus the flow of current (I) will produce a voltage drop across each resistance in the circuit. The resistance of the cell itself is called its internal resistance.

where, Es = source voltage, volts

I = current, amps

R = load resistance, ohms

r = internal resistance, ohms.

The above formula can be stated as under also

where, V = load voltage (I . R.) volts

Example 5.1. No load voltage of a cell is 1.56 while load voltage is 1.26. If the load is 5 ohms then calculate the internal resistance of the cell.

Solution. Given : No load e.m.f., E_s = 1.56 V

Load voltage, V = 1.26 V

5.10. GROUPING OF CELLS

A group of cells is called a 'battery'. Cells can be grouped in the following three ways :

Fig 5.6. Series group of cells

(a) Series Group. Cells are connected in series for obtaining more e.m.f than that of one cell. In this method, the negative terminal of a cell is connected to the positive terminal of the second cell and the negative terminal of the second cell is connected to the positive terminal of the third cell. In this way, any number of cells can be connected in series as per requirements. If the e.m.f. of all the cells is the same then the total e.m.f.

where, E_T = total e.m.f., volts

n = number of cells

E = e.m.f. of one cell, volts

r = internal resistance of one cell, ohms

r_T = total internal resistance, ohms

I = circuit current, amperes.

(b) Parallel Group. Cells are connected in parallel for obtaining more current or current for more time than that with one cell. In this method, the positive terminals of all the cells are joined at one point and the negative terminals at the other. The total e.m.f. of the battery remains the same as of one cell alone. The total internal resistance is reduced in comparison to that of one cell.

Fig 5.7. Parallel group of cells.

(c) Mixed Group. Cells are connected in mixed group for obtaining more e.m.f and more current than that with one cell. For this group, some series groups of cells are connected in parallel. If the e.m.f. of one cell is 2 V and its current supplying capacity is 2 A then for obtaining 90 V I 0 A capacity, 5 series groups of 45 cells each are required to be connected in parallel. Each series group will be able to supply 90 V and 5 such groups connected in parallel will he able to supply 2 x 5 = 10 A current.

Maximum Battery Current.

where, m = no. of series groups

n = no. of cells in a series group.

Example 5.2. If 16 cells of 1.5 V e.m.f. and 0.2 ohms internal resistance each are connected in series across a load resistor of 10 ohms, then calculate the circuit current.

Example 5.3. If 24 cells of 2 V e.m.f. and 0.2 ohms internal resistance parallel across a load resistor of 10 ohms, then calculate the circuit current.

Example 5.4. Flow 20 cells of 1.5 V e.m.f. and 0.5 ohms internal resistance each should be connected across a load resistor of 2.5 ohms so that the maximum current flows through the load ?

Solution. If the cells are connected in series, then the total internal resistance

But, for obtaining a maximum flow of current through the load, the internal resistance of the battery should be equal to the load resistance. Hence, two series groups of 10 cells each are required to be connected in parallel.

Now the total internal resistance,

5.11. CAPACITY OF A BATTERY

The capacity of a battery is expressed in ampere-hours (Ah). Normally 72 Ah, 108 Ah and 144 Ah batteries are in use. Of course, there are batteries above and below the above mentioned capacities. The capacity of a lead-acid battery depends on the number and size of plates which is generally kept 11, 13, 23 or 25 per cell.

If a battery has a capacity of 144 Ah, it means that the battery can supply a current of 144 amperes for one hour or a current of 1 ampere for 144 hours. But, in practice a normal battery can't supply more than 20-25 A current, At more current its plates can buckle and the battery may become useless.

Example 5.5. If a battery can supply 6 amperes of current for 16 hours, calculate its capacity. If the current drainage is 7.5 amperes then how long the battery would be able to work?

Solution.

5.12. BATTERY CHARGING

A secondary cell requires re-charging at frequent intervals of time. A cell or battery can be directly charged with a D.C. source, but if the source is A.C. then it will have to be converted into D.C. first. The methods of battery charging are as follows:

(a) Constant Voltage Method. The magnitude of D.C. charging voltage is kept constant in this method. A dynamo is used D.C, for the purpose. The magnitude of dynamo e.m.f. should be greater 6 V than the battery e.m.f. otherwise the battery will not be charged. If the number of batteries to be charged is greater than one then they are connected in parallel for charging. The magnitude of initial charging current is always maximum and it goes on reducing with an increase in the charge of the battery. In the complete charging process the e.m.f. of the dynamo remains constant, hence the method is called constant voltage method,

Fig. 5.8 Constant voltage battery charging circuit

The above explained method is used for battery charging in motor vehicles and railway trains.

(b) Constant Current Method. In this method the magnitude charging current is kept nearly one tenth of its AhC (ampere hour capacity). When the gas bubbles start to evolve from the battery, the magnitude of charging current is reduced to nearly half of its initial value. A rheostat is Switch connected in the charging circuit for controlling the magnitude of charging current. An ampere meter is also connected in the Lamps circuit for the indication of the magnitude of current. Carbon filament type bulbs are used for the regulation of current, see Fig. 5.9.

Fig 5.9 Constant current battery charging circuit

(c) Trickle Charging Method. When a battery becomes incapable to get the charge due to being left in uncharged condition for a long duration of time, it is then charged at a very low charging current. The method is called trickle charging. In this method, the magnitude of charging current is kept nearly equal to 2% of its normal discharging current value. A battery may require upto one week time for full charging by this method.

5.13. BATTERY CHARGER

Now a day the source of electric supply is A.C. in almost each and every city. Therefore, a battery charger is necessary for battery charging. It consists of mainly

(i) Transformer, (ii) Rectifier, (iii) Ammeter, (iv) Voltmeter, (v) Voltage controller switch, Current controller switch etc.

Generally, an ordinary battery charger has an arrangement for charging 1 to 4 batteries at a time. But, small and large size battery chargers can be constructed as per requirement. A battery charger has an arrangement for supplying 3 A and 6 A current at 6 V, 12 V or 24 V.

A rectifier is the main component of a battery charger. It converts A.C. into D.C. Few years ago, tunger and mercury arc rectifier valves were used for rectification but now a days high current rate solid state rectifiers or silicon controlled rectifiers (SCR) are very commonly used for rectification.

Fig 5.10 Battery charger

5.14. CHARGED AND DISCHARGED CONDITIONS OF A BATTERY

1. Full Charged Condition. In this condition. per cell e.m.f. becomes 2.2 volts and specific gravity of the elctrolyte becomes 1.25 to 1.28. Besides it, battery starts to evolve gas bubbles in sufficient amount.

2. Half Charged Condition. In this condition. per cell e.m.f. becomes 2.1 to 2.15 volts and specific gravity of the electrolyte becomes 1.20 to 1.25.

3. Discharged Condition. In this condition, per cell e.m.f. falls upto 2.0 volts per cell or even lower than this and the specific gravity of the electrolyte falls to 1.18 or even lower than this.

5.15. RELATIVE DENSITY AND HYDROMETER

1. Density. The density of a solid or liquid states its mass per unit volume.

The unit of density is g/cm³ or kg/m³. The density of water is 1.0 g/cm³ or 1.0 x 10³ kg/m³ .

2. Relative Density or Specific Gravity. The relative density of a solid or a liquid is a number which states that the mass of unit volume of the substance is how many times heavier than the mass of unit volume of 4°C water.

Relative density has no unit.

3. Hydrometer. It is a very simple apparatus which is used for the measurement of R.D. of various liquids. It consists of a poly-glass tube, a perforated rubber stopper at the upper end, another perforated rubber stopper with a rubber pipe at the lower end, a ball type rubber bulb at the top and a float inside the tube,

Fig. 5.11 Hydrometer

The inside air of the tube is pushed out by pressing the rubber bulb. The liquid (whose R.D. has to be measured) it then rushes into the tube due atmospheric pressure and the ‘floats’ begins to float in the liquid. The stem of the float is marked with numbers indicating the R.D. of the liquid. In a high R.D. liquid, most of the 'float’ stems Acid proof remains out of the liquid and conversely in a low R.D. liquid the ‘float’ stem remains almost submerged in the liquid.

A hydrometer is used for the measurement of charge condition of lead-acid batteries.

5.16. MAINTENANCE OF BATTERIES

Regular maintenance of lead-acid batteries is necessary otherwise they will spoil up very soon. Following instructions should be strictly observed in this regard:

(i)The normal charging/discharging current rate should not Rubber cork exceed 12 A or I/10 of its Ah. Under special circumstances the rate may be increased upto 25 A. Beyond this rate the buckling defect is Rubber tub likely to develop in the battery.

(ii) A new battery should be used regularly so as to make it smooth.

(iii) A battery should be discharged and then recharged once a week atleast. If it is left unused for a long period, it will get sulphated.

(iv) Distilled water should be added to the battery often on so as to keep the plates fully submerged in the electrolyte otherwise the portion of the plates remained outside the electrolyte may become inactive.

(v) Only distilled water should be used in the batteries. The impurities of ordinary water will disintegrate during electrolysis and will deposit in the bottom as sediments and hence will develop sedimentation defect.

(vi) Both the terminals of the battery should be periodically cleaned with warm water and then dried with a piece of cloth. A small quantity of grease should be applied on the terminals after cleaning. Failing to it, corrosion defect will develop due to atmospheric humidity and battery acid.

(vii) When the R.D. of the electrolyte is reduced to 1.18 or less, the use of battery should be stopped otherwise recharging of the battery would become quite difficult.

(viii) Vent-plugs should be removed during charging so as to provide an easy escape to the gases evolved during charging process.

(ix) The place for battery charging should be separate and airy so that the gases evolved during charging process may not be able to produce suffocation.

(x) The battery charging job should be done with responsibility and a complete record of maintenance work should be maintained so that the batteries may not fail in the emergency.

5.17. SOLAR CELL

A solar cell is a semiconductor device which is capable to convert light energy into electrical-energy. Though, there are number of such substances but most of the solar cells employ 'crystalline-silicon' (Si) for this purpose. The other such substances are Amorphous silicon (a-Si), Gallium-Arsenide (Ga-As) and Cadmium-Selenium (Cd Se). All these substances are light-sensitive substances the efficiency of crystalline - silicon is fairly high about 12%.

The production of e.m.f. by a solar cell depends on the atomic structure as well as on the crystal structure of the semicondutor. When the semiconductor is subjected to light-rays (sun-rays) a photogenerated current flows in the device which is directly proportional to the intensity of the light-rays. Therefore, an e.m.f is developed across the load.

These cells are used in small electronic equipments like pocket calculators. Moreover, these are used as an alternate source of electric supply in villages, small factories, railway stations etc. Such supply system contains a number of solar cells connected in series so as to produce a considerable voltages system also incorporates storage batteries to store the electricity which can be used in the absence of sun-light.

5.18. SILVER OXIDE CELL

These are small sized cells which can he used in wrist watches, pocket calculators, hearing aids etc. These are known as Button Cells. They have the advantage of small size, good performance at high temperatures, good resistance to shock and vibrations and they have a long working life. It is primary alkaline cell.

In these cells, the anode is made of high purity amalgamated zinc and silver oxide is used to act as cathode which is contained in porous separator in the paste form. The electrolyte is a solution (paste) of alkaline hydroxide, Anode