AUTOMOBILE ELECTRICAL SYSTEMS 3. AUTOMOBILE BATTERY
With the exception of some 2-wheelers, all vehicles are provided with a battery for the various electrical systems and for engine cranking; hence the battery is a very important part of the vehicle. Generally a 12-volt lead-acid battery is used in cars. These days modern batteries are maintenance free and very reliable and don’t require much maintenance.
3.0
Objectives After going through this lesson, you will be able to:
i. ii. iii. iv.
3.1
Understand the construction of a lead-acid battery. Know about the electrolyte used in the battery and how its specific gravity changes with temperature. Understand how battery life is affected Know the differences between SMF batteries and conventional lead-acid batteries
Introduction
In this lesson, we will study the construction of the re-chargeable storage type lead-acid battery used in motor vehicles. We will also learn about the electrolyte used in the battery and factors affecting battery life so as to take steps and get the maximum life out of a battery. Finally we will study how modern day sealed maintenance free (SMF) batteries differ from the conventional lead-acid batteries.
3.2
Construction of the Battery
The battery used in an automobile is a lead-acid battery, which is a storage battery of the re-chargeable type. When the energy stored in the battery is used up, it can be restored by providing it with a supply of direct current and this procedure is known as re-charging. The battery is contained in a hard rubber or plastic container and has two tapered terminals projecting out from the top of the container – the positive terminal and the negative terminal. The positive terminal has a ‘+’ or ‘P’ sign on it, a red ring around it and is bigger in size. The negative terminal, on the other hand, has a ‘-‘or ‘N’ sign on it, a black ring around it and is smaller in size. Always connect the correct cable clamp to the terminals of the battery.
Each terminal is connected to a number of plates connected together called the positive plate group and the negative plate group respectively. The total number of plates in a battery is always an odd number, for example, a battery may have seven, nine, eleven or more odd number of plates per cell. The number of negative plates is always one more than the number of positive plates. A 12-volt battery consists of six cells and each cell has a positive plate group and the negative plate group. The positive plates are separated from the negative plates by insulating separators. The group of plates and separators are together called an element. Thus there is one element per cell. The cells are covered by cell covers containing holes, which are closed by vent plugs. These holes are required for topping up with distilled water as and when required and inserting the hydrometer in the electrolyte for taking specific gravity readings. Sealing compound is used to form an acid-tight joint between the covers and container.
Fig. 3.1: (a) Lead acid battery. (b) Odd number of plates.
Each cell is connected by connectors in series i.e. the positive plate group of one cell to negative plate group of the next cell so that finally we have a positive and negative terminal free for external connections. The plates and the separators are immersed in the electrolyte, which is a solution of specific concentration of sulphuric acid in distilled water or de-mineralised water.
Fig. 3.2: (a) Cell voltage is 2 volts. (b) Terminal voltage is 12 volts.
Each cell of the lead acid battery provides a potential difference of a little above 2 volts, which is a characteristic of the chemicals used in the lead acid battery. Since there are six cells in the battery, the terminal voltage of the battery is nearly 12 volts. Even though each lead-acid battery has a potential difference of 12 volts across the positive and the negative terminal, the capacity of the battery varies with its size. A bigger battery with more number of plates, or plates of larger size, and one that contains more electrolyte has a greater capacity. The battery used in the Maruti car has a capacity of 30 ampere-hours at the 20-hour rating, which means that the battery can provide a total of 30 ampere-hours, that is, a current of 30 amperes can be drawn for one hour or a current of 60 amperes can be drawn for half an hour and so on.
Self-check Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
How is energy stored in the battery restored? How would you recognize the positive terminal? How would you recognize the negative terminal? The total number of plates in a battery is always an ______ number. The number of negative plates is always ____ more than the number of positive plates. A 12-volt battery consists of _______ cells Each cell of a battery is connected in __________ One cell of a lead acid battery provides a potential difference of a little above ___ volts The capacity of a battery depends on its _______ A bigger battery with __________ number of plates, or plates of _________ size, and one that contains _________ electrolyte has a greater capacity. The battery used in the Maruti car has a capacity of ______________.
3.2.1 Electrolyte Electrolyte is a highly acidic solution. It is a solution of sulphuric acid with distilled water in a specific concentration. The specific gravity of the electrolyte is a definite value for a given design of battery and it varies with the state of charge on the battery. For example, the specific gravity of the electrolyte used in the battery of the Maruti 800 car is 1.280 at 20째 C when fully charged. When the battery is half charged, its specific gravity falls to 1.220 and when nearly discharged, the specific gravity of the battery drops down to 1.150. Hence, observation of the specific gravity of the battery electrolyte with a hydrometer gives an indication about the state of charge of the battery. Specific gravity of pure distilled water is 1.000. The specific gravity of electrolyte is measured by hydrometer. The hydrometer consists of a glass barrel and bulb syringe for sucking a sample of electrolyte to float an enclosed glass hydrometer calibrated to read in terms of specific gravity. The
hydrometer scale read from 1.100 to 1.350 from top to bottom. After filling it up with the required quantity of electrolyte to float the enclosed glass hydrometer and placing it at eye level, we note the reading at the level of the liquid. This gives us how much heavier is the electrolyte as compared to pure water.
Fig.3.3: Battery hydrometer.
The reading observed by the hydrometer is correct at 27OC. At a higher temperature the reading observed is less because the electrolyte expands and its density decreases so that the electrolyte rises up in the hydrometer and lower value is read. Similarly, at a lower temperature the reading observed is greater. Therefore temperature correction is to be done to get the correct reading. This is done as under: i. Add 0.0007 to the observed hydrometer reading for each OC above 27 OC, and ii. Subtract 0.0007 from the observed hydrometer reading for OC each below 27 OC. 3.2.2 Battery Specification Battery is generally specified by voltage, its rated capacity and number of plates. These specifications are given by the car manufacturers for the particular model and can very from small cars to bigger cars. Therefore we should always use the specified battery as originally provided while replacing the battery. For example a Maruti-800 model car battery has the following specifications: Voltage Rated capacity No of Plates
-
12 V 30 Ah/20 HR 7
Self-check Questions 12. The electrolyte is a solution of specific concentration of ______________ in __________________. 13. The specific gravity of the electrolyte varies with the state of charge on the battery. State whether True/False
14. The specific gravity of the electrolyte used in the battery of the Maruti 800 car is _____________ when fully charged. 15. The specific gravity of electrolyte is measured by ______________. 16. The hydrometer tells us how much heavier the electrolyte is as compared to pure water. State whether True/False 17. The reading observed by the hydrometer is correct at ______. 18. Give electrolyte temperature corrections 19. Maruti-800 model car battery has __ plates per cell 20. Capacity of a battery is given in __________
3.3
Factors Affecting Battery Life
3.3.1 Overcharging Charging a battery greatly in excess of what is required is harmful in several ways, as follows: •
Decomposes water of electrolyte into hydrogen and oxygen gas. Gas bubbles tend to wash active material from the plates and carry moisture and acid from the cells as a fine mist.
•
Decomposition of water leaves acid more concentrated. Concentrated acid is harmful to separators and negative plate material at high temperatures over a prolonged period of time, charring the separators and making the negative material sandy and granular.
•
High internal heat is created, which accelerates corrosion of positive plate grids and damages separators and negatives. Also, containers may be softened and distorted and sealing compound displaced.
•
Overcharging alone or in combination with a previous condition of undercharging may cause severe buckling and warping of positive plates with accompanying perforation of separators.
•
May cause damage by corrosion to cradle, cables and other vital electrical and engine parts by forcing liquid from the cells if charge rates are excessive.
3.3.2 Undercharging •
A battery operated with insufficient charge over a long period of time may develop a type of sulphate in the plates which is dense, hard and crystalline and which cannot be electrochemically converted to normal active material again. Such lead sulphate being less dense than the active material from which it was formed will set up strains in the positive plates so that distortion or bowing of the plates, called buckling, may result. Buckling will be produced especially if the sulphated battery is subjected to sudden prolonged overcharging, as might be
experienced on a long trip or by a generator-regulator system which has got out of adjustment. Severely buckled plates will pinch the separators at the plate corners or chafe the center of the separators. This may result in perforations of an unprotected separator and develop a short circuit of the cell. •
A battery operated in an undercharged condition is not only unable to deliver full power but is liable to freeze during severe winter weather. See section 3.3.7 on Freezing of Electrolyte.
•
Lead sulphate formed on the plates during discharge is relatively insoluble as long as the specific gravity or electrolyte is kept above 1.125; but, if allowed to drop much below this value the lead sulphate becomes increasingly soluble and aided by temperature fluctuations of the electrolyte, may migrate over a considerable period of time into the pores of the separators and deposit as a white crystalline mass. Subsequent charging may convert these crystalline deposits to filamentous metallic lead, which may ‘short’ the positive and negative plates through the areas of the separators affected. These small shorts may cause a condition of low cell voltage when the battery is charged. For this reason automotive battery cells should not be allowed to stand idle in a more than 75% discharged condition for very long periods of time.
3.3.3 Lack of Water Water is one of the four essential chemicals of a lead-acid storage battery and under normal conditions of operation is the only component of the battery which is lost as the result of charging. It should be replaced as soon as the liquid level falls to the top of the separators. If water is not replaced, and the plates are soon exposed, the acid will reach a dangerously high concentration that may char and disintegrate the separators and may permanently sulphate and impair the performance of the plates. Plates cannot take full part in the battery action unless they are completely covered by the electrolyte. Sulphuric acid need never be added to a cell unless it has been lost due to spillage. 3.3.4 Loose Hold-downs Hold-downs if not properly adjusted may allow the battery to bounce around in the cradle. This may cause the bridges on which the elements rest to notch the bottom of the separators and may cause the plates to notch the bridge tops, causing a severe disarrangement of the element. The bouncing of the battery may also crack or wear the container badly and cause sealing compound to open up and leak acid. Leaking acid corrodes terminals and cables and makes high resistance battery connections, thereby weakening the battery's power and shortening its life. Hold-downs, on the other hand, can be too tight, distort or crack the container and loosen sealing compound, allowing loss of acid from the cells, particularly the end cells, and this may cause loss of battery capacity.
3.3.5 Battery ‘Dopes’ No satisfactory substitute electrolyte has been found for the simple mixture of sulphuric acid in water. Use no substitutes. to do so may injure the battery and void the manufacturer's guarantee. 3.3.6 Excessive Loads A battery should never be used to propel the car by the use of the starting motor with clutch engaged except in a great emergency. This may produce extremely high internal battery temperature and damage the starting motor. 3.3.7 Freezing of Electrolyte The electrolyte of a battery in various stages of charge will start to freeze at temperatures indicated below. The given temperatures indicate the approximate points at which the first ice crystals begin to appear in the solution. The solution does not freeze solid until a lower temperature is reached. Solid freezing of the electrolyte may crack the container and damage the positive plates. At ¾ charged automotive battery is in no danger of damage from freezing. Therefore, keep batteries better than ¾ charged, especially during winter weather. Specific gravity (corrected to 27OC) 1.280 1.230 1.200 1.150 1.100
3.4
Freezing temperatures - 71o C -40 o C -26.5o C -15 o C -7o C
Sealed Maintenance Free (SMF) Batteries
A new type of battery, called ‘maintenance-free’, is seen on cars now. It can easily be identified by its lack of filler caps; the top of the battery is a solid cover. These batteries are designed to operate without periodic additions of water throughout their normal service life. Since the addition of water is the only maintenance normally performed on batteries, the elimination of this service results in a truly maintenance free battery. Such batteries are still classed as lead-acid batteries and still function in the same manner as the conventional variety. The only difference is in the lead alloy used to make the plates. Conventional batteries use a lead antimony alloy for plate construction because antimony increases the strength and casting qualities of lead. Pure lead, by itself, is not a suitable plate material. Although antimony adds the necessary mechanical properties to lead, it also affects the electrical properties of the battery. It tends to increase a battery's normal
self-discharge rate and also to lower its ‘gassing’ potential. Self-discharge is what causes an otherwise normal battery to gradually run down if left without charging for long periods, especially in temperatures over 15.5o C. Normally, this is not a problem for batteries in regular service, but can be for new batteries left in stock or for vehicles left unattended for long periods. Eliminating antimony minimizes this internal self-discharge. ‘Gassing’ is what happens to the electrolyte when a battery is under normal charge. This is what causes the water in the electrolyte to gradually ‘boil away’. Actually, the water does not boil but is broken down by the charging voltage into its two elements, hydrogen and oxygen. By eliminating antimony from the plates, the voltage at which water breaks down, or ‘gasses’, is raised. Therefore, at normal charging voltages, the original water in the electrolyte of a maintenance-free battery can last several years or more before it finally boils away. In place of antimony, the lead in a maintenance-free battery is alloyed with calcium to give it the necessary manufacturing characteristics. Calcium, however, is a more difficult material to process and results in higher battery costs. But the result is a battery with a very low self-discharge rate and a greatly reduced tendency to boil off its water. Although such batteries do not require servicing as do conventional ones, there are times when they must be tested to see if they are still serviceable.
Self-check Questions 21. Both overcharging and undercharging are harmful to battery life. State whether True/False. 22. As the electrolyte specific gravity drops, its freezing temperature ____________. 23. How will you identify ‘maintenance-free’ batteries? 24. During the charging process, water is broken into its two elements, hydrogen and oxygen hence electrolyte level comes down. State whether True/False
3.5
Assignments
3.5.1 Class Assignment 1. 2. 3. 4. 5.
Discuss Discuss Discuss Discuss Discuss
why cells are connected in series and not in parallel. how terminal voltage is affected by the No. of cells. the ill-effects of overcharging. the ill-effects of undercharging. how SMF batteries differ from conventional lead-acid batteries.
3.5.2 Home Assignment 1. Write the construction of a battery.
3.6
Summing Up
In this lesson we studied the construction of the lead acid battery and how battery plates are arranged in each cell and connected to the external terminals. We also understood the importance of the electrolyte and how its specific gravity changes with the state of charge. We also understood why the hydrometer gives an incorrect reading at temperatures other than 27OC and how to correct the same. Next we studied the factors affecting battery life and finally got to know about the maintenance free battery. Now in next lesson we will study about handling and care of the battery.
3.7
Possible Answers to Self-check Questions
1. it can be restored by providing it with a supply of direct current 2. The positive terminal has a ‘+’ or ‘P’ sign on it, a red ring around it and is bigger in size. 3. The negative terminal, on the other hand, has a ‘-‘or ‘N’ sign on it, a black ring around it and is smaller in size. 4. odd 5. one 6. six 7. series 8. 2 9. size 10. more, larger, more 11. 30 ampere-hours. 12. sulphuric acid, distilled water 13. True 14. 1.280 at 20OC 15. hydrometer 16. True 17. 27OC 18. Add 0.0007 to the observed hydrometer reading for each OC above 27 OC, or subtract 0.0007 from the observed hydrometer reading for OC each below 27 OC. 19. 7 20. amp-hours 21. True 22. increases 23. by absence of filler caps 24. True
3.8
Terminal Questions
1. Describe the construction of a lead-acid battery. 2. Write a note on battery electrolyte. 3. Explain in detail the effect of overcharging and undercharging a battery.
3.9
References
1. Battery Service Manual, 3rd Ed., 1961. Standard Batteries Ltd, Bombay.
3.10 Suggested Further Reading 1. Kohli, P.L., 1987. Automotive Electrical Equipment. First Reprint. Tata McGrawHill Publishing Co. Ltd, New Delhi. 2. Crouse, W. H., 1966. Automotive Electrical Equipment. McGraw-Hill Book Co., New York.
3.11 Glossary Ampere-hours Battery
Unit of capacity of battery; it is the product of No. of amperes and time in hours for which the current can be drawn. An electrochemical device for storing energy in chemical form so that it can be released as electricity; a group of cells connected together.
Capacity of the battery
Ability of the battery to deliver electric current, which depends upon no. of plates, size and thickness of plates, and the quantity of electrolyte.
Cell
An element (group of negative and positive plates, and separators) immersed in electrolyte and capable of producing voltage.
De-mineralised water
Water which does not contain minerals also called battery water and sometimes distilled water.
Direct current
Electric current that flows in one direction only.
Distilled water
Water which is heated to vapour form and cooled i.e. condensed to water again. It does not contain any mineral.
Dope
Any substitute for battery electrolyte.
Electrolyte/battery
A mixture of sulphuric acid and distilled water used as a
acid
charge carrier.
Element
A group of negative and positive plates, and separators assembled in a cell. Rotary motion given to the crankshaft of an IC engine by self-starter or by any other method for starting the engine is termed as engine cranking.
Engine cranking
Gassing potential
The voltage at which water of the electrolyte breaks into hydrogen and oxygen gas during charging or discharging.
Hydrometer
An instrument used to measure the specific gravity of a liquid. A battery hydrometer is calibrated to read the specific gravity of battery electrolyte.
Insulating separator
A material that does not permit electrons to flow through it and is used to separate positive and negative plates in the cell of a battery.
Lead-acid battery
A chemical device that produces a voltage from two dissimilar materials submerged in an electrolyte.
Positive terminal
The terminal is projected out of the battery container to which external circuit is connected The positive terminal is connected to the group of positive plates in the battery container.
Potential difference
Voltage drop across two points.
Re-chargeable storage type leadacid battery
The re-chargeable lead-acid battery is a storage type battery. Once the energy stored in the battery is used up, it can be re-charged with a supply of direct current.
Sealing compound
Bituminous material used to seal the cell cover on the container so that electrolyte does not splash out.
Self-discharge
A lead-acid battery discharges by itself if kept unused for long.
SMF battery
Sealed Maintenance Free battery – in this the gassing potential is high and self-discharge is low so that water loss is minimal and hence topping up is required infrequently, so it is called maintenance free.
Specific gravity
The ratio of the density of a liquid divided by the density of water i.e. 1.
Temperature correction
Correction of electrolyte specific gravity reading taken by hydrometer for temperature since electrolyte being a liquid, its density changes with increase or decrease in its temperature whereas that of hydrometer float does not change much.