Theoretically, the discharge may be continued until the voltage drops to zero, but practically, the discharge should be stopped when the voltage of each cell has dropped to 1.7 (on low discharge rates). If the discharge is carried on beyond this point much of the spongy lead and lead peroxide have either been changed into lead sulphate, or have been covered up by the sulphate so effectively that they are almost useless. Plates in this condition require a very long charge in order to remove all the sulphate.
The limiting value of 1.7 volts per cell applies to a continuous discharge at a moderate rate. At a very high current flowing for only a very short time, it is not only safe, but advisable to allow a battery to discharge to a lower voltage, the increased drop being due to the rapid dilution of the acid in the plates.
The cell voltage will rise somewhat every time the discharge is stopped. This is due to the diffusion of the acid from the main body of electrolyte into the plates, resulting in an increased concentration in the plates. If the discharge has been continuous, especially if at a high rate, this rise in voltage will bring the cell up to its normal voltage very quickly on account of the more rapid diffusion of acid which will then take place.
The voltage does not depend upon the area of the plate surface but upon the nature of the active materials and the electrolyte. Hence, although the plates of a cell are gradually being covered with sulphate, the voltage, measured when no current is flowing, will fall slowly and not in proportion to the amount of energy taken out of the cell. It is not until the plates are pretty thoroughly covered with sulphate, thus making it difficult for the acid to reach the active material, that the voltage begins to drop rapidly. This is shown clearly in Fig. 22, which shows that the cell voltage has dropped only a very small amount when the cell is 50% discharged. With current flowing through the cell, however, the increased internal resistance causes a marked drop in the voltage. Open circuit voltage is not useful, therefore to determine how much energy has been taken from the battery.
Acid Density. The electrolyte of a lead storage battery is a mixture of chemically pure sulphuric acid, and chemically pure water, the acid forming about 30 per cent of the volume of electrolyte when the battery is fully charged. The pure acid has a "specific gravity" of 1.835, that is, it is 1.835 times as heavy as an equal volume of water. The mixture of acid and water has a specific gravity of about 1.300. As the cell discharges, acid is abstracted from the electrolyte, and the weight of the latter must therefore grow less, since there will be less acid in it. The change in the weight, or specific gravity of the electrolyte is the best means of determining the state of discharge of a cell, provided that the cell has been used properly. In order that the value of the specific gravity may be used as an indication of the amount of energy in a battery, the history of the battery must be known. Suppose, for instance, that in refilling the battery to replace the water lost by the natural evaporation which occurs in the use of a battery, acid, or a mixture of acid and water has been used. This will result in the specific gravity being too high, and the amount of energy in the battery will be less than that indicated by the specific gravity. Again, if pure water is used to replace electrolyte which has been spilled, the specific gravity will be lower than it should be. In a battery which has been discharged to such an extent that much of the active material has been covered by a layer of tough sulphate, or if a considerable amount of sulphate and active material has been loosened from the plates and has dropped to the bottom of the cells, it will be impossible to bring the specific gravity of the electrolyte up to 1.300, even though a long charge is given. There must, therefore, be a reasonable degree of certainty that a battery has been properly handled if the specific gravity readings are to be taken as a true indication of the condition of a battery. Where a battery does not give satisfactory service even though the specific gravity readings are satisfactory, the latter are not reliable as indicating the amount of charge in the battery.
As long as a discharge current is flowing from the battery, the acid within the plates is used up and becomes very much diluted. Diffusion between the surrounding electrolyte and the acid in the plates keeps up the supply needed in the plates in order to, carry on the chemical changes. When the discharge is first begun, the diffusion of acid into the plates takes place rapidly because there is little sulphate clogging the pores in the active material, and because there is a greater difference between the concentration of acid in the electrolyte and in the plates than will exist as the discharge progresses. As the sulphate begins to form and fill up the pores of the plates, and as more and more acid is abstracted from the electrolyte, diffusion takes place more slowly.
If a battery is allowed to stand idle for a short time after a partial discharge, the specific gravity of the electrolyte will decrease because some, of the acid in the electrolyte will gradually flow into the pores of the plates to replace the acid used up while the battery was discharging. Theoretically the discharge can be continued until all the acid has been used up, and the electrolyte is composed of pure water. Experience has shown, however, that the discharge of the battery should not be continued after the specific gravity of the electrolyte has fallen to 1.150. As far as the electrolyte is concerned, the discharge may be carried farther with safety. The plates determine the point at which the discharge should be stopped. When the specific gravity has dropped from 1.300 to 1.150, so much sulphate has been formed that it fills the pores in the active material on the plates. Fig. 23 shows the change in the density of the acid during discharge.
[Fig. 23: Variation of Capacity with Specific Gravity]
Changes at the Negative Plate. Chemically, the action at the negative plate consists only of the formation of lead sulphate from the spongy lead. The lead sulphate is only slightly soluble in the electrolyte and is precipitated as soon as it is formed, leaving hydrogen ions, which then go to the lead peroxide plate to form water with oxygen ions released at the peroxide plate. The sulphate forms more quickly on the surface of the plate than in the inner portions because there is a constant supply of acid available at the surface, whereas the formation of sulphate in the interior of the plate requires that acid diffuse into the pores of the active materials to replace that already used up in the formation of sulphate. In the negative plate, however, the sulphate tends to form more uniformly throughout the mass of the lead, because the spongy lead is more porous than the lead peroxide, and because the acid is not diluted by the formation of water as in the positive plate.
Changes at the Positive Plate. In a fully charged positive plate we have lead peroxide as the active material. This is composed of lead and oxygen. From this fact it is plainly evident that during discharge there is a greater chemical activity at this plate than at the negative plate, since we must find something to combine with the oxygen in order that the lead may form lead sulphate with the acid. In an ideal cell, therefore, the material which undergoes the greater change should be more porous than the material which does not involve as great a chemical reaction. In reality, however, the peroxide is not as porous as the spongy lead, and does not hold together as well.
The final products of the discharge of a positive plate are lead sulphate and water. The lead peroxide must first be reduced to lead, which then combines with the sulphate from the acid to form lead sulphate, while the oxygen from the peroxide combines with the hydrogen of the acid to form water. There is, therefore, a greater activity at this plate than at the lead plate, and the formation of the water dilutes the acid in and around the plate so that the tendency is for the chemical actions to be retarded.
The sulphate which forms on discharge causes the active material to bulge out because it occupies more space than the peroxide. This causes the lead peroxide at the surface to begin falling, to the bottom of the jar in fine dust-like particles, since the peroxide here holds together very poorly.
CHAPTER 6. WHAT TAKES PLACE DURING CHARGE. ———————————————-
Voltage. Starting with a battery which has been discharged until its voltage has decreased to 1.7 per cell, we pass a current through it and cause the voltage to rise steadily. Fig. 24 shows the changes in voltage during charge. Ordinarily the voltage begins to rise immediately and uniformly. If, however, the battery has been left in a discharged condition for some time, or has been "over discharged," the voltage rises very rapidly for a fraction of the first minute of charge and then drops rapidly to the normal value and thereafter begins to rise steadily to the end of the charge. This rise at the beginning of the charge is due to the fact that the density of the acid in the pores of the plates rises rapidly at first, the acid thus formed being prevented from diffusing into the surrounding electrolyte by the coating of sulphate. As soon as this sulphate is broken through, diffusion takes place and the voltage drops.
[Fig. 24 Graph: voltage changes during charge]
As shown in Fig. 24, the voltage remains almost constant between the points M and N. At N the voltage begins to rise because the charging chemical reactions are taking place farther and farther in the inside parts of the plate, and the concentrated acid formed by the chemical actions in the plates is diffusing into the main electrolyte. This increases the battery voltage and requires a higher charging voltage.
At the point marked 0, the voltage begins to rise very rapidly. This is due to the fact that the amount of lead sulphate in the plates is decreasing very rapidly, allowing the battery voltage to rise and thus increasing the charging voltage. Bubbles of gas are now rising through the electrolyte.
At P, the last portions of lead sulphate are removed, acid is no longer being formed, and hydrogen and oxygen gas are formed rapidly. The gas forces the last of the concentrated acid out of the plates and in fact, equalizes the acid concentration throughout the whole cell. Thus no further changes can take place, and the voltage becomes constant at R at a voltage of 2.5 to 2.7.
Density of Electrolyte. Discharge should be stopped when the density of the electrolyte, as measured with a hydrometer, is 1.150. When we pass a charging current through the battery, acid is produced by the chemical actions which take place in the plates. This gradually diffuses with the main electrolyte and causes the hydrometer to show a higher density than before. This increase in density continues steadily until the battery begins to "gas" freely.
The progress of the charge is generally determined by the density of the electrolyte. For this purpose in automobile batteries, a hydrometer is placed in a glass syringe having a short length of rubber tubing at one end, and a large rubber bulb at the other. The rubber tube is inserted in the cell and enough electrolyte drawn up into the syringe to float the hydrometer so as to be able to obtain a reading. This subject will be treated more fully in a later chapter.
Changes at Negative Plate. The charging current changes lead sulphate into spongy lead, and acid is formed. The acid is mixed with the diluted electrolyte outside of the plates. As the charging proceeds the active material shrinks or contracts, and the weight of the plate actually decreases on account of the difference between the weight and volume of the lead sulphate and spongy lead. If the cell has had only a normal discharge and the charge is begun soon after the discharge ended, the charge will proceed quickly and without an excessive rise in temperature. If, however, the cell has been discharged too far, or has been in a discharged condition for some time, the lead sulphate will not be in a finely divided state as it should be, but will be hard and tough and will have formed an insulating coating over the active material, causing the charging voltage to be high, and the charge will proceed slowly. When most of the lead sulphate has been reduced to spongy lead, the charging current will be greater than is needed to carry on the chemical actions, and will simply decompose the water into hydrogen and oxygen, and the cell "gasses." Spongy lead is rather tough and coherent, it, and the bubbles of gas which form in the pores of the negative plate near the end of the charge force their way to the surface without dislodging any of the active material.
Changes at the Positive Plate. When a cell has been discharged, a portion of the lead peroxide has been changed to lead sulphate, which has lodged in the pores of the active material and on its surface. During charge, the lead combines with oxygen from the water to form lead peroxide, and acid is formed. This acid diffuses into the electrolyte as fast as the amount of sulphate will permit. If the discharge has been carried so far that a considerable amount of sulphate has formed in the pores and on the surface of the plate, the action proceeds very slowly, and unless a moderate charging current is used, gassing begins before the charge is complete, simply because the sulphate cannot absorb the current. The gas bubbles which originate in the interior of the plate force their way to the surface, and in so doing cause numerous fine particles of active material to break off and fall to the bottom of the jar. This happens because the lead peroxide is a granular, non-coherent substance, with the particles held together very loosely, and the gas breaks off a considerable amount of active material.
CHAPTER 7. CAPACITY OF STORAGE BATTERIES. ———————————————
The capacity of a storage battery is the product of the current drawn from a battery, multiplied by the number of hours this current flows. The unit in which capacity is measured is the ampere-hour. Theoretically, a battery has a capacity of 40 ampere hours if it furnishes ten amperes for four hours, and if it is unable, at the end of that time, to furnish any more current. If we drew only five amperes from this battery, it should be able to furnish this current for eight hours. Thus, theoretically, the capacity of a battery should be the same, no matter what current is taken from it. That is, the current in amperes, multiplied by the number of hours the battery, furnished this current should be constant.
In practice, however, we do not discharge a battery to a lower voltage than 1.7 per cell, except when the rate of discharge is high, such as is the case when using the starting motor, on account of the increasing amount of sulphate and the difficulty with which this is subsequently removed and changed into lead and lead peroxide. The capacity of a storage battery is therefore measured by the number of ampere hours it can furnish before its voltage drops below 1.7 per cell. This definition assumes that the discharge is a continuous one, that we start with a fully charged battery and discharge it continuously until its voltage drops to 1.7 per cell.
The factors upon which the capacity of storage batteries depend may be grouped in two main classifications:
1. Design and Construction of Battery 2. Conditions of Operation
Design and Construction.
Each classification may be subdivided. Under the Design and Construction we have:
(a) Area of plate surface. (b) Quantity, arrangement, and porosity of active materials. (c) Quantity and strength of electrolyte. (d) Circulation of electrolyte.
These sub-classifications require further explanation. Taking them in order:
(a) Area of Plate Surface. It is evident that the chemical and electrical activity of a battery are greatest at the surface of the plates since the acid and active material are in intimate contact here, and a supply of fresh acid is more readily available to replace that which is depleted as the battery is discharged. This is especially true with high rates of discharge, such as are caused in starting automobile engines. Therefore, the capacity of a battery will be greater if the surface area of its plates is increased. With large plate areas a greater amount of acid and active materials is available, and an increase in capacity results.
(b) Quantity, Arrangement, and Porosity of Active Materials. Since the lead and lead peroxide are changed to lead sulphate on discharge, it is evident that the greater the amount of these materials, the longer can the discharge continue, and hence the greater the capacity.
The arrangement of the active materials is also important, since the acid and active materials must be in contact in order to produce electricity. Consequently the capacity will be greater in a battery, all of whose active materials are in contact with the acid, than in one in which the acid reaches only a portion of the active materials. It is also important that all parts of the plates carry the same amount of current, in order that the active materials may be used evenly. As a result of these considerations, we find that the active materials are supported on grids of lead, that the plates are made thin, and that they have large surface areas. For heavy discharge currents, such as starting motor currents, it is essential that there be large surface areas. Thick plates with smaller surface areas are more suitable for low discharge rates.
Since the inner portions of the active materials must have a plentiful and an easily renewable supply of acid, the active materials must be porous in order that diffusion may be easy and rapid.
(c) Quantity and Strength of Electrolyte. It is important that there be enough electrolyte in order that the acid may not become exhausted while there is still considerable active material left. An insufficient supply of electrolyte makes it impossible to obtain the full capacity from a battery. On the other hand, too much electrolyte, due either to filling the battery too full, or to having the plates in a jar that holds too much electrolyte, results in an increase in capacity up to the limit of the plate capacity. There is a danger present, however, because with an excess of electrolyte the plates will be discharged before the specific gravity of the electrolyte falls to 1.150. This results in over discharge of the battery with its attendant troubles as will be described more fully in a later chapter.
It is a universal custom to consider a battery discharged when the specific gravity of the electrolyte has dropped to 1.150, and that it is fully charged when the specific gravity of the electrolyte has risen to 1.280-1.300. This is true in temperate climates. In tropical countries, which may for this purpose be defined as those countries in which the temperature never falls below the freezing point, the gravity of a fully charged cell is 1.200 to 1.230. The condition of the plates is, however, the true indicator of charged or discharged condition. With the correct amount of electrolyte, its specific gravity is 1.150 when the plates have been discharged as far as it is considered safe, and is 1.280-1.300 when the plates are fully charged. When electrolyte is therefore poured into a battery, it is essential that it contains the proper proportion of acid and water in order that its specific gravity readings be a true indicator of the condition of the plates as to charge or discharge, and hence show accurately how much energy remains in the cell at any time.
A question which may be considered at this point is why in automobile, work a specific gravity of 1.280-1.300 is adopted for the electrolyte of a fully charged cell. There are several reasons. The voltage of a battery increases as the specific gravity goes up. Hence, with a higher density, a higher voltage can be obtained. If the density were increased beyond this point, the acid would attack the lead grids and the separators, and considerable corrosion would result. Another danger of high density is that of sulphation, as explained in a later chapter. Another factor which enters is the resistance of the electrolyte. It is desirable that this be as low as possible. If we should make resistance measurements on various mixtures of acid and water, we should find that with a small percentage of acid, the resistance is high. As the amount of acid is increased, the resistance will grow less up to a certain point. Beyond this point, the resistance will increase again as more acid is added to the mixture. The resistance is lowest when the acid forms 30% of the electrolyte. Thus, if the electrolyte is made too strong, the plates and also the separators will be attacked by the acid, and the resistance of the electrolyte will also increase. The voltage increases as the proportion of acid is increased, but the other factors limit the concentration. If the electrolyte is diluted, its resistance rises, and the amount of acid is insufficient to give much capacity. The density of 1.280-1.300 is therefore a compromise between the various factors mentioned above.
(d) Circulation of Electrolyte. This refers to the passing of electrolyte from one plate to another, and depends upon the ease with which the acid can pass through the pores of the separators. A porous separator allows more energy to be drawn from the battery than a nonporous one.
Considering now the operating conditions, we find several items to be taken into account. The most important are:
(e) Rate of discharge. (f) Temperature.
(e) Rate of Discharge. As mentioned above, the ampere hour rating of a battery is based upon a continuous discharge, starting with a specific gravity of 1.280-1.300, and finishing with 1.150. The end of the discharge is also considered to be reached when the voltage per cell has dropped to 1.7. With moderate rates of discharge the acid is abstracted slowly enough to permit the acid from outside the plates to diffuse into the pores of the plates and keep up the supply needed for the chemical actions. With increased rates of discharge the supply of acid is used up so rapidly that the diffusion is not fast enough to hold up the voltage. This fact is shown clearly by tests made to determine the time required to discharge a 100 Amp. Hr., 6 volt battery to 4.5 volts. With a discharge rate of 25 amperes, it required 160 minutes. With a discharge rate of 75 amperes, it required 34 minutes. From this we see that making the discharge rate three times as great caused the battery to be discharged in one fifth the time. These discharges were continuous, however, and if the battery were allowed to rest, the voltage would soon rise sufficiently, to burn the lamps for a number of hours.
The conditions of operation in automobile work are usually considered severe. In starting the engine, a heavy current is drawn from the battery for a few seconds. The generator starts charging the battery immediately afterward, and the starting energy is soon replaced. As long as the engine runs, there is no load on the battery, as the generator will furnish the current for the lamps, and also send a charge into the battery. If the lamps are not used, the entire generator output is utilized to charge the battery, unless some current is furnished to the ignition system. Overcharge is quite possible.
When the engine is not running, the lamps are the only load on the battery, and there is no charging current. Various drivers have various driving conditions. Some use their starters frequently, and make only short runs. Their batteries run down. Other men use the starter very seldom, and take long tours. Their batteries will be overcharged. The best thing that can be done is to set the generator for an output that will keep the battery charged under average conditions.
From the results of actual tests, it may be said that modem lead-acid batteries are not injured in any way by the high discharge rate used when a starting motor cranks the engine. It is the rapidity with which fresh acid takes the place of that used in the pores of the active materials that affects the capacity of a battery at high rates, and not only limitation in the plates themselves. Low rates of discharge should, in fact, be avoided more than the high rates. Battery capacity is affected by discharge rates, only when the discharge is continuous, and the reduction in capacity caused by the high rates of continuous discharge does not occur if the discharge is an intermittent one, such as is actually the case in automobile work. The tendency now is to design batteries to give their rated capacity in very short discharge periods. If conditions should demand it, these batteries would be sold to give their rated capacity while operating intermittently at a rate which would completely discharge them in three or four minutes. The only change necessary for such high rates of discharge is to provide extra heavy terminals to carry the heavy current.
The present standard method of rating starting and lighting batteries, as recommended by the Society of Automotive Engineers, is as follows:
"Batteries for combined lighting and starting service shall have two ratings. The first shall indicate the lighting ability, and shall be the capacity in ampere hours of the battery when discharged continuously at the 5 hour rate to a final voltage of not less than 1.7 per cell, the temperature of the battery beginning such discharge being 80 deg.F. The second rating shall indicate the starting ability and shall be the capacity in ampere-hours when the battery is discharged continuously at the 20-minute rate to a final voltage of not less than 1.5 per cell, the temperature of the battery beginning such discharge being 80 deg.F."
The discharge rate required under the average starting conditions is higher than that specified above, and would cause the required drop in voltage in about fifteen minutes. In winter, when an engine is cold and stiff, the work required from the battery is even more severe, the discharge rate being equivalent in amperes to probably four or five times the ampere-rating of the battery. On account of the rapid recovery of a battery after a discharge at a very high rate, it seems advisable to allow a battery to discharge to a voltage of 1.0 per cell when cranking an engine which is extremely cold and stiff.
(f) Temperature. Chemical reactions take place much more readily at high temperatures than at low. Furthermore, the active materials are more porous, the electrolyte lighter, and the internal resistance less at higher temperatures. Opposed to this is the fact that at high temperatures, the acid attacks the grids and active materials, and lead sulphate is formed, even though no current is taken from the battery. Other injurious effects are the destructive actions of hot acid on the wooden separators used in most starting and lighting batteries. Greater expansion of active material will also occur, and this expansion is not, in general, uniform over the surface of the plates. This results in unequal strains and the plates are bent out of shape, or "buckled." The expansion of the active material will also cause much of it to fall from the plates, and we then have "shedding."
[Fig. 25 Graph: Theoretical temperature changes during charge and discharge]
When sulphuric acid is poured into water, a marked temperature rise takes place. When a battery is charged, acid is formed, and when this mixes with the diluted electrolyte, a temperature rise occurs. In discharging, acid is taken from the electrolyte, and the temperature has a tendency to drop. On charging, therefore, there is danger of overheating, while on discharge, excessive temperatures are not likely. Fig. 25 shows the theoretical temperature changes on charge and discharge. The decrease in temperature given-in the curve is not actually obtained in practice, because the tendency of the temperature to decrease is balanced by the heat caused by the current passing through the battery.
Age of Battery.
Another factor which should be considered in connection with capacity is the age of the battery. New batteries often do not give their rated capacity when received from the manufacturer. This is due to the methods of making the plates. The "paste" plates, such as are used in automobiles, are made by applying oxides of lead, mixed with a liquid, which generally is dilute sulphuric acid, to the grids. These oxides must be subjected to a charging current in order to produce the spongy lead and lead peroxide. After the charge, they must be discharged, and then again charged. This is necessary because not all of the oxides are changed to active material on one charge, and repeated charges and discharges are required to produce the maximum amount of active materials. Some manufacturers do not charge and discharge a battery a sufficient number of times before sending it out, and after a battery is put into use, its capacity will increase for some time, because more active material is produced during each charge.
Another factor which increases the capacity of a battery after it is put into use is the tendency of the positive active material to become more porous after the battery is put through the cycles of charge and discharge. This results in an increase in capacity for a considerable time after the battery is put into use.
When, a battery has been in use for some time, a considerable portion of the active material will have fallen from the positive plates, and, a decrease in capacity will result. Such a battery will charge faster than a new one because the amount of sulphate which has formed when the battery is discharged is less than in a newer battery. Hence, the time required to reduce this sulphate will be less, and the battery will "come up" faster on charge, although the specific gravity of the electrolyte may not rise to 1.280.
CHAPTER 8. INTERNAL RESISTANCE. ——————————
The resistance offered by a storage battery to the flow of a current through it results in a loss of voltage, and in heating. Its value should be as low as possible, and, in fact, it is almost negligible even I in small batteries, seldom rising above 0.05 ohm. On charge, it causes the charging voltage to be higher and on discharge causes a loss of voltage. Fig. 26 shows the variation in resistance.
[Fig. 26 Graph: Changes in internal resistance during charge and discharge]
The resistance as measured between the terminals of a cell is made up of several factors as follows:
1. Grids. This includes the resistance of the terminals, connecting links, and the framework upon which the active materials are pasted. This is but a small part of the total resistance, and does not undergo any considerable change during charge and discharge. It increases slightly as the temperature of the grids rises.
2. Electrolyte. This refers to the electrolyte between the plates, and varies with the amount of acid and with temperature. As mentioned in the preceding chapter, a mixture of acid and water in which the acid composes thirty per cent of the electrolyte has the minimum resistance. Diluting or increasing the concentration of the electrolyte will both cause an increase in resistance from the minimum I value. The explanation probably lies in the degree to which the acid is split up into "ions" of hydrogen (H), and sulphate (SO4). These "ions" carry the current through t he electrolyte. Starting with a certain amount of acid, let us see how the ionization progresses. With very concentrated acid, ionization does not take place, and hence, there are no ions to carry current. As we mix the acid with water, ionization occurs. The more water used, the more ions, and hence, the less the resistance, because the number of ions available to carry the current increases. The ionization in creases to a certain maximum degree, beyond which no more ions are formed. It is probable that an electrolyte containing thirty per cent of acid is at its maximum degree of ionization and hence its lowest resistance. If more water is now added, no more ions are formed. Furthermore, the number of ions per unit volume of electrolyte will now decrease on account of the increased amount of water. There Will therefore be fewer ions per unit volume to carry the current, and the resistance of the electrolyte increases.
With an electrolyte of a given concentration, an increase of temperature will cause a decrease in resistance. A decrease in temperature will, of course, cause an increase in resistance. It is true, in general, that the resistance of the electrolyte is about half of the total resistance of the cell. The losses due to this resistance generally form only one per cent of the total losses, and area practically negligible factor.
3. Active Material. This includes the resistance of the active materials and the electrolyte in the pores of the active materials. This varies considerably during charge and discharge. It has been found that the resistance of the peroxide plate changes much more than that of the lead plate. The change in resistance of the positive plate is especially marked near the end of a discharge. The composition of the active material, and the contact between it and the grid affect the resistance considerably.
During charge, the current is sent into the cell from an external source. The girds therefore carry most of the current. The active material which first reacts with the acid is that near the surface of the plate, and the acid formed by the charging current mixes readily with the main body of electrolyte. Gradually, the charging action takes place in the inner portions of the plate, and concentrated acid is formed in the pores of the plate. As the sulphate is removed, however, the acid has little difficulty in mixing with the main body of electrolyte. The change in resistance on the charge is therefore not considerable.
During discharge, the chemical action also begins at the surface of the plates and gradually moves inward. In this case, however, sulphate is formed on the surface first, and it becomes increasingly difficult for the fresh acid from the electrolyte to diffuse into the plates so as to replace the acid which has been greatly diluted there by the discharge actions. There is therefore an increase in resistance because of the dilution of the acid at the point of activity. Unless a cell is discharged too far, however, the increase in resistance is small.
If a battery is allowed to stand idle for a long time it gradually discharges itself, as explained in Chapter 10. This is due to the formation of a tough coating of crystallized lead sulphate, which is practically an insulator. These crystals gradually cover and enclose the active material. The percentage change is not high, and generally amounts to a few per cent only. The chief damage caused by the excessive sulphation is therefore not an increase in resistance, but consists chiefly of making a poor contact between active material and grid, and of removing much of the active material from action by covering it.
CHAPTER 9. CARE OF THE BATTERY ON THE CAR. ———————————————-
The manufacturers of Starting and Lighting Equipment have designed their generators, cutouts, and current controlling devices so as to relieve the car owner of as much work as possible in taking care of batteries. The generators on most cars are automatically connected to the battery at the proper time, and also disconnected from it as the engine slows down. The amount of current which the generator delivers to the battery is automatically prevented from exceeding a certain maximum value. Under the average conditions of driving, a battery is kept in a good condition. It is impossible, however, to eliminate entirely the need of attention on the part of the car owner, and battery repairman.
The storage battery requires but little attention, and this is the very reason why many batteries are neglected. Motorists often have the impression that because their work in caring for a battery is quite simple, no harm will result if they give the battery no attention whatever. If the battery fails to turn over the engine when the starting switch is closed, then instruction books are studied. Thereafter more attention is paid to the battery. The rules to be observed in taking care of the battery which is in service on the car are not difficult to observe. It is while on the car that a battery is damaged, and the damage may be prevented by intelligent consideration of the battery's housing and living conditions, just as these conditions are made as good as possible for human beings.
1. Keep the Interior of the Battery Box Clean and Dry. On many cars the battery is contained in an iron box, or under the seat or floorboards. This box must be kept dry, and frequent inspection is necessary to accomplish this. Moisture condenses easily in a metal box, and if not removed will cause the box to become rusty. Pieces of rust may fall on top of the battery and cause corrosion and leakage of current between terminals.
Occasionally, wash the inside of the box with a rag dipped in ammonia, or a solution of baking soda, and then wipe it dry. A good plan is to paint the inside of the box with asphaltum paint. This will prevent rusting, and at the same time will prevent the iron from being attacked by electrolyte which may be spilled, or may leak from the battery.
Some batteries are suspended from the car frame under the floor boards or seat. The iron parts near such batteries should be kept dry and free from rust. If the battery has a roof of sheet iron placed above it, this roof should also be kept clean, dry and coated with asphaltum paint.
[Fig. 27 "Do not drop tools on top of battery"]
2. Put Nothing But the Battery in the Battery Box. If the battery is contained in an iron box, do not put rags, tools, or anything else of a similar nature in the battery box. Do not lay pliers across the top of the battery, as shown in Fig. 27. Such things belong elsewhere. The battery should have a free air space all around it, Fig. 28. Objects made of metal will short-circuit the battery and lead to a repair bill.
3. Keep the battery clean and dry. The top of the battery should be kept free of dirt, dust, and moisture. Dirt may find its way into the cells and damage the battery. A dirty looking battery is an unsightly object, and cleanliness should be maintained for the sake of the appearance of the battery if for no other reason.
Moisture on top of the battery causes a leakage of current between the terminals of the cells and tends to discharge the battery. Wipe off all moisture and occasionally go over the tops of the cell connectors, and terminals with a rag wet with ammonia or a solution of baking soda. This will neutralize any acid which may be present in the moisture.
The terminals should be dried and covered with vaseline. This protects them from being attacked by acid which may be spilled on top of the battery. If a deposit of a grayish or greenish substance is found on the battery terminals, handles or cell connectors, the excess should be scraped off and the parts should then be washed with a hot solution of baking soda (bicarbonate of soda) until all traces of the substance have been removed. In scraping off the deposit, care should be taken not to scrape off any lead from terminals or connectors. After washing the parts, dry them and cover them with vaseline. The grayish or greenish substance found on the terminals, connectors, or handles is the result of "corrosion," or, in other words, the result of the action of the sulphuric acid in the electrolyte upon some metallic substance.
[Fig. 28 Battery installed with air space on all sides]
The acid which causes the corrosion may be spilled on the battery when hydrometer readings are taken. It may also be the result of filling the cells too full, with subsequent expansion and overflowing as the temperature of the electrolyte increases during charge. Loose vent caps may allow electrolyte to be thrown out of the cell by the motion of the car on the road. A poorly sealed battery allows electrolyte to be thrown out through the cracks left between the sealing compound and the jars or posts. The leaks may be caused by the battery cables not having sufficient slack, and pulling on the terminals.
The cap which fits over the vent tube at the center of the top of each cell is pierced by one or more holes through which gases formed within the cell may escape. These holes must be kept open; otherwise the pressure of the gases may blow off the top of the cell. If these holes are found to be clogged with dirt they should be cleaned out thoroughly.
The wooden battery case should also be kept clean and dry. If the battery is suspended from the frame of the car, dirt and mud from the road will gradually cover the case, and this mud should be scraped off frequently. Occasionally wash the case with a rag wet with ammonia, or hot baking soda solution. Keep the case, especially along the top edges, coated with asphaltum or some other acid proof paint.
[Fig. 29 Battery held in place by "hold-down" bolts]
4. The battery must be held down firmly. If the battery is contained in an iron box mounted on the running-board, or in a compartment in the body of the car having a door at the side of the running-board, it is usually fastened in place by long bolts which hook on the handles or the battery case. These bolts, which are known as "hold-downs," generally pass through the running board or compartment, Fig. 29, and are generally fastened in place by nuts. These nuts should be turned up so that the battery is held down tight.
Other methods are also used to hold the battery in place, but whatever the method, it is vital to the battery that it be held down firmly so that the jolting of the car cannot cause it to move. The battery has rubber jars which are brittle, and which are easily broken. Even if a battery is held down firmly, it is jolted about to a considerable extent, and with a loosely fastened battery, the jars are bound to be cracked and broken.
5. The cables connected to the battery must have sufficient slack so that they will not pull on the battery terminals, as this will result in leaks, and possibly a broken cover.
The terminals on a battery should be in such a position that the cables may be connected to them easily, and without bending and twisting them. These cables are heavy and stiff, and once they are bent or twisted they are put under a strain, and exert a great force to straighten themselves. This action causes the cables to pull on the terminals, which become loosened, and cause a leak, or break the cover.
[Fig. 30 Measure height of electrolyte in battery]
6. Inspect the Battery twice every month in Winter, and once a week in Summer, to make sure that the Electrolyte covers the plates. To do this, remove the vent caps and look down through the vent tube. If a light is necessary to determine the level of the electrolyte, use an electric lamp. Never bring an open flame, such as a match or candle near the vent tubes of a battery. Explosive gases are formed when a battery "gasses," and the flame may ignite them, with painful injury to the face and eyes of the observer as a result. Such an explosion may also ruin the battery.
During the normal course of operation of the battery, water from the electrolyte will evaporate. The acid never evaporates. The surface of the electrolyte should be not less than one-half inch above the tops of the plate. A convenient method of measuring the height of the electrolyte is shown in Fig. 30. Insert one end of a short piece of a glass tube, having an opening not less than one-eighth inch diameter, through the filling hole, and allow it to rest on the upper edge of the plates. Then place your finger over the upper end, and withdraw the tube. A column of liquid will remain in the lower end of the tube, as shown in the figure, and the height of this column is the same as the height of the electrolyte above the top of the plates in the cell. If this is less than one-half inch, add enough distilled water to bring the electrolyte up to the proper level. Fig. 31 shows the correct height of electrolyte in an Exide cell.
Never add well water, spring water, water from a stream, or ordinary faucet water. These contain impurities which will damage the battery, if used. It is essential that distilled water be used for this purpose, and it must be handled carefully so as to keep impurities of any kind out of the water. Never use a metal can for handling water or electrolyte for a battery, but always use a glass or porcelain vessel. The water should be stored in glass bottles, and poured into a porcelain or glass pitcher when it is to be used.
[Fig. 31 Correct height of electrolyte in Exide cell]
A convenient method of adding the water to the battery is to draw some up in a hydrometer syringe and add the necessary amount to the cell by inserting the rubber tube which is at the lower end into the vent hole and then squeezing the bulb until the required amount has been put into the cell.
In the summer time it makes no difference when water is added. In the winter time, if the air temperature is below freezing (32 deg. F), start the engine before adding water, and keep it running for about one hour after the battery begins to "gas." A good time to add the water is just before starting on a trip, as the engine will then usually be run long enough to charge the battery, and cause the water to mix thoroughly with the electrolyte. Otherwise, the water, being lighter than the electrolyte, will remain at the top and freeze. Be sure to wipe off water from the battery top after filling. If battery has been wet for sometime, wipe it with a rag dampened with ammonia or baking soda solution to neutralize the acid.
Never add acid to a battery while the battery is on the car. By "acid" is meant a mixture of sulphuric acid and water. The concentrated acid, is of course, never used. The level of the electrolyte falls because of the evaporation of the water which is mixed with the acid in the electrolyte. The acid does not evaporate. It is therefore evident that acid should not be added to a cell to replace the water which has evaporated. Some men believe that a battery may be charged by adding acid. This is not true, however, because a battery can be charged only by passing a current through the battery from an outside source. On the car the generator charges the battery.
It is true that acid is lost, but this is not due to evaporation, but to the loss of some of the electrolyte from the cell, the lost electrolyte, of course, carrying some acid with it. Electrolyte is lost when a cell gasses; electrolyte may be spilled; a cracked jar will allow electrolyte to leak out; if too much water is added, the expansion of the electrolyte when the battery is charging may cause it to run over and be lost, or the jolting of the car may cause some of it to be spilled; if a battery is allowed to become badly sulphated, some of the sulphate is never reduced, or drops to the bottom of the cell, and the acid lost in the formation of the sulphate is not regained.
If acid or electrolyte is added instead of water, when no acid is needed, the electrolyte will become too strong, and sulphated plates will be the result. If a battery under average driving conditions never becomes fully charged, it should be removed from the car and charged from an outside source as explained later. If, after the specific gravity of the electrolyte stops rising, it is not of the correct value, some of the electrolyte should be drawn off and stronger electrolyte added in its place. This should be done only in the repair shop or charging station.
Care must be taken not to add too much water to a cell, Fig. 32. This will subsequently cause the electrolyte to overflow and run over the top of the battery, due to the expansion of the electrolyte as the charging current raises its temperature. The electrolyte which overflows is, of course, lost, taking with it acid which will later be replaced by water as evaporation takes place. The electrolyte will then be too weak. The electrolyte which overflows will rot the wooden battery case, and also tend to cause corrosion at the terminals.
If it is necessary to add water very frequently, the battery is operating at too high a temperature, or else there is a cracked jar. The high temperature may be due to the battery being charged at too high a rate, or to the battery being placed near some hot part of the engine or exhaust pipe. The car manufacturer generally is careful not to place the battery too near any such hot part. The charging rate may be measured by connecting an ammeter in series with the battery and increasing the engine speed until the maximum current is obtained. For a six volt battery this should rarely exceed 14 amperes. If the charging, current does not reach a maximum value and then remain constant, or decrease, but continues to rise as the speed of the engine, is increased, the regulating device is out of order. An excessive charging rate will cause continuous gassing if it is much above normal, and the temperature of the electrolyte will be above 100 deg. F. In this way an excessive charging current may be detected.
[Fig. 32 Cell with level of electrolyte too high]
In hot countries or states, the atmosphere may have such a high temperature that evaporation will be more rapid than in temperate climates, and this may necessitate more frequent addition of water.
If one cell requires a more frequent addition of water than the others, it is probable that the jar of that cell is cracked. Such a cell will also show a low specific gravity, since electrolyte leaks out and is replaced by water. A battery which has a leaky jar will also have a case which is rotted at the bottom and sides. A battery with a leaky jar must, of course, be removed from the car for repairs.
From time to time within the past two years, various solutions which are supposed to give a rundown battery a complete charge within five or ten minutes have been offered to the public. The men selling such "dope" sometimes give a demonstration which at first sight seems to prove their claims. This demonstration consists of holding the starting switch down (with the ignition off) until the battery can no longer turn over the engine. They then pour the electrolyte out of the battery, fill it with their "dope," crank the engine by hand, run it for five minutes, and then get gravity readings of 1.280 or over. The battery will also crank the engine. Such a charge is merely a drug-store charge, and the "dope" is generally composed mainly of high gravity acid, which seemingly puts life into a battery, but in reality causes great damage, and shortens the life of a battery. The starting motor test means nothing. The same demonstration could be given with any battery. The high current drawn by the motor does not discharge the battery, but merely dilutes the electrolyte which is in the plates to such an extent that the voltage drops to a point at which the battery can no longer turn over the starting motor. If any battery were given a five minutes charge after such a test, the diluted electrolyte in the plates would be replaced by fresh acid from the electrolyte and the battery would then easily crank the engine again. The five minutes of running the engine does not put much charge into the battery but gives time for the electrolyte to diffuse into the plates.
Chemical analysis of a number of dope electrolytes has shown that they consist mainly of high gravity acid, and that this acid is not even chemically pure, but contains impurities which would ruin a battery even if the gravity were not too high. The results of some of the analyses are as follows:
No. 1. 1.260 specific gravity sulphuric acid, 25 parts iron, 13.5 parts chlorine, 12.5, per cent sodium sulphate, 1 per cent nitric acid.
No. 2. 1.335 specific gravity sulphuric acid, large amounts of organic matter, part of which consisted of acids which attack lead.
No. 3. 1.340 specific gravity sulphuric acid, 15.5 per cent sodium sulphate.
No. 4. 1.290 specific gravity sulphuric acid, 1.5 per cent sodium sulphate.
No. 5. 1.300 specific gravity sulphuric acid.
If such "dope" electrolytes are added to a discharged battery, the subsequent charging of the battery will add more acid to the electrolyte, the specific gravity of which will then rise much higher than it should, and the plates and separators are soon ruined.
Do not put faith in any "magic" solution which is supposed to work wonders. There is only one way to charge a battery, and that is to send a current through it, and there is only one electrolyte to use, and that is the standard mixture of distilled water and chemically pure sulphuric acid.
7. The specific gravity of the electrolyte should be measured every two weeks and a permanent record of the readings made for future reference.
The specific gravity of the electrolyte is the ratio of its weight to the weight of an equal volume of water. Acid is heavier than water, and hence the heavier the electrolyte, the more acid it, contains, and the more nearly it is fully charged. In automobile batteries, a specific gravity of 1.300-1.280 indicates a fully charged battery. Generally, a gravity of 1.280 is taken to indicate a fully, charged cell, and in this book this will be done. Complete readings are as follows:
1.280-1.200—More than half charged.
1.200-1.150—Less than half charged.
1.150 and less—Completely discharged.
[Fig. 33 and Fig. 34: battery hydrometers]
For determining the specific gravity, a hydrometer is used. This consists of a small sealed glass tube with an air bulb and a quantity of shot at one end, and a graduated scale on the upper end. This scale is marked from 1.100 to 1.300, with various intermediate markings as shown in Fig. 33. If this hydrometer is placed in a liquid, it will sink to a certain depth. In so doing, it will displace a certain volume of the electrolyte, and when it comes to rest, the volume displaced will just be equal to the weight of the hydrometer. It will therefore sink farther in a light liquid than in a heavy one, since it will require a greater volume of the light liquid to equal the weight of the hydrometer. The top mark on the hydrometer scale is therefore 1.100 and the bottom one 1.300. Some hydrometers are not marked with figures that indicate the specific gravity, but are marked with the words "Charged," "Half Charged," "Discharged," or "Full," "Half Full," "Empty," in place of the figures.
The tube must be held in a vertical position, Fig. 35, and the stem of the hydrometer must be vertical. The reading will be the number on the stem at the surface of the electrolyte in the tube, Fig. 36. Thus if the hydrometer sinks in the electrolyte until the electrolyte comes up to the 1.150 mark on the stem, the specific gravity is 1.150.
[Fig. 35 Using hydrometer for reading specific gravity]
For convenience in automobile work, the hydrometer is enclosed in a large tube of glass or other transparent, acid proof material, having a short length of rubber tubing at its lower end, and a large rubber bulb at the upper end. The combination is called a hydrometer-syringe, or simply hydrometer. See Figure 34. In measuring the specific gravity of the electrolyte, the vent cap is removed, the bulb is squeezed (so as to expel the air from it), and the rubber tubing inserted in the hole from which the cap was removed. The pressure on the bulb is now released, and electrolyte is drawn up into the glass tube. The rubber tubing on the hydrometer should not be withdrawn from the cell. When a sufficient amount of electrolyte has entered the tube, the hydrometer will float. In taking a reading, there should be no pressure on the bulb, and the hydrometer should be floating freely and not touching the walls of the tube. The tube must not be so full of electrolyte that the upper end of the hydrometer strikes any part of the bulb.
The tube must be held in a vertical position, Fig. 35, and the stem of the hydrometer must be vertical. The reading will be the number on the stem at the surface of the electrolyte in the tube, Fig. 36. Thus if the hydrometer sinks in the electrolyte until the electrolyte comes up to the 1.150 mark on the stem, the specific gravity is 1.150.
If the battery is located in such a position that it is impossible to hold the hydrometer straight up, the rubber tube may be Pinched shut with the fingers, after a sufficient quantity of electrolyte has been drawn from the cell and the hydrometer then removed and held in a vertical position.
Specific gravity readings should never be taken soon after distilled water has been added to the battery. The water and electrolyte do not mix immediately, and such readings will give misleading results. The battery should be charged several hours before the readings are taken. It is a good plan to take a specific gravity reading before adding any water, since accurate results can also be obtained in this way.
[Fig. 36 Hydrometer reading showing cell charged, half-charged, and discharged]
Having taken a reading, the bulb is squeezed so as to return the electrolyte to the cell.
Care should be taken not to spill the electrolyte from the hydrometer syringe when testing the gravity. Such moisture on top of the cells tends to cause a short circuit between the terminals and to discharge the battery.
In making tests with the hydrometer, the electrolyte should always be returned to the same cell from which it was drawn.
Failure to do this will finally result in an increased proportion of acid in one cell and a deficiency of acid in others.
The specific gravity of all cells of a battery should rise and fall together, as the cells are usually connected in series so that the same current passes through each cell both on charge and discharge.
If one cell of a battery shows a specific gravity which is decidedly lower than that of the other cells in series with it, and if this difference gradually increases, the cell showing the lower gravity has internal trouble. This probably consists of a short circuit, and the battery should be opened for inspection. If the electrolyte in this cell falls faster than that of the other cells, a leaky jar is indicated. The various cells should have specific gravities within fifteen points of each other, such as 1.260 and 1.275.
If the entire battery shows a specific gravity below 1.200, it is not receiving enough charge to replace the energy used in starting the engine and supplying current to the lights, or else there is trouble in the battery. Use starter and lights sparingly until the specific gravity comes up to 1.280-1.300. If the specific gravity is less than 1.150 remove the battery from the car and charge it on the charging bench, as explained later. The troubles which cause low gravity are given on pages 321 and 322.
It is often difficult to determine what charging current should be delivered by the generator. Some generators operate at a constant voltage slightly higher than that of the fully charged battery, and the charging current will change, being higher for a discharged battery than for one that is almost or fully charged. Other generators deliver a constant current which is the same regardless of the battery's condition.
In the constant voltage type of generator, the charging current automatically adjusts itself to the condition of the battery. In the constant current type, the generator current remains constant, and the voltage changes somewhat to keep the current constant. Individual cases often require that another current value be used. In this case, the output of the generator must be changed. With most generators, a current regulating device is used which may be adjusted so as to give a fairly wide range of current, the exact value chosen being the result of a study of driving conditions and of several trials. The charging current should never be made so high that the temperature of the electrolyte in the battery remains above 90 deg. F. A special thermometer is very useful in determining the temperature. See Fig. 37. The thermometer bulb is immersed in the electrolyte above the plates through the filler hole in the tops of the cells.
Batteries used on some of the older cars are divided into two or more sections which are connected in parallel while the engine is running, and in such cases the cables leading to the different sections should all be of exactly the same length, and the contacts in the switch which connect these sections in parallel should all be clean and tight. If cables of unequal length are used, or if some of the switch contacts are loose and dirty, the sections will not receive equal charging currents, because the resistances of the charging circuits will not be equal. The section having the greatest resistance in its circuit will receive the least amount of charge, and will show lower specific gravity readings than for other sections. In a multiple section battery, there is therefore a tendency for the various sections to receive unequal charges, and for one or more sections to run down continually. An ammeter should be attached with the engine running and the battery charging, first to one section and then to each of the others in turn. The ammeter should be inserted and removed from the circuit while the engine remains running and all conditions must be exactly the same; otherwise the comparative results will not give reliable indications. It would be better still to use two ammeters at the same time, one on each section of the battery. In case the amperage of charge should differ by more than 10% between any two sections, the section receiving the low charge rate should be examined for proper height of electrolyte, for the condition of its terminals and its connections at the starting switch, as described. Should a section have suffered considerably from such lack of charge, its voltage will probably have been lowered. With all connections made tight and clean and with the liquid at the proper height in each cell, this section may automatically receive a higher charge until it is brought back to normal. This high charge results from the comparatively low voltage of the section affected.
In case the car is equipped with such a battery, each section must carry its proper fraction of the load and with lamps turned on or other electrical devices in operation the flow from the several sections must be the same for each one. An examination should be made to see that no additional lamps, such as trouble lamps or body lamps, have been attached on one side of the battery, also that the horn and other accessories are so connected that they draw from all sections at once.
Some starting systems have in the past not been designed carefully in this respect, one section of the battery having longer cables attached to it than the others. In such systems it is impossible for these sections to receive as much charging current as others, even though all connections and switches are in good condition. In other systems, all the cells of the battery are in series, and therefore must receive the same charging current, but have lighting wires attached to it at intermediate points, thus dividing the battery into sections for the lighting circuits. If the currents taken by these circuits are not equal, the battery section supplying the heavier current will run down faster than others. Fortunately, multiple section batteries are not being used to any great extent at present, and troubles due to this cause are disappearing.
The temperature of the electrolyte affects the specific gravity, since heat causes the electrolyte to expand. If we take any battery or cell and heat it, the electrolyte will expand and its specific gravity will decrease, although the actual amount of acid is the same. The change in specific gravity amounts to one point, approximately, for every three degrees Fahrenheit. If the electrolyte has a gravity of 1.250 at 70 deg.F, and the temperature is raised to 73 deg.F, the specific gravity of the battery will be 1.249. If the temperature is decreased to 67 deg.F, the specific gravity will be 1.251. Since the change of temperature does not change the actual amount of acid in the electrolyte, the gravity readings as obtained with the hydrometer syringe should be corrected one point for every three degrees change in temperature. Thus 70 deg.F is considered the normal temperature, and one point is added to the electrolyte reading for every three degrees above 70 deg.F. Similarly, one point is subtracted for every three degrees below 70 deg.F. For convenience of the hydrometer user, a special thermometer has been developed by battery makers. This is shown in Fig. 37. It has a special scale mounted beside the regular scale. This scale shows the corrections which must be made when the temperature is not 70 deg.F. Opposite the 70 deg. point on the thermometer is a "0" point on the special scale. This indicates that no correction is to be made. Opposite the 67 deg. point on the regular scale is a -1, indicating that 1 must be subtracted from the hydrometer reading to find what the specific gravity would be if the temperature were 70 deg.F. Opposite the 73 deg. point on the regular scale is a +1, indicating that 1 point must be added to reading on the hydrometer, in order to reduce the reading of specific gravity to a temperature of 70 deg.F.
[Fig. 37 Special thermometer]
8. Storage batteries are strongly affected by changes in temperature. Both extremely high and very low temperatures are to be avoided. At low temperatures the electrolyte grows denser, the porosity of plates and separators decreases, circulation and diffusion of electrolyte are made difficult, chemical actions between plates and acid take place very slowly, and the whole battery becomes sluggish, and acts as if it were numbed with cold. The voltage and capacity of the battery are lowered.
As the battery temperature increases, the density of the electrolyte decreases, the plates and separators become more porous, the internal resistance decreases, circulation and diffusion of electrolyte take place much more quickly, the chemical actions between plates and electrolyte proceed more rapidly, and the battery voltage and capacity increase. A battery therefore works better at high temperatures.
Excessive temperatures, say over 110 deg. F, are, however, more harmful than low temperatures. Evaporation of the water takes place very rapidly, the separators are attacked by the hot acid and are ruined, the active materials and plates expand to such an extent that the active materials break away from the grids and the grids warp and buckle. The active materials themselves are burned and made practically useless. The hot acid also attacks the grids and the sponge lead and forms dense layers of sulphate. Such temperatures are therefore extremely dangerous.
A battery that persistently runs hot, requiring frequent addition of water, is either receiving too much charging current, or has internal trouble. The remedy for excessive charge is to decrease the output of the generator, or to burn the lamps during the day time. Motorists who make long touring trips in which considerable day driving is done, with little use of the starter, experience the most trouble from high temperature. The remedy is either to decrease the charging rate or burn the lamps, even in the day time.
Internal short-circuits cause excessive temperature rise, both on charge and discharge. Such short circuits usually result from buckled plates which break through the separators, or from an excessive amount of sediment. This sediment consists of active material or lead sulphate which has dropped from the positive plate and fallen to the bottom of the battery jar. All battery jars are provided with ridges which keep the plates raised an inch or more from the bottom of the jar, and which form pockets into which the materials drop. See Fig. 10. If these pockets become filled, and the sediment reaches the bottom of the plates, internal short circuits result which cause the battery to run down and cause excessive temperatures.
If the electrolyte is allowed to fall below the tops of the plates, the parts of the plates above the acid become dry, and when the battery is charged grow hot. The parts still covered by the acid also become hot because all the charging current is carried by these parts, and the plate surface is less than before. The water will also become hot and boil away. A battery which is thus "charged while dry" deteriorates rapidly, its life being very short.
If a battery is placed in a hot place on the car, this heat in addition to that caused by charging will soften the plates and jars, and shorten their life considerably.
In the winter, it is especially important not to allow the battery to become discharged, as there is danger of the electrolyte freezing. A fully charged battery will not freeze except at an extremely low temperature. The water expands as it freezes, loosening the active materials, and cracking the grids. As soon as a charging current thaws the battery, the active material is loosened, and drops to the bottom of the jars, with the result that the whole battery may disintegrate. Jars may also be cracked by the expansion of the water when a battery freezes.
To avoid freezing, a battery should therefore be kept charged, The temperatures at which electrolyte of various specific gravities freezes are as follows:
Specific Gravity Freezing Pt. Specific Gravity Freezing Pt. ———————— —————— ———————— —————— 1.000 32 deg. F 1.200 -16 deg. F 1.050 26 deg. F 1.250 -58 deg. F 1.100 18 deg. F 1.280 -92 deg. F 1.150 5 deg. F 1.300 -96 deg. F
9. Care of Storage Battery When Not in Service. A storage battery may be out of service for a considerable period at certain times of the year, for example, when the automobile is put away during the winter months, and during this time it should not be allowed to stand without attention. When the battery is to be out of service for only three or four weeks, it should be kept well filled with distilled water and given as complete a charge as possible the last few days, the car is in service by using the lamps and starting motor very sparingly. The specific gravity of the electrolyte in each cell should be tested, and it should be somewhere between 1.280 and 1.300. All connections to the battery should be removed, as any slight discharge current will in time completely discharge it, and the possibilities of such an occurrence are to be avoided. If the battery is to be put out of service for several months, it should be given a complete charge by operating the generator on the car or by connecting it to an outside charging circuit. During the out-of-service period, water should be added to the cells every six or eight weeks and the battery given what is called a freshening charge; that is, the engine should be run until the cells have been gassing for perhaps one hour, and the battery may then be allowed to stand for another similar period without further attention. Water should be added and the battery fully charged before it is put back into service. It is desirable to have the temperature of the room where the battery is stored fairly constant and as near 70 degrees Fahrenheit as possible.
CHAPTER 10. STORAGE BATTERY TROUBLES. ————————————-
The Storage Battery is a most faithful servant, and if given even a fighting chance, will respond instantly to the demands made upon it. Given reasonable care and consideration, it performs its duties faithfully for many months. When such care is lacking, however, it is soon discovered that the battery is subject to a number of diseases, most of which are "preventable," and all of which, if they do not kill the battery, at least, greatly impair its efficiency.
In discussing these diseases, we may consider the various parts of which a battery is composed, and describe the troubles to which they are subject. Every battery used on an automobile is composed of:
1. Plates 2. Separators 3. Jars in which Plates, Separators, and Electrolyte are placed 4. Wooden case 5. Cell Connectors, and Terminals 6. Electrolyte
Most battery diseases are contagious, and if one part fails, some of the other parts are Affected. These diseases may best be considered in the order in which the parts are given in the foregoing list.
Plates are the "vitals" of a battery, and their troubles affect the life of the battery more seriously than those of the other parts. It is often difficult to diagnose their troubles, and the following descriptions are given to aid in the diagnosis.
1. Over discharge. Some battery men say that a battery is suflphated whenever anything is wrong with it. Sulphation is the formation of lead sulphate on the plates. As a battery of the lead acid type discharges, lead sulphate must form. There can be no discharge of such a battery without the formation of lead sulphate, which is the natural product of the chemical reactions by virtue of which current may be drawn from the battery. This sulphate gradually replaces the lead peroxide of the positive plate, and the spongy lead of the negative plate. When a battery has been discharged until the voltage per cell has fallen to the voltage limits, considerable portions of the lead peroxide and spongy lead remain on the plates. The sulphate which is then present is in a finely divided, porous condition, and can readily be changed back to lead peroxide and spongy lead by charging the battery.
If the discharge is continued after the voltage has fallen to the voltage limits, an excessive amount of sulphate forms. It fills up the pores in the active materials, and covers up much of the active material which remains, so that it is difficult to change the sulphate back to active material. Moreover, the expansion of active material which takes place as the sulphate forms is then so great that it causes the active material to break off from the plate and drop to the bottom of the jar.
2. Allowing a Battery to Stand Idle. When lead sulphate is first formed, it is in a finely divided, porous condition, and the electrolyte soaks through it readily. If a battery which has been discharged is allowed to stand idle without being charged, the lead sulphate crystals grow by the combination of the crystals to form larger crystals. The sulphate, instead of having a very large surface area, upon which the electrolyte may act in changing the sulphate to active material, as it does when it is first formed, now presents only a very small surface to the electrolyte, and it is therefore only with great difficulty that the large crystals of sulphate are changed to active material. The sulphate is a poor conductor, and furthermore, it covers up much of the remaining active material so that the electrolyte cannot reach it.
A charged battery will also become sulphated if allowed to stand idle, because it gradually becomes discharged, even though no wires of any kind are attached to the battery terminals. How this takes place is explained later. The discharge and formation of sulphate continue until the battery is completely discharged. The sulphate then gradually forms larger crystals as explained in the preceding paragraph, until all of the active material is either changed to sulphate, or is covered over by the sulphate so that the electrolyte cannot reach it. The sulphate thus forms a high resistance coating which hinders the passage of charging current through the battery and causes heating on charge. It is for this reason that sulphated plates should be charged at a low rate. The chemical actions which are necessary to change the sulphate to active material can take place but very slowly, and thus only a small current can be absorbed. Forcing a large current through a sulphated battery causes heating since the sulphate does not form uniformly throughout the plate, and the parts which are the least sulphated will carry the charging current, causing them to become heated. The heating damages the plates and separators, and causes buckling, as explained later.
If batteries which have been discharged to the voltage limits are allowed to stand idle without being charged, they will, of course, continue to discharge themselves just as fully charged batteries do when allowed to stand idle.
3. Starvation. If a battery is charged and discharged intermittently, and the discharge is greater than the charge, the battery will never be fully charged, and lead sulphate will always be present. Gradually this sulphate forms the large tough crystals that cover the active material and remove it from action. This action continues until all parts of the plate are covered with the crystalline sulphate and we have the same condition that results when a battery is allowed to stand idle without any charge.
4. Allowing Electrolyte to Fall Below Tops of Plates. If the electrolyte is allowed to fall below the tops of the plates, so that the active materials are exposed to the air, the parts thus exposed will gradually become sulphated. The spongy lead of the negative plate, being in a very finely divided state, offers a very large surface to the oxygen of the air, and is rapidly oxidized, the chemical action causing the active material to become hot. The charging current, in passing through the parts of the plates not covered by the electrolyte also heats the active materials. The electrolyte which occasionally splashes over the exposed parts of the plates and which rises in the pores of the separators, is heated also, and since hot acid attacks the active materials readily, sulphation takes place quickly. The parts above the electrolyte, of course, cannot be charged and sulphate continues to form. Soon the whole exposed parts are sulphated as shown in Fig. 209.
As the level of the electrolyte drops, the electrolyte becomes stronger, because it is only the water which evaporates, the acid remaining and becoming more and more concentrated. The remaining electrolyte and the parts of the plates covered by it become heated by the current, because there is a smaller plate area to carry the current, and because the resistance of the electrolyte increases as it grows more concentrated. Since hot acid attacks the active materials, sulphation also takes place in the parts of the plates still covered by the electrolyte.
The separators in a battery having the electrolyte below the tops of the plates suffer also, as will be explained later. See page 346.
5. Impurities. These are explained later. See page 76.
6. Adding Acid Instead of Water. The sulphuric acid in the electrolyte is a heavy, oily liquid that does not evaporate. It is only the water in the electrolyte which evaporates. Therefore, when the level of the electrolyte falls, only water should be added to bring the electrolyte to the correct height. There are, however, many car owners who still believe that a battery may be charged by adding acid when the level of the electrolyte falls. Batteries in which this is done then contain too much acid. This leads to two troubles. The first is that the readings taken with a hydrometer will then be misleading. A specific gravity of 1.150 is always taken to indicate that a battery is discharged, and a specific gravity of 1.280 that a battery is charged. These two values of specific gravity indicate a discharged and charged condition of the battery ONLY WHEN THE PROPORTION OF ACID IN THE ELECTROLYTE IS CORRECT. It is the condition of the plates, and not the specific gravity of the electrolyte which determines when a battery is either charged or discharged. With the correct proportion of acid in the electrolyte, the specific gravity of the electrolyte is 1.150 when the plates are discharged and 1.280 when the plates are charged, and that is why specific gravity readings are generally used as an indication of the condition of the battery.
If there is too much acid in the electrolyte, the plates will be in a discharged condition before the specific gravity of the electrolyte drops to 1.150, and will not be in a charged condition until after the specific gravity has risen beyond the usual value. As a result of these facts a battery may be over-discharged, and never fully charged, this resulting in the formation of sulphate.
The second trouble caused by adding acid to the electrolyte is that the acid will then be too concentrated and attacks both plates and separators. This will cause the plates to become sulphated, and the separators rotted.
7. Overheating. This was explained in Chapter 9. See page 66.
Buckling is the bending or twisting of plates due to unequal expansion of the different parts of the plate, Figs. 207 and 208. It is natural and unavoidable for plates to expand. As a battery discharges, lead sulphate forms. This sulphate occupies more space than the lead peroxide and spongy lead, and the active materials expand. Heat expands both active materials and grids. As long as all parts of a plate expand equally, no buckling will occur. Unequal expansion, however, causes buckling.
1. Over discharge. If discharge is carried too far, the expansion of the active material on account of the formation of lead sulphate will bend the grids out of shape, and may even break them.
2. Continued Operation with Battery in a Discharged Condition. When a considerable amount of lead sulphate has, formed, and current is still drawn from the battery, those portions of the plate which have the least amount of sulphate will carry most of the current, and will therefore become heated and expand. The parts covered with sulphate will not expand, and the result is that the parts that do expand will twist the plate out of shape. A normal rate of discharge may be sufficient to cause buckling in a sulphated plate.
3. Charging at High Rates. If the charging rate is excessive, the temperature will rise so high that excessive expansion will take place. This is usually unequal in the different parts of the plate, and buckling results. With a battery that has been over discharged, the charging current will be carried by those parts of the plates which are the least sulphated. These parts will therefore expand while others will not, and buckling results.
4. Non-Uniform Distribution of Current Over the Plates. Buckling may occur in a battery which has not been over-discharged, if the current carried by the various parts of the plate is not uniform on account of faulty design, or careless application of the paste. This is a fault of the manufacturers, and not the operating conditions.
5. Defective Grid Alloy. If the metals of which the grids are composed are not uniformly mixed throughout the plate, areas of pure lead may be left here and there, with air holes at various points. The electrolyte enters the air holes, attacks the lead and converts the grid partly into active material. This causes expansion and consequent distortion and buckling.
Buckling will not necessarily cause trouble, and batteries with buckled plates may operate satisfactorily for a long time. If, however, the expansion and twisting has caused much of the active material to break away from the grid, or has loosened the active material from the grids, much of the battery capacity is lost. Another danger is that the lower edges of a plate may press against the separator with sufficient force to cut through it, touch the next plate, and cause a short-circuit.
Shedding, or Loss of Active Material
The result of shedding, provided no other troubles occur, is simply to reduce the capacity of the plates. The positives, of course, suffer more from shedding than the negatives do, shedding being one of the chief weaknesses of the positives. There is no remedy for this condition. When the shedding has taken place to such an extent that the capacity of the battery has fallen very low, new plates should be installed. After a time, the sediment space in the bottom of the jar becomes filled with sediment, which touches the plates. This short-circuits the cell, of course, and new plates must be installed, and the jars washed out thoroughly.
1. Normal Shedding. It is natural and unavoidable for the positives to shed. Lead Peroxide is a powder-like substance, the particles of which do not hold together. A small amount of sulphate will cement the particles together to a considerable extent. At the surface of the plate, however, this sulphate is soon changed to active material, and the peroxide loses its coherence. Particles of peroxide drop from the plates and fall, into the space in the bottom of the jar provided for this purpose.
Bubbles of gas which occur at the end of a charge blow some of the peroxide particles from the plate. The electrolyte moving about as the battery is jolted by the motion of the car washes particles of peroxide from the positive plates. Any slight motion between positive plates and separators rubs some peroxide from the plates. It is therefore entirely natural for shedding to occur, especially at the positives. The spongy lead of the negatives is much more elastic than the peroxide, and hence very little shedding occurs at the negative plates. The shedding at the positives explains why the grooved side of the separator is always placed against the positive plate. The grooves, being vertical, allow the peroxide to fall to the bottom of the jar, where it accumulates as sediment, or "mud."
2. Excessive Charging Rate, or Overcharging. If a battery is charged at too high a rate, only part of the current is used to produce the chemical actions by which the battery is charged. The balance of the current decomposes the water of the electrolyte into hydrogen and oxygen, causing gassing. As the bubbles of gas force their way out of the plates, they blow off particles of the active material.
When a battery is overcharged, the long continued gassing has the same effect as described in the preceding paragraph.
3. Charging Sulphated Plates at too High a Rate. In sulphated plates, the chemical actions which take place as a battery is charged can proceed but very slowly, because the sulphate, besides being a poor conductor, has formed larger crystals which present only a small surface for the electrolyte to act upon, and has also covered up much of the remaining active material. Since the chemical actions take place slowly, the charging current must be kept at a low value. If too heavy a charging current is used, the battery will be overheated, and some of the current will simply cause gassing as explained in No. 2 above. The gas bubbles will break off pieces of the sulphate, which then fall to the bottom of the jars as "mud."
4. Charging Only a Part of the Plate. If the electrolyte falls below the tops of the plates, and the usual charging current is sent into the battery, the current will be too great for the plate area through which it passes, and hence gassing and shedding will result as already explained.
The same condition exists in a battery in which one or more plates have been broken from the strap, either because of mechanical vibration or because of impurities such as acetic acid in improperly treated separators. The remaining plates are called upon to do more work, and carry the entire charging current. Gassing and shedding will result.
5. Freezing. If a battery is given any care whatever, there is little danger of freezing. The electrolyte of a fully charged battery with a specific gravity of 1.280 freezes at about 92 deg. below zero. With a specific gravity of 1.150, the electrolyte freezes at about 5 deg. above zero. A frozen battery therefore indicates gross neglect.
As the electrolyte freezes, the water of the electrolyte expands. Since there is electrolyte in all the inner parts of the plate, the expansion as the water in the paste freezes forces the pastes out of the grids. The expansion also cracks the rubber jars, and sometimes bulges out the ends of the battery case.
Loose Active Material
This refers to a condition in which the active materials are no longer in contact with the grid. Corrosion, or sulphation, of the grids themselves is generally present at the same time, since the chemical actions are shifted from the active material to the grids themselves.
1. Over discharge. As a battery discharges, the lead sulphate which forms causes an expansion of the active material. If a battery is repeatedly over-discharged, this results in the positives shedding. In the negatives, the spongy lead is puffed out, resulting in the condition known as "bulged negatives" as illustrated in Fig 122.
2. Buckling. As a plate grid is bent out of shape, the active material, especially the peroxide, breaks loose from the grid, since the peroxide cannot bend as much as the grids. This occurs in the negatives also, though not to such an extent as in the positives.
If the plates are buckled to such an extent that the element will not go back into the jar, the positives should be discarded. If the positives are buckled, the negatives will be also, but not to the extent that the positives are.
In the case of the positives, there is no remedy, and the plates should be discarded. The negatives, however, may be fully charged, and then straightened, and the active material forced back flush with the grids by pressings, as described in Chapter 15.
Impurities may be divided into two general classes. The first class includes those which do not attack the separators or grids, but merely cause internal self-discharge. The second class includes those which attack the grids or separators.
1. Impurities Which Merely Cause Self-discharge. This includes metals other than lead. If these metals are in solution in the electrolyte, they deposit on the negative plate, during charge, in their ordinary metallic state, and form small cells with the spongy lead. These small cells discharge as soon as the charging circuit is opened, and some of the lead is changed to lead sulphate. This, of course, causes a loss in capacity. Free hydrogen is given off by this local discharge, and so much of it is at times given off that the hydrogen bubbles give the electrolyte a milky appearance.
Silver, gold, and platinum are the most active in forming small local cells. These metals form local cells which have comparatively high voltages, and which take away a considerable portion of the energy of a cell. Platinum is especially active, and a small amount of platinum will prevent a negative plate from taking a charge. Gradually, however, the spongy lead covers up the foreign metal and prevents it from forming local cells.
Iron also forms local cells which rob the cell of a considerable portion of its capacity. This may be brought into the cell by impure acid or water. Iron remains in solution in the electrolyte, and is not precipitated as metallic iron. The iron in solution travels from the positive to the negative plate, and back again, causing a local discharge at each plate. It is, moreover, very difficult to remove the iron, except by pouring out all of the electrolyte. Manganese acts the same as the iron.
2. Impurities Which Attack the Plates. In general, this class includes acids other than sulphuric acid, compounds formed from such acids, or substances which will readily form acids by chemical action in the cell. Nitric acid, hydrochloric or muriatic acid, and acetic acid belong in this class of impurities. Organic matter in a state of decomposition attacks the lead grids readily.
Impurities in the second class dissolve the lead grids, and the plate disintegrates and falls to pieces, since its backbone is destroyed. When a battery which contains these impurities is opened, it will be found that the plates crumble and fall apart at the slightest touch. See Fig. 210.
Separators which have not been treated properly introduce acetic acid into a cell. The acetic acid attacks and rots the lead, especially the lugs projecting above the electrolyte, and the plate connecting straps. The plates will generally be found broken from the connecting strap, with the plate lugs broken and crumbled.
As for remedies, there is not much to be done. Impurities in the first class merely decrease the capacity of the battery. If the battery is fully charged, and the negatives then washed thoroughly, some of the impurities may be removed. Impurities of the second class have generally damaged the plates beyond repairs by the time their presence is suspected.
The best thing to do is to keep impurities out of the battery. This means that only distilled water, which is known to be absolutely free from impurities should be used.
Impurities which exist in the separators or acid cannot be detected readily, but in repairing a battery, separators furnished by one of the reliable battery makers should be used. Pure acid should also be used. This means that only chemically pure, or "C. P." acid, also known as battery acid should be used. In handling the acid in the shop, it should always be kept in its glass bottle, and should be poured only into a glass, porcelain, earthenware, lead, or rubber vessel. Never use a vessel made of any other material.
When the grids of a plate are attacked chemically, they become thin and weak, and may be spoken of as being corroded.
1. Impurities. Those impurities which attack the lead grids, such as acids other than sulphuric acid, compounds formed from these acids, or substances which will readily form acids dissolve some of the lead which composes the grids. The grids gradually become weakened. The decrease in the amount of metal in the grids increases the internal resistance of the cell and give a tendency for temperatures to be higher in the cell. The contact between grids and active material is in time made poor. If the action of the impurities continues for any length of time, the plate becomes very weak, and breaks at the slightest touch.
2. High Temperatures. Anything that raises the temperature of the electrolyte, such as too high a charging rate, causes the acid to attack the grids and form a layer of sulphate on them. The sulphate is changed to active material on charge, and the grids are thereby weakened.