Demonstrate construction and working of automobile batteries.

Demonstrate construction and working of automobile batteries.

INTRODUCTION

Battery is device that converts chemical energy into electrical energy. Storage battery is most important component of the entire electrical system vehicle.

The battery is the main part of the electrical system in an automobile. Without the battery, the engine cannot be started with the starting motor. The battery supplies current for operation of the starting motor and ignition system when the engine is being cranked for starting. It also supplies current for light, radio, heater and several other accessory units when the generator is not operating fast enough to handle the electric load.

TYPES OF BATTERY

1. Lead acid battery.

2. Alkaline battery.

(a) Nickel-iron type.

(b) Nickel-cadmium type.

3. Zinc-air battery.

4. Dry charged battery.

LEAD ACID BATTERY

The lead- -acid battery is most widely used in automobiles.

Construction

A lead-acid battery consists of the following components:

i. Container ii. Plates

iii. Separators iv. Cell Covers

v. Electrolyte vi. Grids

vii. Cell connectors viii. Tapered Terminals

ix. Sealing compound 

i. Container

The container of a lead-acid battery is a single piece construction and is made of hard rubber, plastic or bituminous composition. It must withstand extreme of heat and cold as well as mechanical shocks and must be resistant to the absorption of acid. It is divided into compartments by partitions for different cells. Bridges are formed at the bottom of each compartment on which battery plates rest. The spaces between the bridge ribs are provided to collect sediments. This minimizes the danger of short circuits due to the sediment.

The repeated charging and discharging of a battery will, of course, gradually wear it out so that after a time the active material of the positive plates, which consists of extremely fine grains of lead peroxide, gradually disintegrates and loses physical contact with the plate on which it has been held. This loosened material unless held in place, is free to fall off the plate and deposit in sediment spaces between the bridges at the bottom of each cell. By the time the sediment spaces fill up the bottom of the element, the life of the cell is usually spent, since the shedded material will gradually form an electrical path or a short circuit between the positive and negative plates and will interfere with the charging and retention of charge of the battery. The battery may, of course, fail before this condition occurs.

Gasoline and oils should be kept away from bituminous composition containers as they will dissolve the bituminous binders.

ii. Plates

In a battery, several similar plates are properly spaced and welded or lead—burned to a strap to form plate group. The plates consist of perforated grids into which lead or lead peroxide has been pressed. The grids are made of an alloy of lead and antimony, which makes them resistant to electrochemical corrosion, and gives them strength and rigidity. There are two types of plate groups in each cell—positive plate group and negative plate group. The plate group connected to the positive terminal of the cell consists of grids filled with a paste of lead peroxide (brown in color). The plate group connected to the negative terminal on the cell consists of grids filled with metallic lead. It is spongy and dull grey in color. Each group of plates is held together by a post strap, to which each individual plate is welded. These straps extended up through the cell cover to provide the cell terminals to connect one cell to the other. The plate groups are arranged in the cell so that the positive and negative plates alternate.

It is to be noted that die positive plates are filled with lead peroxide active material. This is a dark brown crystalline material which consists of very small grains or particles, disposed so as to provide a high degree of porosity in order to allow the electrolyte to penetrate the plate freely. Negative plates are filled with a porous mass of lead, in spongy form, in which electrolyte can penetrate freely. The active material also contains so called expanders included to prevent the spong lead from contracting and reverting to the solid inactive state during the life of the battery.

iii. Separators

Separators are placed between the negative and positive plates to keep them separate between each other. The separators are designed to hold the plates apart so that they do not touch and same time they must be porous enough to permit liquid to circulate between the plates. Separators are usually made of specially treated wood, hard rubber, resin impregnated fibre alone or in combination with rubber or mats of glass fibres. Some batteries have separators made of polyvinyl chloride or polyethylene saturated cellulose.

Separators have ribs on the side facing the positive plates to provide greater acid volume next to the positive for reasons of improved efficiency and to facilitate acid circulation within the cell. The ribs also minimize the area of contact with the positive plate which has a highly oxidizing effect on most separators. Glass fibre retainer mats or perforated rubber or plastic sheets are sometimes placed between the positive plate and the separator to retard the loss of active material from the plate and to protect the separator from oxidation.

iv. Cell Cover

Each cell is sealed by a cover of hard rubber through which the positive and negative terminals project. Adjacent negative and negative terminals are connected by connector straps. Each cover has an opening through which liquid can be added. A filler cap is screwed on this opening. The filler cap has an air vent for the escape of gas. In many late model batteries one-piece cover is provided that covers all the cells,

Vent plugs of special designs co-operate with the cover vent opening to baffle the gases and electrolyte splashed and sprayed against the underside of the cover, to prevent loss of acid from the cell.

v. Electrolyte

The sponge lead and lead peroxide which fill the respective plates are referred to as the active materials of the battery. But these materials cannot become active until they are covered by an aqueous solution of sulphuric acid called the electrolyte. The sulphuric acid of the electrolyte supplies the sulphate ions which combines with each of the plate materials and releases the electrical energy. The sulphuric acid electrolyte is also the carrier for the electric current inside the battery between the positive and negative plates through the separators. The antimony - lead alloy of the grid framework of the plates carries the electric current to and from the active materials to the outside terminals.

The electrolyte of a fully charged battery contains about 31% sulphuric acid by weight or about 21% by volume in distilled water. This corresponds to a specific gravity of 1.230 at 27C.

The electrolyte used in a lead acid battery is the solution of sulphuric acid. It consists of 40% sulphuric acid and 60% distilled water. The level of the electrolyte in the container is about 10 mm above the tops of the plates. When the electrolyte has been added and the battery is given an initial forming charge, it is ready for operation.

vi. Grids

The plates of a lead-acid storage battery consist of an electrically conducting grid frame work in the meshes of which the active materials are incorporated by electrochemical process. These grids serve to conduct the current to and from the active materials of the positive and negative plates. An alloy consisting essentially of lead and antimony is used for the grids. The antimony stiffens and strengthens the soft lead. The presence of antimony also facilitates casting the fine detail of the wire structure of the grids and enables the battery weight to be kept to a minimum.

vii. Cell Connectors

To connect the cells of a battery, in series, the elements are placed in each cell so that the negative terminal of one cell will be adjacent to the positive terminal of the next cell and so on throughout the battery. Cell connectors are placed over the protruding terminal parts and welded to them to connect the cells in series. Connectors must be heavy enough to carry the high current required for starting without overheating.

viii. Tapered terminals

Battery terminals are of special design, being tapered to specified dimensions in accordance with standards agreed upon by the industry so that all positive and negative cable clamp terminals will fit any corresponding battery terminal interchangeably. The positive terminal is slightly larger (17.5 mm dia.) at the top than the negative terminal (16 mm dia.) at the top so as to minimize the danger of installing a battery in reverse.

ix. Sealing compound

Sealing compounds are used to form an acid tight joint between covers and containers. They are blends of socially processed bituminous substances having resistance to flow at high summer temperatures and resistance to cracking at low winter temperatures. In special constructions rigid plastic resin seals arc sometimes used which are permanent and cannot be removed by heating.

Working

The chemicals used in a battery are as follows:

1. Lead oxide PbO2 on Positive plate

2. Spong lead Pb on Negative plate

3. Sulphuric acid H2SO4 as Electrolyte

The sponge lead and lead peroxide are held in plate grids to form negative and positive plates. Sulphuric acid mixed with water is filled in the container and the plates alongwith the separators are held in it The three substances thus react chemically to produce a flow of current The plate grid consists of a frame work of antimony-lead alloy with interlocking horizontal and vertical bars, which serves to hold the paste in the plate.

The chemical reactions take place between the three chemicals in the battery. In the presence of sulphuric acid, the electrons from one group of plates collect on the other group of plates. This transfer of electrons is continued until there is sufficient imbalance of electrons to create a 2 volts pressure between the two groups of plates. This results in a pressure of 2 volts between the terminals of the battery cell. If the two terminals are connected by a circuit the electrons (current) will flow. They flow from terminal where the chemical reaction has collected them, through the circuit to the other terminal where the chemical reaction has taken them away. The chemical reactions use up the sponge lead, lead peroxide and sulphuric acid. Thus, after a certain amount of current has been withdrawn, the battery is discharged or dead or run-down. When it is discharged, it is not capable of delivering any additional current It may then be recharged.

When the battery is discharging the sulphuric acid ( H2SO4 ) is broken up into two parts - hydrogen (H2) and sulphate (SO4). The hydrogen is librated at the lead oxide (PbO), which combines with parts of sulphuric acid to form lead sulphate (PbS04) and water ( H2O). The sulphate is librated at the spongy lead plates (Pb) and combines sulphate (PbSO4). During this process tire electrolyte becomes absorption of SO4 by the spong lead plates. with them to form lead dilute because of the

When the battery is charged the chemical reaction as described above, is reversed. The lead sulphate (PbSO4), on one plate is again converted to lead peroxide (PbO2), and the lead sulphate on the other plate is reduced to spongy lead (Pb). The electrolyte becomes concentrated because of increased amount of sulphuric acid.

Thus, the battery cell is a means of converting electrical energy into chemical energy during charging and chemical energy into electrical energy during discharging.

Alkaline batteries are of two types, the Edison type or the nickel – iron type, and the nickel – cadmium type. The construction of these is similar except which are different. However, only the latter type is suitable for automobile service.

The basic construction of the alkaline battery is similar to that of lead- acid battery. In both the nickel and the nickel cadmium types, the active material on positive plates is nickel hydroxide. On negative plates, it is metallic iron in nickel iron cell and cadmium oxide in nickel cadmium cells. The electrolyte used is potassium hydroxide solution. The active material for positive as well as negative plate is containing in finally perforated steel tubes, which combine together to form a plate. Due to this reason, even under severe jolts no active material is lost from the plate and thus no sediment is produced.

The electrolyte does not take part in chemical reaction on charging or discharging, unlike in the lead acid batteries. Its specific – gravity remains, therefore, constant at about 1.20. For this reason, smaller quantities of the electrolyte are required. However, due to the same reason, there is no induction of the state of charge of the battery.

ZINC-AIR BATTERY

Zinc–air batteries (non-rechargeable), and zinc–air fuel cells (mechanically rechargeable) are metal-air batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. Sizes range from very small button cells for hearing aids, larger batteries used in film cameras that previously used mercury batteries, to very large batteries used for electric vehicle propulsion.

During discharge, a mass of zinc particles forms a porous anode, which is saturated with an electrolyte. Oxygen from the air reacts at the cathode and forms hydroxyl ions which migrate into the zinc paste and form zincate, releasing electrons to travel to the cathode. The zincate decays into zinc oxide and water returns to the electrolyte. The water and hydroxyl from the anode are recycled at the cathode, so the water is not consumed. The reactions produce a theoretical 1.65 volts, but this is reduced to 1.35–1.4 V in available cells.

Zinc–air batteries have some properties of fuel cells as well as batteries: the zinc is the fuel, the reaction rate can be controlled by varying the air flow, and oxidized zinc/electrolyte paste can be replaced with fresh paste.

The electrolyte is supplied separately packed in an acid proof carton. When the battery is to put in services, open the vent holes, put electrolyte in the battery and closed the vent holes by plugs. After one or two hours the battery is ready for service. When the electrolyte is not put in the battery it may be stored for about 3 years without damage.