BATTERIES

FUNDAMENTALS OF ELECTRICAL ENGINEERING

3RD SEMESTER

ELECTRICAL ENGINEERING

• Presentation By :

BARJINDER SINGH LECTURER ELECTRICAL ENGG.GOVERNMENT POLYTECHNIC COLLEGGURU TEG BAHADURGARH DISTT. MOGA

BATTERIES

CONTENTS

• Electric Cell & How the cell is formed • E.M.F. developed in a Cell• Types of Cells• Important terms of a Cell • Grouping of Cells• Examples

CONTENTS

• Battery• Lead- Acid Battery• Construction of lead-acid battery• Working principles of lead-acid battery• Chemical action during discharging• Chemical action during charging• Charge indicators• Hydrometer

CONTENTS• Charging of lead-acid battery• Care and maintenance of lead-acid battery• Applications of lead-acid battery• Nickel- Iron Cell• Advantages & disadvantages of Nickel-Iron Cell• Nickel-Cadmium Cell• Advantages & disadvantages • Silver Oxide Cell• Solar Cell

What is a Cell ?• A cell is a source of electrical energy (d.c.

supply). The e.m.f. and current supplied by a single cell is very small. For instant, the e.m.f. is 1.5 V and current to be supplied is 125 mA. So to obtain higher voltage and current , a number of cells are connected in series, parallel or series-parallel combination. Such combination of cells is known as battery.

Today we shall confine our attention on

•Grouping of cells

•Lead acid batteries

•Other secondary cells

•Their practical applications

ELECTRIC CELL

A source of e.m.f. (d.c.) in which chemical energy is converted into electrical energy is called an electric cell.

Forming of a cell

An electric cell consists of the following :1.Two metal plates (electrodes) of different

materials. So that different potentials are build up when chemical action takes place on them.

2.A suitable solution (electrolyte) such as acid or salt solution. The solution must be capable to react chemically with two electrodes.

E.M.F. developed in a cell

When the two electrodes are immersed in the electrolyte, different chemical actions takes place on them and a potential difference is produced between them.

The magnitude of e.m.f. of a cell depends upon : 1. nature and material of the plates used as electrodes 2. nature or type of electrolyte used

TYPES OF CELLSElectric cells may be of two types1.Primary cells : The cell in which chemical

action is not reversible are called as primary cells. e.g. voltaic cell, denial cell, dry cell etc.

In this type, during discharging one of the plate is consumed which can not recovered by reversing the direction of flow of current. In this case cell is not recharged. Thus chemical action is not reversible. So primary cells are expansive source of energy.

TYPES OF CELLS2. Secondary cells : The cells in which chemical

action is reversible are called secondary cells e.g. lead acid cell, nickel iron cell, nickel cadmium cell etc.

In these cells, no electrode is consumed during discharging, however chemical composition of the plates is changed. When the direction of flow of current is reversed, the plates regain their original composition. Thus the cells can be recharged. That why the are called as storage cells.

IMPORTANT TERMS OF A CELL E r

V

Electromotive force : The energy supplied by a cell to one coulomb of charge is called e.m.f. It is the potential difference between two electrodes.

Internal Resistance : The opposition offered to the flow of current by the internal composition of the cell itself is called internal resistance.

Terminal voltage : The potential difference across the terminals of the cells at load is called terminal voltage.

Thus V = E – I x r

Grouping of Cells

A single cell can supply a very small current at low voltage. Generally, to operate electrical and electronic circuits, a large current at high voltage is required. Therefore, a number of cells are connected in :

1.Series grouping2.Parallel grouping3.Series- parallel grouping

Series groupingWhen a number of cells are connected in such a way that the negative terminal of one cell is connected to the positive terminal of the other and so on, the cells are said to be connected in series grouping.

In this case, n = no. of cells connected in series E = e.m.f. of each cell r = internal resistance R = load resistance Current delivered to the load, I = n E

R + nr

Parallel groupingWhen a number of cells are connected in such a way that the positive terminals of all the cells are connected together and negative terminals are connected together separately, the cells are said to be connected in parallel .

Where, m = no. of cells connected in parallel E = e.m.f. of each cells r = internal resistance of each cell R = load resistance Current delivered to the load , I = E

R + (r/m) Current supplied by each cell = I/m.

Series- parallel groupingThe grouping in which a number of cells are connected

in series in one row and a number of such rows are connected in parallel is called series-parallel grouping of cells.Where,

n = no. of cells in seriesm = no. of rows connected in parallel r = internal resistance of each cellE = e.m.f. of each cellR = load resistance

Current delivered to the load, I = n E R + (nr/m)

Example 1 : How many cells, each having an e.m.f. of 1.5 V and internal resistance of 0.25 ohm would be required to pass a current of 1.5 A through a resistance of 15 ohm, when connected in series ?

Solution :Let n be the number of cells connected in series.current delivered to load , I = nE / (R + nr)Here, E = 1.5 V, R = 15 ohm, r = 0.25 ohm and I= 1.5 A

1.5 = n X 1.5 / (15 + n X 0.25) n = 20 Ans.

Example 2 : Four dry cells each of which has an e.m.f. of 1.5 V and an internal resistance of 0.06 ohm are connected in parallel. Determine current and power dissipated by external load of 2.985 ohms.

Solution :In parallel circuit, current delivered to load resistance, I = E/ (R+(r/m))

Here, E=1.5 V, R=2.985 ohm, r =0.06 ohm & m=4

I = 1.5/ (2.985 + (0.06/4)) = 0.5 APower dissipated in external load resistor,

P = I²R = (0.5)² X 2.985 = 0.74625 W

Example 3 : A battery of 24 cells is required to send the current through a resistance of 3 ohms.Find type of grouping which will give largest current. The internal resistance of each cell is 1 ohm and e.m.f. is 2 Volt.

Solution : Let a series-parallel grouping is applied with n is no. of cells in series in one row and m be the no. of such rows.nr/m =R or n X 1/m=3 or n/m=3

now Total no. of cells, n X m = 24so n=8 and m=3current supplied to load,

I = nE/(R + (nr/m)) = 2.8235 A

What is a Battery ?A series, parallel or series-parallel grouping of cells is called a

battery.Generally, a cell can deliver a small current at low voltage.

• If higher voltage is required- a battery containing number of cells connected in series.

• If higher current is required – a battery containing number of cells connected in parallel.

• If large current at high voltage is required- a battery containing number of cells in series and further connected in parallel.

Usually a no. of cells connected in series placed in single container is called a battery.

LEAD – ACID BATTERYA lead acid battery consists of the following parts• Container• Plates• Separator• Electrolyte• Battery cover• Vent pipes• Inter cell connector• Cell terminals

CONTAINER

It is the outer body of the battery. It is made of hard rubber or plastic material and is sealed at the top to prevent spilling of the electrolyte. A large space is left at the bottom of the container so that the sediment that drops from the plate are collected here and may not short circuit the positive and negative plates.

PLATES

Generally, alloy of lead sheets covered with lead peroxide are used as electrodes. To increase the capacity of the battery a large numbers of plates in each cell is used. The numbers of positive and negative plates (i.e. 11,13, 15 or 17) of each cell are alternatively placed and sandwiched with an insulator called separator. A separate compartment is provided for each cell in the battery container.

SEPARATOR

To reduce the internal resistance of the cell and to save the space, the plates are placed very close to each other. To prevent touching of plates with each other, they are separated by a rubber sheet having large number of small holes called separator.

ELECTROLYTE

Dilute sulphuric acid is used as an electrolyte in lead acid batteries. Sulphuric acid is added to water in such a proportion that with a fully charged battery, its specific gravity is about 1.28 to 1.29.

BATTERY COVER

Each cell compartment is covered usually with a molded hard rubber and the joints between cover and container are sealed with an acid resistant material. In each cell cover openings are provided- two for positive and negative terminals and third for vent.

VENT CAPS

The vent cap has a hole to allow free exit of the gases formed in the cell during charging. The vent pipe can be removed for adding pure water in the cells. The vent cap is also removed to insert the nozzle of hydrometer for checking specific gravity of electrolyte.

INTER CELL CONNECTOR

The cells placed in the same container are connected in series with a lead alloy link called inter – cell connector.

CELL TERMINALS

Each cell has two terminals which are made of lead. The positive terminal of the battery is marked with a red color or by a large positive (+) sign.

WORKING PRINCIPLE OF LEAD ACID BATTERY

When a lead acid cell is ready for use, its positive plate is of lead peroxide (PbO2)- chocolate brown in color and negative plate is of spongy lead (Pb)- grey in color. Both the plates are immersed in a dilute sulphuric acid of specific gravity 1.28. When the load is connected across the terminals of the cell, it starts delivering current to the load and this process is called as discharging of cell. In this process chemical energy is converted into electrical energy.

CHEMICAL ACTION DURING DISCHARGING

When the load is connected, the sulphate ions moves towards cathode and hydrogen ions moves toward anode. The following chemical action takes place ;

At cathode, Pb + SO4 PbSO4

At anode, PbO + H2SO4 PbSO4 + H2OThus during discharging :1.Both plates are converted to lead sulphate2.Specific gravity of sulphuric acid is 1.153.Terminal voltage fall from 2.0 V to 1.8 V4.Chemical energy changes to electrical energy

CHEMICAL ACTION DURING RECHARGING

For recharging anode is connected to positive terminal of source and cathode is connected to negative terminal. During this hydrogen ions moves towards cathode and sulphate ions towards anode.

At anode, PbSO4+ O + H2O PbO2 + H2SO4

At cathode, PbSO4 + 2H Pb + H2SO4

During recharging :1.Plates regain their original composition2.Specific gravity of acid become 1.283.Terminal voltage increases from 1.8 V to 2.0 V4.Electrical energy converted to chemical energy which is stored in cell.

CAPACITY OF A BATTERY

The quantity of electricity which a battery can deliver during single discharge until its terminal voltage falls to 1.8 V per cell is called the capacity of a battery.

The capacity of battery or cell is expressed in ampere-hours and denoted by AH.

CHARGE INDICATION

A fully charged battery has a specific gravity of 1.28. However when it falls to 1.15, the battery is fully discharged. To get good life of battery keep the specific gravity more than 1.18.

Specific gravity Condition1.280 to 1.290 100 % charged1.230 to 1.250 75 % charged1.190 to 1.200 50 % charged1.150 to 1.160 25 % chargedBelow 1.130 Fully discharged

HYDROMETER

To check the specific gravity of sulphuric acid , an instrument called hydrometer is used. Which works on Archmedeies principle. However the state of battery can be checked by checking :

1.Voltage : When the terminal voltage is 2.1 to 2.5 V per cell, the battery is fully charged. But when it become 1.8 V per cell, the battery is fully discharged.

2.Colors of plates : When battery is fully charged , the anode is of chocolate color and cathode is of grey color. When battery is fully discharged the color of both plates is whitish.

CHARGING OF LEAD ACID BATTERYWhen terminal voltage falls below 1.8 V per cell, Battery is put under recharging. The following steps must be kept in mind while charging :

1.Only d.c. voltage source is applied.2.Positive terminal of battery is connected with

positive terminal of source and negative with negative.

3.Charging voltage of source should be 2.5 V per cell

4.The charging current should be 1 A

CARE AND MAINTENANCE OF LEAD ACID BATTERY

To obtain longer life of battery the following points must be kept in view :

1. The battery should not be allowed to use when e.m.f. falls below 1. 8 V per cell.

2. The specific gravity of acid is more than 1.15.3. Battery should never be left standing in a discharged condition.4. When not in use , battery must be fully charged5. The level of electrolyte must be checked regularly.6. When the level of electrolyte decreases then only distilled water is

added.7. The battery should be charged or discharged at low rate so that

temperature may not rise above 45 degree C.

CARE AND MAINTENANCE OF LEAD ACID BATTERY

8.The battery terminals should not be short circuited.

9.Keep battery terminals clean. 10.While charging polarity must be checked

carefully.11.Keep battery in well ventilated room.12.Keep flames away from vent of battery.

APPLICATIONS Some important applications of lead acid battery are :

1.Used in automobiles for starting & lighting.2.For lighting of railway trains.3.Used at generating station or sub station to

operate protective devices4.Used in telephone exchanges.5.Used in emergency tube lights6.Used for lighting purposes in remote rural areas

NICKEL IRON ALKALINE CELLIt contains two plates. The active material of anode is Nickel Ni(OH)4 and cathode is iron (Fe) when fully charged . The electrolyte is a solution of potassium hydroxide KOH. A small quantity of Lithium hydrate is added to increase life of the cell. Following are the main features of this cell :

1.e.m.f. of fully charged cell is 1.4 V and 1.0 V during fully discharged.

2.Internal resistance is quite high nearly 5 times to that of lead acid cell

3.The AH capacity is nearly 80 %

ADVANTAGES

It has following advantages :1.Longer life about 5 years2.Its electrolyte is not harmful as in lead acid 3.Specific gravity remains same in both charging

and discharging states4.Lower weight almost half to that of lead acid5.No damage during high recharging 6.It can withstand high temperature7.It withstand more mechanical stress

DISADVANTAGES

The following are the disadvantages :1.Higher cost nearly double to that of lead acid2.As e.m.f. in each cell is less so more cells are

required to get particular voltage3.Higher internal resistance almost 5 time to

that of lead acid cell

NICKEL CADMIUM CELLIt has anode made up of Nickel Ni(OH)4 and cathode of cadmium Cd. The electrolyte is potassium hydroxide KOH of specific gravity 1.2.

The following are the main point about this :1.E.M.F. fully charged cell is 1.4 V and 1.0 V for

fully discharged cell.2.Internal resistance is very low.

ADVANTAGES

It has following advantages ;

1.Very long active life almost 20 years.2.No change in specific gravity of electrolyte.3.These cell can be charged in a short period.

DISADVANTAGES

The following are the disadvantages :

1.It is very costly.2.Low average e.m.f. so more cells are required

to get particular voltage.

SILVER OXIDE CELLIn silver oxide cell, cathode is silver oxide and anode is of zinc with an potassium hydroxide as an electrolyte.This type of cells are generally made in button size and working potential of 1.5 V.* These cells are leak proof (sealed)* Having very small internal resistance* Very handy and occupy less space* Used in cameras, watches, small electronic

toys and other electronic circuits

SOLAR CELLSA device that converts light energy (sunlight) directly into electrical energy is called solar cell.A pure silicon (semiconductor) is doped with a specific amount of arsenic (donor impurity) and this makes N-type semiconductor. Similarly make the top layer as P-type semiconductor.When the light falls on the top of P-type layer and penetrates into N-type material and thus free electrons start moving. This continuous movement generate electric current.

The operating voltage of one solar cell is about 0.39 V and current varies between 30 to 40 mA. They have very long life estimated to be thousands of years.

Applications :1.To charge Nickel cadmium batteries in

satellites2.To provide power for calculator, clocks etc.3.To provide power to control devices such as

movie cameras, microwave relay stations etc.

SOLAR CELLWORKING OF SOLAR CELL AS SUN LIGHT FALLS ON p – TYPE LAYER AND PENTRATE TO n-TYPE LAYER AND FORMING A PN JUNCTION

LIGHT (SUN RAY)

p TYPE LAYER

n TYPE LAYER

SUMMARY1. Secondary cell : The cells in which chemical action is

reversible.2. EMF of cell : The energy supplied by a cell to one

coloumb of charge.3. Internal resistance : The opposition offered to the

flow of current by the internal resistance of a cell.4. Primary cell : The cells in which chemical action is

not reversible.5. Terminal voltage : The potential difference across

the terminals of a cell.

SUMMARY6. Series grouping : Current in all the cells is

same.7. Parallel grouping : current is divided but

voltage is same.8. Series-parallel grouping : There are n no; of

cells in series in each row and m no; of such rows are connected in parallel.

9. Battery : A number of cells connected in series and placed in a single container.

SUMMARY10.Capacity of battery : The quantity of

electricity which a battery can deliver in single discharge is called the capacity.

11.Charge indication : If specific gravity is 1.280 to 1.290 the battery is 100 % charged.

12.Voltage of a cell is more than 2.0 V the battery is fully charged and voltage less than 1.8 V the battery is fully discharged for lead acid battery.

SUMMARY13.For Nickel-Iron cell, EMF of fully charged cell

is 1.4 V and fully discharged cell is 1.0 V.14.High internal resistance – 5 times to lead acid

cell.15.Silver cells have a very long life span.16.Solar cells are used to charge sattelite

batteries.