cells to be used in series. As the field of the storage battery is very broad, so the design varies greatly, including almost every con ceivable combination from the small three-cell battery for auto starting (Fig. 26) to the large central station battery of 150 cells, as shown in Fig. 30.
connecting the individual cells together electrically various forms of "connectors" and "bus bars" are in use, usually made of lead or lead-antimony alloy, occasionally of copper, protected by a coating In the large stationary batteries the plates of adjacent cells are as nearly as possible welded directly together, each plate (Fig. 29) having a projecting Mug" which reaches part way over the edge of the cell where all are united to a substantial lead bus bar, as in Fig. 30 and at al:)" in Fig. 28. The bus bar 60) in this figure is one of the battery terminals, and the lead is reinforced by a large tapered plate of' copper, to which is bolted the copper bar that connects the battery to the switchboard.
In Fig. 30 are shown a number of these terminals with the large coppers leading off. In this instance the battery is so arranged that the number of cells in circuit may be varied 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.
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, 6A." Charactenstics of the Storage Battery, The primary useful quantities furnished by a storage battery are electromotive force, or P.D. (measured in volts), and current (meas ured 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 fur nish it. Frequently the relation of these three
primal), 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 quan tities one with another. In the following dis cussion 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 dur ing 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 inj ury.
The capacity is limited by the fall of voltage to a point where usefulness ceases, this point being again arbitrary, but through large experi ence 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 mod ern storage batteries, and each design possesses its own characteristics; the curves which fol low are chosen as fairly representative, but of course cannot pretend to fit all cases.
Characteristics of the Edison 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 tem perature, to an extent which varies with the discharge rate. This variation with tempera ture is so great that there results a cntical 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 grad ually warm itself ; but for immediate action at low temperatures it is unworkable.