Aircraft technical Basics: TM 1-406, Aircraft Electrical Systems, 1940: 1. Aircraft Storage Batteries

TM 1-406, TECHNICAL MANUAL,  AIRCRAFT ELECTRICAL SYSTEMS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON, October 18, 1940.

Section 1. - Aircraft storage batteries

1. General. - a. The purpose of the aircraft storage battery is to provide a source of electrical energy for operating the various electrical units used as aircraft equipment. These units include starters, boosters, lights, radio, heating equipment, electrical instruments, and numerous other electrical devices. An engine-driven generator is usually used in conjunction with the storage battery to maintain it in a charged condition and, in the event of generator failure, acts as a source of energy.

b. The two common types of storage batteries in use are the Edison-cell and the lead-acid types. The Edison-cell storage battery, which is an alkaline type, is not used in aircraft ; therefore, no space will be given to its discussion. The lead-acid type (fig. 1) is the conventional aircraft storage battery.

c. The storage battery consists of a number of individual cells connected in series. This design results in a complete unit which has a terminal voltage equal to the sum of the voltage of the individual cells connected in series, and an ampere-hour capacity equal to that of the individual cell. Batteries may be connected in parallel, series, or series-parallel depending upon the voltage desired and the peak-load that the battery may be called upon to service. At the present time the batteries utilized in aircraft are 12 and 24 volts, and when more than one battery is used they are connected in parallel.

Figure 1.-Typical aircraft storage battery.

d. The capacity of the battery is given in ampere-hours. As the term indicates, an ampere is furnished to or taken from a cell when an ampere flows through the cell for an hour. The capacity discharge rate of a battery or a cell is usually based upon a 5- or 8-hour discharge ; for example, a battery of 96 ampere-hours' capacity could furnish 12 amperes for 8 hours. Discharging the battery in less than 8-hour periods will cause the battery to give less than its rated capacity, and a longer period of discharge will cause the battery to give somewhat more than its rated capacity. The total capacity of the battery divided by eight or five is often called the normal current rate of the battery for an 8-hour or a 5-hour discharge period.

e. The voltage of a battery cell depends upon the character of the metals or metallic compounds forming the elements or plates, and on the density or concentration of the electrolyte in which the plates are immersed. The rate of current flow at which a battery may be discharged depends upon the area of plate surface acted upon by the electrolyte. The active area in any battery is equal to the sum of the individual positive plate areas multiplied by two, because both sides of each individual positive plate are active.

f. The internal resistance of an individual cell in a charged state is small and for practical purposes may be considered negligible. However, consideration is given total internal resistance in a storage battery made up of individual cells and is due to a number of factors. These factors are the resistance of the electrolyte, the resistance of the active material which varies during charge and discharge, and the resistance of the grids and terminals. The total internal resistance of a storage battery increases toward the end of discharge to more than double its resistance when fully charged. There is a considerable change in the density and resistance of the electrolyte when the battery is in charged and discharged state. As the battery is discharged the resistance of the electrolyte increases. The change in resistance taking place at the plates of a lead-acid battery is largely due to lead sulphate; that is, during discharge the lead and peroxide particles become more densely covered with a layer of nonconducting lead sulphate, and if this sulphate is allowed to accumulate, the internal resistance of the battery will rise to a very high value.

g. The efficiency of a storage battery is usually expressed as the ampere-hour or watt-hour efficiency. In some cases the term "voltage efficiency" is also used. The ampere-hour efficiency is the ratio of the ampere-hours of output to the ampere-hours of input. The voltage efficiency is the ratio of the average voltage of discharge to the average voltage of charge. The watt-hour efficiency is the ampere-hour efficiency multiplied by the voltage efficiency. The efficiency of the battery will always be less than 100 percent, because there is always some energy lost due to the internal resistance of the plates and electrolyte.

2. Construction.-a. The lead-acid type storage battery is composed of

(1) Cells or jars.

(2) Plates (negative and positive).

(3) Separators.

(4) Electrolyte.

(5) Cell covers and vent plugs.

(6) Cell connectors.

(7) Terminals (negative and positive).

b. The jars or cell containers of separate cells are of hard-rubber compound. The bottom of the cells contains ribs or bridges upon which the plates rest. These ribs form pockets or spaces into which sediment, gradually thrown off the plates in service, settles. This prevents "shorting" of the plates as long as the rib channels remain unfilled.

e. In any cell there must be two elements of unlike materials or substances immersed in an electrolyte. In the lead-acid cell, the negative plate is pure sponge lead (Pb), and the positive plate, lead peroxide (PbO₂). Both the lead peroxide and sponge lead are slightly soluble in sulphuric acid and are called the active materials. As the lead peroxide and sponge lead are relatively poor conductors of electricity, and are not sufficiently hard to be made into durable plates, it is necessary to attach the active materials to frames or grids of some harder material which is a good conductor of electricity. More-over, the material of this grid must have the property of not acting as a third plate, otherwise the grid would produce local action between the grid and sponge lead or between the grid and lead peroxide whenever it came in contact with them. The material usually chosen for the grids is an alloy of lead and antimony. This alloy is structurally strong and produces no local action in the presence of the other substances contained in the cell. There are two methods of attaching the active materials to the grids. These two methods are known as the Plante and the Faure types. At present the Faure method is used in the construction of aircraft storage battery cells.

d. (1) The negative and positive plates of a cell are assembled into groups, with a positive plate alternately placed between two negative plates. The total number of plates is always an odd number, as each positive plate surface must be adjacent to a negative surface. To prevent the plates from touching, separators of wood or rubber materials are used.

(2) The space between adjacent positive and negative plates is reduced to a minimum in order to lower the internal resistance of the cell. Adjacent plates are prevented from coming in contact with each other by separators. These separators are made of either wood, rubber, or a composition and are ribbed on one side. The ribbed side is placed against the positive plate to permit free circulation of the electrolyte and provide a passage for sediment to settle to the bottom of the cell. Wooden separators are made of especially treated wood. Rubber separators are made of hard rubber and perforated with small holes to permit free circulation of the electrolyte. They are not normally used alone but in conjunction with wooden separators. The rubber separators are placed adjacent to the positive plates thus increasing the life of the wooden separators; the action of oxygen in the positive plate active material deteriorates wood. Modern air-craft storage batteries use the composition separator exclusively. The composition is much superior to the other types in efficiency and life. The separators do not prevent the passage of current through the electrolyte as it flows from plate to plate.

e. The process whereby chemical compounds are decomposed by an electric current is called "electrolysis." The liquid which undergoes this decomposition in the storage battery is known as the electrolyte. In the aircraft storage battery, the electrolyte is a dilute solution of sulphuric acid and pure distilled water in such proportion that the solution satisfies the required specific gravity of approximately 1.300. It has been found that by reducing the specific gravity of the electrolyte for batteries used in tropical climates that the service life is considerably increased. The battery manufacturers furnish instructions for reducing the specific gravity on all batteries shipped direct to activities located in tropical climates.

(1) Sulphuric acid has a powerful affinity for water and readily absorbs it in either a liquid or vapor form. This acid should always be poured into water and not the reverse, because of the extremely high temperatures encountered as the acid combines with the water. The sulphuric acid used in an electrolyte must be free of or have a minimum of any impurities as required by specification. Sulphuric acid will attack all metals except gold and platinum. Needless to say, it will burn clothing and the skin. The use of water and the application of bicarbonate of soda will neutralize the acid in the event it accidently comes into contact with the skin. Any oil or grease introduced into the electrolyte will cause a foaming or frothing. Acid should never be added to an electrolyte, but should first be combined with water in the proper proportion and then the prepared electrolyte added to the cell.

(2) Due to the impurities found in well, tap, rain, and surface water, distilled water is the only pure water which is satisfactory for storage battery electrolyte.

f. The cover of the storage battery cell is constructed of hard rubber and is sealed with a special sealing compound (fig. 2). The cover provides for an expansion space above the electrolyte level, and also permits the installation of a nonspill vent plug assembly and terminal posts. The vent plugs are arranged so as to provide openings for testing or adjusting the strength of the electrolyte and for the escape of collected gas. The holes may also be used for inspection of the interior of the cell. Aircraft batteries are so designed that the electrolyte will not spill out of the battery in any position the aircraft might assume during flight. In early types of aircraft batteries, provisions were made for sufficient space above the plate baffles and a long vent plug the opening of which could not be covered by the electrolyte in any position of the battery (fig. 3).

FIGURE 2. Construction of an aircraft storage battery cell.

FIGURE 3. Early type nonspill design.

The present type of nonspill vent plug is illustrated in figure 4. This type is actuated by gravity as the airplane assumes various positions in flight and is very efficient. It simplifies considerably the size and shape of the storage battery.

3. Operating principles.-a. When the cell of a lead-acid storage battery has been completely charged, the active material of the positive plate (lead peroxide) and that of the negative plate (spongy lead) form the basis of a simple voltaic cell; that is, two conducting substances are immersed in a liquid which attacks one substance more freely than it does the other substance. When the circuit is closed (fig. 5), the liquid (electrolyte) attacks the negative plate (Pb) producing lead sulphate (PbSO₄). Hydrogen (H₂) released from the sulphuric acid (H₂SO₄) at the positive plate (PbO₂) is converted into water at the expense of the oxygen in the peroxide, therefore the peroxide is the depolarizer of the cell.

FIGURE 4.-Modern nonspill vent plug design

FIGURE 5.-Cell discharging.

(1) When the peroxide has thus been deoxidized, the remaining lead in the positive plate is attacked by the electrolyte, producing lead sulphate, and the action ceases. In practice, however, the cell is recharged before this limit is reached. The reaction which takes place may be written thus :

(2) During discharge the acid (H₂SO₄) is withdrawn from the electrolyte and goes into combination with the plates, and the water (H₂O) is released. Therefore, as the electromotive force (e. m. f.) of the cell decreases, the resistance of the electrolyte in-creases and the specific gravity of the electrolyte decreases.

b. The reactions on charge (fig. 6) are the reverse of those shown on discharge. Assume that the positive and negative plates have become lead sulphate (PbSO₄) and the electric generator is sending current through the cell. The water (H₂O) of the electrolyte is de-composed into hydrogen (H₂) and oxygen (0). The hydrogen re-moves the (SO₄) from both plates, forming sulphuric acid (H₂SO₄), and the oxygen released at the positive plate reconverts the lead sulphate (PbSO₄) into lead peroxide (PbO₂). The reaction which takes place may be written thus :

As a result of this reaction, the e. m. f. of the cell rises, the resistance of the electrolyte decreases and the specific gravity of the electrolyte increases.

4. Charging methods.-a. A battery can be charged only with direct current. If alternating current only is available, it must be converted into direct current by the use of a rectifier or motor generator set.

b. There are two systems in general use for charging batteries: the constant current system and the constant potential system.

(1) The constant current system of charging (employing a rectifier) utilizes a current that is maintained constant (at a normal rate) by adjusting the rectifier rheostat., which increases or decreases the resistance as the battery charge progresses. The value of the current flowing through the battery on charge is dependent on the difference between its voltage and the voltage of the charging equipment. As the charge progresses and the battery voltage gradually increases, the charging equipment voltage must be increased to maintain a constant value of charging current. The initial charging rate is roughly one-tenth of the ampere-hour capacity of the battery and is maintained until all cells are gassing freely, after which the rate is reduced to a low value (designated as the finishing rate) until the battery reaches its full charge. An objectional feature of this system is that all batteries in the circuit receive the same charge regardless of their capacity or state of charge.

FIGURE 6.-Cell charging.

(2) The constant potential system of charging (employing a motor generator set) utilizes a charging voltage approximately 2 percent greater than the open circuit voltage of a fully charged battery. In this system the charging voltage is maintained at a constant fixed value (fig. 7). When the battery is first placed on charge, the low internal voltage of the battery and the low resistance of the circuit permit a high flow of charging current, often reaching 30 to 50 amperes on a completely discharged battery. As the internal voltage of the battery increases, the flow of charging current decreases until the battery becomes completely charged, at which point the internal voltage of the battery very nearly equals that of the charging source. The constant potential system of charging does not require manual control of the charging current, provided the voltage regulator is properly adjusted. The charging equipment in aircraft (generator and control box) is of the constant potential type; however, the resistance of the electrical wiring limits the flow of current. Under such conditions the charging system is termed "modified constant potential." Since the constant potential system of charging automatically tapers off the input current as the battery becomes charged, batteries of a group being charged in the same circuit are connected in parallel, and each battery receives, independent of the others, a charge corresponding to its internal voltage.

FIGURE 7.-Battery charging curve, constant potential system.

c. (1) Batteries undergoing charge should be inspected at intervals of from 1/2 to 1 hour, and a temperature and gravity reading taken of one cell of each battery. The temperature of the battery, obtained by a thermometer reading of the electrolyte, should never exceed 110° F. and should be kept below that point by reducing the charging rate, if necessary. Also, when excessive gassing is found, the charging rate should be reduced. Batteries are permitted to remain on charge until the specific gravity approaches normal (1.275 to 1.300), and three successive 30-minute interval inspections reveal no increase in gravity reading.

(2) Batteries of different ampere-hour capacities are not to be charged in the same circuit, if a constant current system of charging is used. Those of like capacity will be connected together in series and each group connected to a separate charger. In connecting batteries to the charging equipment, like terminals of the battery and charging equipment will always be connected together (positive to positive and negative to negative).

5. Testing methods.-a. Storage batteries should not be allowed to fall below a certain state of discharge; therefore, periodical tests are required to check their condition. These are known as the specific gravity or hydrometer test, the high rate discharge test, and the cadmium test.

b. The specific gravity test is accomplished by the use of a hydrometer (fig. 8). The hydrometer consists of a syringe containing a small sealed glass bulb weighted with a quantity of shot and having a graduated scale on its upper end. This scale is marked from 1.100 to 1.300 with various intermediate graduations. In measuring the specific gravity of the electrolyte, the battery vent cap is removed, the. rubber bulb squeezed, and the rubber tubing inserted in the hole of the cell cover. The pressure on the bulb is then released and the electrolyte is drawn into the glass syringe tube. When a sufficient amount of the electrolyte has entered the tube the hydrometer bulb will float, and the reading will be the number showing on the stem at the surface of the electrolyte in the tube (fig. 9). These readings should not be taken immediately after adding distilled water. The chart (fig. 10) indicates the state of charge of a battery at various specific gravity readings. A gravity reading

Below 1.150 indicates a completely discharged battery.

Between 1.150 and 1.200 indicates a half charged battery.

Between 1.275 and 1.300 indicates a fully charged battery.

In cold weather it is especially important not to allow the specific gravity of the electrolyte to fall below a certain figure as there is danger of freezing. The temperatures at which electrolyte will freeze are as follows:

e. The high rate discharge test is performed to determine the exact condition of a storage battery, particularly when the hydrometer reading may give a false indication, such as in the case when a hydrometer test is made after adding water to the electrolyte.

FIGURE 8.-Specific gravity hydrometer.

(1) A conventional high rate discharge tester (fig. 11) consists of a handle carrying two heavy prongs which are bridged by a length of heavy michrome conductor of approximately 100-ampere capacity. Each prong is connected to a low reading voltmeter which is usually mounted conveniently and securely to the tester. By locating the zero reading in the center of the voltmeter scale and a range of 2.5 volts on both sides of the zero mark, no precaution need be taken in positioning the tester on a positive and negative terminal post.

FIGURE 9.-Reading specific gravity of a cell.

(2) When the ends of the tester prongs are pressed down on the terminal posts of a cell, a current of 100 amperes is discharged across the michrome conductor and a voltage reading taken while this discharge current is flowing. This condition assimilates the current drawn by some part of the electrical equipment. Not less than 15 seconds nor more than 20 seconds should be consumed in the testing of each cell, and as only two or three percent of the battery capacity is consumed by the test, it is not usually necessary to recharge after making the test on a fully charged battery. When the hydrometer readings of a battery are all below 1.200 and the high rate discharge reading below 1.6 volts, the battery needs recharging. If the voltage reading of the cells differs 0.10 volt or more and the battery is fairly well charged, there is evidence of trouble in the cell or cells having the lower readings. With a discharged battery, the difference in cell voltage will be greater, depending on the extent of discharge ; however, a cell which reads more than 0.10 volt less than the remaining cells in a battery is generally defective.

FIGURE 10.-Specific gravity chart.

FIGURE 11.-High rate discharge tester.

(3) After a battery has been given a bench charge and the electrolyte adjusted for correct reading, no cell should show a voltage less than 1.75 volts, and the voltage should remain fairly constant during the 15-second test. If every cell in the battery reads less than 1.75 volts, the battery has not been fully charged.

FIGURE 12.-Cadmium tester.

d. Another method of testing the condition of a storage battery cell is by the use of a cadmium tester (fig. 12). It consists of a low reading voltmeter and two brass prods which are connected to a flexible wire. Fastened at right angles to one of the prods is a rod of pure cadmium which is connected to the negative terminal of the voltmeter. The purpose of the cadmium test is to determine the condition of the individual plate groups while the battery is on charge or discharge. When making the cadmium test, insert the cadmium prod through the opening in the top of the cell after removing the plug, care being exercised that the cadmium touches nothing but the electrolyte. It should be allowed to remain in the electrolyte for a short time before making the reading in order that local chemical action, which may take place at the cadmium rod, will make no change in its condition during the test. When making a test on the positive plates of the cell, the other test prod should be connected to the positive terminal of the cell (fig. 13). When making a test on the negative plates, the cadmium rod is inserted through the filler plug as before, and the other prod connected to the negative terminal of the cell (fig. 14).

FIGURE 13.-Cadmium testing positive plates.

(1) When testing a charged battery with a cadmium tester, start with the positive plates while the battery is on the finish charging rate. These plates should test in the positive direction on the volt-meter from 2.3 to 2.45 volts. If for any reason the voltage on the positive plates should test below these figures, it will indicate that the plates are defective. The negative plates should test as far below the zero line as possible, usually 0.1 to 0.2 volt. If the reading is near the zero line or above it, it indicates that the negative plates are defective, usually due to sulphation. When testing a discharged battery, it should be discharging at a constant fixed rate. A test on the positive plates of each cell should give a positive reading on the voltmeter scale of approximately 2.05 volts. The test on the negative plates should give a positive reading on the voltmeter at 0.25 volt. If both sets of plates give practically a zero reading, it will indicate an internal short circuit.

FIGURE 14.-Cadmium testing negative plates.

(2) When taking the readings, the sum or the difference of the two cadmium readings, as the case may be, should be equal to the cell voltage; however, in actual practice there may be a small difference. To know whether to add or to subtract the readings to get cell voltage, the following rules should be applied:

(a) When both readings on the voltmeter deflect in the same direction (as when making test on discharge), the voltage of the cell is equal to their difference.

(b) When the readings deflect in the opposite direction, the cell voltage is equal to their sum.

(3) Examples in the use of a cadmium prod for obtaining readings are as follows:

(a) During discharge, both deflections on the voltmeter are in the same direction :

Cadmium negative, +0.25. Cadmium positive, +2.05.

Cell voltage, 2.05—0.25=1.8 volts.

(b) During charge, both deflections are in the opposite direction :

Cadmium negative, -0.1. Cadmium positive, +2.45.

Cell voltage, 2.45+0.1=2.55 volts.

(4) A voltmeter especially designed to make cadmium tests is used. Figure 15 shows the face of the voltmeter and the four positions on the scale. There are four lines marked on the face, "negative charged," "negative discharged," "positive charged," and "positive discharged." When testing the positive plates of a battery on charge. the pointer of the instrument will move to the line marked "positive charged" if the plates are fully charged. When testing the negative plates. the pointer will move to the line marked "negative charged." Also when testing the plates of a battery in discharged condition, when the positive plates are tested the pointer will move to the line marked "positive discharged," and when testing the negative plates the pointer will move to the line marked "negative discharged."

FIGURE 15.--Cadmium tester voltmeter.

6. Battery deterioration.-The principal forms of deterioration to which a battery is subjected are loss of capacity ; corrosion of plates; fracture or buckling; shedding of active material; sulphation; and internal discharge.

a. Loss of capacity, as distinguished from loss of discharge, may arise from clogging of pores of the lead sponge with sulphates or impurities; contraction of the pores of the mass; loss of active material from the grid ; formation of a layer of sulphate between the grid and the active material ; or loss of electrolyte.

b. Corrosion of plates may arise from two causes: the chemical action resulting from electrolyte decomposition of highly diluted acid in pores of the active material; or the presence of lead-dissolving acids or their salts in the electrolyte.

e. Fracture or buckling is due to excessive or unequal expansion and indicates excessive charging or discharging.

d. Shedding of active material results when the material disintegrates or loosens from the grid, and is caused by expansion or contraction which the grid cannot follow; by the rapid release of gases when charging is accomplished at high rates; or by overcharging of the plates.

e. Sulphation of the injurious kind differs from the normal sulphation of discharge in that it is irreducible and causes shedding of the active material plate, buckling, loss of capacity, and increase of internal resistance with consequent reductions of efficiency. It also causes a rapid increase of temperature with passing current. The causes of oversulphation are overcharging or rapid discharge, either of the entire mass of active material or only certain portions of it. The injurious effects are those which arise from the great increase in resistance and excessive expansion and contraction.

f. Internal discharge which takes place under certain conditions between the active material and the grid or portions of the active material, and metallic impurities on the same plate which may be at different potentials, is termed "local action."

7. Maintenance.-a. Owing to the relatively light duty and low capacities of aircraft type batteries as compared to heavy duty commercial types, the useful life of the former is considerably less than can be expected of batteries with heavier plates. Therefore, service repair operations will be confined to operations associated with service maintenance and testing.

b. (1) When batteries are installed in containers, the terminals are opposite the cable outlet holes. The cables leading to the battery terminals are checked to see that they are protected from acid, where they cross the top of the battery and pass through the container, by rubber tubing of a size that will allow the container cover to squeeze down on the rubber and seal the outlet. When a battery is installed in a container and does not fit snugly. wooden blocks are inserted between the battery and container. In no case, however, are the wooden blocks forcibly driven into place. Check the battery container drain tube and the rubber sponge of the battery tray, if installed, for condition.

(2) Check the rubber tubing gas vents on integrally shielded batteries installed in aircraft and note the condition of the metal tubing used for protection over the rubber tubes, especially at all bends where there is danger of the rubber tubes becoming pinched. Also check the outlet to see that the slipstream has not whipped and damaged the rubber tube.

(3) All parts of the airplane or engine within a 10-inch radius of each side and top of the battery and all parts immediately below on which electrolyte is likely to spill (except where shielded batteries are used) are inspected to see that they are well protected with asphalt varnish.

(4) Check the battery terminals and remove all corrosion. To remove corrosion from the battery terminals, leave the vent plugs tightly in place, disconnect the battery leads and brush the terminals with a wire brush, after which apply baking soda mixed to the consistency of a thin cream to the corroded parts. This neutralizing solution must not be permitted to enter any of the cells. Fresh applications should be repeated until all bubbling action ceases, which indicates that all the acid has been neutralized by the soda solution. Wash the parts and top of the battery with water. When dry, brush the terminals (except on shielded batteries) and apply a light coat of vaseline to the terminals only. Corrosion can be minimized by keeping the connections tight. Where wing nuts are used, the battery leads are secured by using one plain washer and one lock washer. The proper procedure is first to place the battery lead terminal over the battery post, then install the lock washer and the plain washer in the order named, and securely tighten the wing nut.

c. Test procedure is as follows :

(1) When testing the specific gravity of a storage battery remove the vent caps and test each cell with a battery hydrometer. Be sure to make this test before adding distilled water. If the battery is drained, return the electrolyte to the cell from which it is removed. If the specific gravity is 1.200 or below and all cells are approximately the same, a normal discharged condition is indicated and the battery should be replaced. Add distilled water, when necessary. as the electrolyte level must be maintained from 1/8 inch to 3/8 inch above the plates. In case of batteries having a baffle installed just above the plates, the level of the electrolyte will be maintained at a level with the baffle.

(2) A high rate discharge test is required when a battery does not hold its charge under normal use or does not appear to function properly; the voltage of the cells is taken when the battery is discharging. Connect the ends of the discharge tester to the positive and negative posts of a battery cell; this causes the cell to discharge. If the voltage of any cell is found to be lower than the others, it is evidence of a defective cell, and battery should be replaced. When using the high rate discharge tester, not less than 15 seconds nor more than 20 seconds should be consumed in testing each cell. When the hydrometer readings of a battery are all below 1.200 and the high rate discharge voltage of the cells is below 1.7 volts, the battery needs recharging.