Large Stand-by Battery, 150 Cells, 167 Plates (Fig. 29) each. Capacity 12,400 amp. for one hour. Each cell stands about four feet high.

by means of a motor operated "end cell switch,» so as to vary the voltage at will, or more usually to maintain the voltage constant by throwing in additional cells as the E M F of each falls off during the progress of discharge.

Insulation. In the small portable batteries, such as the auto starting type, insulation of the individual cells is easily accomplished by the rubber jars themselves, which accordingly are placed side by side in a box of suitable size. When, however, a number of such cells, say 40 or more, are connected in series, as in Fig. 36, it is found necessary to divide the series amongst a number of boxes, or "trays," each one preferably containing not more than 8 or 10 cells.

In small stationary batteries a flat glass tray under each cell is often used, as shown in Fig. 25, a little sand being placed in the bottom to give an even seat for the glass jar. Larger stationary cells are usually provided with double insulation of some kind; thus in Fig. 28 the tank rests upon a glass-oil insulator, "B," which in turn is supported by a large inverted stoneware cup, "A."

Characteristics of the Storage Battery.The primary useful quantities furnished by a storage battery are electromotive force, or P.D. (measured in volts), and current (measured in amperes); since the time during which a given current may be maintained is frequently of controlling importance there arises a third quantity called the capacity, the product of the current and the time which the battery can furnish it. Frequently the relation of these three primary attributes to the weight of the battery is a vital factor; while the effects of internal resistance and temperature are scarcely less

important. The characteristics of a battery therefore consist of the relations of these quantities one with another. In the following discussion the unit considered is in every case a single cell.

In speaking of the discharge of a battery, the term discharge rate is very frequently used, commonly expressed in terms of the time during which the discharge can be maintained; the four-hour rate for instance being that rate which the battery can hold for four hours. The so-called "normal rate," originally that for which the battery was intended, is actually of but little significance, since the modern battery may be discharged at almost any rate without injury.

The capacity is limited by the fall of voltage to a point where usefulness ceases, this point being again arbitrary, but through large experience fairly well defined as about 1.60-1.80 for the lead cell, about 0.6-1.0 for the Edison type.

Many variations exist in the design of modern storage batteries, and each design possesses its own characteristics; the curves which follow are chosen as fairly representative, but of course cannot pretend to fit all cases.

Characteristics of the Edison Cell.Capacity-Temperature.- The capacity of a given Edison cell is very nearly a constant quantity independent of the rate of discharge, amounting under ordinary conditions to about 11.5 ampere hours per pound. The capacity is, however, very markedly dependent upon temperature, to an extent which varies with the discharge rate. This variation with temperature is so great that there results a critical point, below which the Edison cell becomes practically inoperative; and since this point is from 30-50° F., depending on conditions, it constitutes one of the chief limitations to the usefulness of this form of battery. If given a chance to discharge rapidly, when slightly below the critical temperature, the battery will gradually warm itself; but for immediate action at low temperatures it is unworkable.

Voltage-"P.D."- Fig. 31 shows a typical voltage curve of an Edison cell during its "normal" or five-hour rate of discharge, and during the corresponding charge. At lower rates of discharge the voltage is higher, while at higher rates it becomes lower. Fig. 32 summarizes a number of discharge curves by giving the initial, the mean and the final voltage at rates up to six times the normal. It is noticeable that the voltage falls off very rapidly with increasing discharge rates and that the maximum current obtainable is only about 14 times the normal, while the maximum watt output is reached at about seven times the normal rate.

It is of interest to notice that the mean voltage of the Edison cell is about 60 per cent that of the lead type and that the percentage drop during discharge is about triple with the Edison. It is thus necessary to employ at least 65 per cent more cells of Edison type for a given discharge voltage; and still more than this if the discharge rate be high.

Efficiency.- Comparing the mean values of the two curves of Fig. 31 we arrive at the value-72 per cent as the mean volt efficiency; the corresponding ampere hour efficiency is approximately 88 per cent, while the watt hour, or energy efficiency, the product of these two, is 63 per cent.

tic of all lead batteries, though differences in design modify it appreciably. Thinner plates tend to give a flatter curve, thicker ones a more sloping one.

A very important corollary of the variation of capacity with rate exists in the fact that a lead cell which has been completely discharged at a high rate, if allowed to stand for some hours, will largely recover, so as to give a considerable further discharge. In the case of a continuous discharge of diminishing rate, the ultimate capacity approaches that which would have obtained had the final rate been maintained throughout. In the operation of an electric vehicle the rates on starting, up grades, etc., exceed the normal rate by five to one or more; yet owing to the periods of rest, or low rate, the capacity attainable is practically identical with that of a continuous normal rate discharge. The normal rate for batteries of this kind is usually that corre

proaches a slanting straight line. Referring again to Fig. 33, the three upper lines, with the scale of ordinates at the right hand side, summarize the effect upon voltage of various discharge rates up to 10 times the normal. It is of interest to note that at 10 times normal the mean voltage has lost but 20 per cent; that the maximum watt output occurs at about 25 times normal; and that short circuit gives about 50 times normal discharge current. Čomparison between these curves and the corresponding ones for the Edison battery, Fig. 32, is very significant. The Edison battery is inferior (a) in that it has a much greater percentage drop in voltage during discharge at any given rate, and (b) in that the lead battery can discharge at about three times as high a rate as the Edison.

2. Acid Change ("Gravity").- It has been pointed out in discussing electric-chemical equations, that the amount of free sulphuric

sponding to a continuous discharge of four or five hours.

2. Temperature at time of discharge exercises a direct influence upon capacity to the extent of about 61⁄2 per cent per 10 degrees F. It thus comes about that at 0° the lead battery has about 54 per cent normal capacity, and that it is perfectly workable at temperatures much lower even than this, especially where the discharge rate is lower than normal.

Discharge Phenomena.-1. Voltage (or PD). Of equal importance with the capacity of a battery is its voltage characteristic, a typical curve of which is shown in Fig. 34. Starting off at approximately two volts, there is a gradual falling off, till the end approaches, when the voltage rapidly drops below a useful value. The curve shown is for the normal rate, but is fairly indicative of the general behavior of a lead cell on discharge. With higher rates, however, the curve is lower throughout its length, and more nearly ap

acid varies as discharge proceeds, and the third curve of Fig. 34 shows for a particular case what this change amounts to. Barring the fact of a lag at the start, the change of acid, measured by hydrometer, varies directly with the ampere hours drawn from the cell; but the amount of change depends so entirely upon the relative volume of acid contained in a given cell, that the numerical values of this curve in Fig. 34 have no general significance.

3. Temperature Change.-The lead cell to a slight extent is a thermo-electric accumulator, inasmuch as a slight disappearance of heat accompanies the discharge. This phenomena is graphically shown in the lower curve of Fig. 34, where it is seen that the temperature dropped 9° F. during discharge. At higher rates, the heat generated by internal electric friction overbalances that absorbed, and at the one hour rate the temperature rises about to the same amount as it dropped at normal rate. The absorption of heat on dis

charge, while of much theoretic interest, is of little practical value.

Internal Resistance.- One of the most valuable attributes of the lead battery is its high conductivity, which enables it to yield up its stored energy at extremely high rates. It is impossible to state the resistance definitely owing to variation of design; but by way of illustration it may be said that a cell of the type from which Fig. 34 was taken, having a normal rate of 35 amperes, has an internal resistance of about .0014 ohms at beginning, and .0028 ohms at end of discharge. Since it is mainly through its influence upon voltage that internal resistance is of interest, the data furnished by the curves of Fig. 33 give the practical information required better than an attempted formula for calculating resistance.

Efficiency. During charge, the P. D. of a lead cell starting at about two volts rises gradu

ditions, the volt efficiency is about 75 per cent, the watt hour efficiency about 65 per cent. These conditions are the most prevalent, except when a battery is charged directly from a generator, whose voltage is made to vary according to the charging curve.

Capacity Weight Ratios.- The capacity per unit weight of lead storage batteries varies all the way between 1.4 ampere hour per pound of cell in the heavier stationary types, such as Fig. 25, to about 5.5 in the lightest thin plate types for portable service, Fig. 26. These figures, as a basis of comparison, refer in all cases to a discharge rate approximating the five-hour. To find the corresponding values for other rates, reference should be had to the capacity curve of Fig. 33, bearing in mind that 100 per cent in this figure corresponds to an actual capacity of 4.6 ampere hours per pound.

Since the mean discharge voltage under

ally and finally becomes constant at a rather indefinite value, from 2.5 to 2.6 volts, following the general trend of the upper curve of Fig. 34. The mean height of this curve is 2.3 volts; that of the discharge curve 1.95 volts. Hence the volt efficiency is 85 per cent.

In commercial operation, it is found necessary that the charge exceed the discharge by about 15 per cent, so that the ampere hour efficiency is about 87 per cent, the watt hour efficiency about 75 per cent. When worked to less than 100 per cent capacity, both voltage and current efficiency are higher; so that in such cases it may reach or even exceed 90 per cent, as in regulating service, where charge and discharge succeed each other rapidly and for a few minutes duration only.

Where a battery is charged from a fixed voltage, on the other hand, this voltage must at least equal that at the end of the charge; hence where worked to full capacity under these con

these conditions is approximately 1.95 it follows that the energy capacity of a conservatively designed battery for portable service (as in Fig. 24) is about 9 watt hours per pound; for the lightest types in commercial use (Fig. 26) about 11.

It is a prevalent and quite natural idea that because the ordinary storage battery is made of lead, it is therefore unduly heavy. But when it is stated that each pound weight of battery can store up 24,000 foot-pounds, it may be readily appreciated that the electric storage battery of to-day is by far the most effective piece of mechanism known for storing energy. Put in slightly different form, it may be stated that the modern battery of conservative and substantial design, as in Fig. 24, can give out, in the space of five-hours, electrical energy sufficient to lift itself approximately five miles high.

Care and Operation.- To give a complete treatise on battery operation, covering the whole

varied field to which batteries are applied, would be beyond the scope of this article; but a few fundamental principles which are applicable to all cases may be briefly stated.

1. Care of Electrolyte.- Evaporation of the water of the electrolyte is constantly taking place, especially while the cell is gassing freely at the end of charge; this must be made up by periodic additions of water. Since the amount of water thus added during the life of a battery is many times the original amount contained, any impurities in the water accumulate quite rapidly. Hence it is very important to use only pure water for this purpose, and among the impurities commonly present chlorine and iron in any form are especially to be avoided. Addition of acid should be avoided.

2. Regulation of Charge. If a battery be habitually charged too little, the active material becomes gradually more and more converted into lead sulphate, until in time it ceases to function. Very long continued charge is then necessary to restore it to a working condition. If on the other hand it be charged too much, the gas bubbles liberated from the plates give rise to a softening and eroding action upon the positive material, which detaches it from the plates, and in time leaves the grids bare, and no longer workable. Experience has demonstrated that best results are obtained when each charge exceeds in ampere hours the previous discharge by about 15 per cent.

Several methods are in use for determining the correct amount of charge, as follows: (a) The "ampere hour" metre shows directly, both the current withdrawn on discharge and that put in on charge, from which the latter may be regulated; a very generally effective method when the discharge current is not too low, say 10 per cent of the normal. (b) The battery may be charged till the voltage ceases to rise; one of the older and less reliable methods. (c) The best indication of the state of charge is that based upon specific gravity of the electrolyte. If the specific gravity be read at regular intervals during charge, it will be found to rise steadily for a time, and then become constant.

When three successive readings covering a period of about an hour show no change in gravity, it means that chemical action between plates and electrolyte has ceased, and hence that the charge is complete, and should be stopped.

This method of determining charge is far the most reliable, and should be used wherever possible; and in any case should be used from time to time, to check up and make certain that charging is being done correctly. A single cell is usually selected as a "pilot" for taking readings with this method.

While the life of a battery may be much increased by careful regulation of the charge, according to the above principles, yet many hundreds of thousands of batteries are in successful operation, where the only care observed in charging is to arrange that it is ample, regardless of other considerations.

The strength of electrolyte used in storage batteries is not standardized, but varies with different makes and designs, and even with the individual cells of a given battery. In general where weight is a prime factor, higher gravity, usually about 1.280 specific gravity, is used; where weight and bulk are not important lower gravity, 1.200 or even 1.180, are preferably em

ployed. In the former case the volume of acid is small, and the drop of acid during discharge is correspondingly large, so that in discharged condition it may be 1,150; in the case of a stationary cell, where there is no close limit to bulk, the bacid which reads perhaps/1,200 on charge will drop during discharge to about the same point as the other, namely, 1,150-1,170. It is thus impossible to give any generally applicable values for the specific gravity of the electrolyte, but the theory of charging till a maximum is reached holds universally true.

The specific gravity of sulphuric acid of the concentration used in batteries varies with temperature, a rise of 10° F. causing a drop of .003 specific gravity; and as a matter of reference it is usual to correct all readings to 70° F.

Applications of the Storage Battery.The field of the storage battery to-day is so broad that a few of the most important applications only may be enumerated, as follows:

Propulsion of Automobiles and Commercial Trucks Usually 40 or 42 cells, capacity of 100-250 ampere hours, types of cell as m per Fig. 24.usta 10 9415201, 2 28 boeufted noitsie Propulsion of Mine Locomotives.-40-88 cells, 200-300 ampere hours capacity. Fig. 36. 10 Propulsion of Small Industrial Trucks. Usually 12-16 cells, 100-200 bampere hours capacity, type of cell as per Fig. 24.

FIG. 35.

Two Cell Unit of Train Lighting Battery, which
consists of 16 Cells in all. Approx. 1/10 size.

Starting and Lighting of Automobile.Usually 3 cells, designed to give 150-250 amperes for short intervals of a few minutes duration. Fig. 26.

Airplane Motor Ignition.- Usually 4 small cells, to insure motor reliability.

Mine Lamps.- One or two cells, 8-12 ampere hours capacity.

Railway Signal Service, for operating the signals which control the movements of trains.

Wireless Telegraphy as the source of power, both ashore and afloat, in army and navy, as well as commercial service.

Telephone Stations furnishing power for the telephone systems. Practically every central is provided with a battery, charged from a small dynamo, and for the purpose of assuring con