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size of the battery or its relative state of discharge, but depend only on the actual state of discharge. The curve in Fig. 52 shows in percentage the theoretical variation in charging rate and also in state of charge if a battery were charged strictly in accordance with this rule, and this represents the method by which a battery may be safely charged in a minimum time in regular operation.

It is evident that very wide latitude for proper charging is offered from which to select the best way to charge a battery under any given local conditions. If the vehicle is equipped with an ampere-hour meter the readings of this meter may be taken as the basis for selecting a charging rate which may be used for a particular length of time, so that at the end of that time the charging rate will be at the maximum permissible rate, at which time the rate should, of course, be reduced. It follows from the general rule for charging that, if R=permissible charging rate, D=ampere-hours out of the battery at the start of the charge (reading of the ampere-hour meter) and T=time in hours until the current can be adjusted, then RED:(+T)=maximum permissible charging rate for T hours, and at the end of T hours the charging rate will equal the reading of the amperehour meter. This value can again be divided by 1+T for the new charging rate, and so on until the charge can be finished at the finishing rate. If it is desired to charge the battery rapidly, the time T should be taken as short as possible. For convenience Table I, given in Fig. 51, calculated from this formula, is given. This table is of use not only in the charging-room, but also for the determination of the best manner for charging vehicles under any contemplated conditions and for the selection of charging equipment to meet the requirements of these conditions.

Ampere-hour Meter Indications as Basis for Charging: It should be carefully noted that if an ampere-hour meter is made the basis for charging a battery, care must be taken to be sure that the meter indicates as nearly as possible the real state of charge of the battery. An accurate record of the ampere-hours discharge from a battery does not give an accurate measure of the ampere-hours necessary to fully recharge it, for there are certain variable losses in the battery which the ampere-hour meter cannot

measure.

In fact, there is no accurate way to predetermine exactly how many ampere-hours charge may be necessary to fully charge a battery, nor is it necessary in ordinary service that the battery be really completely recharged daily. An ordinary clock is not an accurate instrument for measuring time, yet if it is set correctly occasionally it is sufficiently accurate for ordinary purposes. It is the same with an ampere-hour meter. It is necessary that a battery be fully charged occasionally, say, once a week, if the battery is subjected to hard daily use, as on a commercial truck, and this furnishes an opportunity to set the ampere-hour meter.

A battery is fully charged only when all the sulphate has been driven out of the plates into the electrolyte, and this is most easily indicated by the specific gravity of the electrolyte. As long as sulphate is being thrown out of the plates into the electrolyte during charge, the specific gravity of the latter must continue to rise, and when the rise stops the battery is fully charged. Most battery manufacturers recommend that a battery be given such a charge (called an equalizing charge), regardless of the indication of the ampere-hour meter, once a week or once in two weeks. When it is known that the battery is full, the charge is discontinued and then the meter is set to indicate a full battery, and the meter is then a sufficiently accurate indicator of the state of battery charge to be used for a week or two weeks until another equalizing charge is given the battery, when the meter should again be set.

Ampere-hour meters require cleaning and regulation at intervals as does a clock, and if they are treated in this manner they are of great assistance in the proper handling of a battery. These meters are frequently furnished with a contact-making device, so arranged as to interrupt the charging circuit when the meter indicates that the battery is fully charged, and this is a valuable protection again unnecessary charging and gassing of the battery during ordinary operation. This tripping device should, of course, be disconnected during the equalizing charge.

CHAPTER V

Uses of Storage Batteries—Internal Combustion Engine Ignition-Auto

mobile Starting and Lighting Systems Shifting Gears by Electricity -Electric Pleasure and Commercial Automobiles—Isolated Lighting Plants — Train Lighting — Storage-Battery Locomotives — BatteryDriven Street Cars—Submarine Boats—Miscellaneous Marine Applications—Railway Switch and Signal ServiceStand-by and Booster Service-Drawbridge Operation-Mine Lamp Battery.

Uses of Storage Batteries. In this chapter the writer intends to describe briefly the most interesting of the many possible applications of storage batteries, some of which are unusual, to say the least. A rather flexible imagination is needed to consider that the cheerfully lighted farm home; the lifting of a massive drawbridge; the roaring start of a powerful automobile or hydroaeroplane engine and the noiseless movement of the electric automobile are all accomplished by the same agency. Storage batteries propel numerous electric launches on the water's surface, and are the sole power for the submarine lurking in its depths. Hundreds of palatial yachts are illuminated by this means, and the penetrating shaft of light from the miner's lamp is produced by the same energizing source. A wireless call for help from a ship in distress, with its life-saving possibilities, is produced from a few cells of a storage battery. The headlights of the myriad automobiles now using our highways after nightfall attest to the practical value of this current-producer.

The magnitude of the industry and the great possibilities of the field for storage batteries can be only briefly touched upon, but it is evident, from a perusal of the following list, that they can be used anywhere a dependable source of current is required. We find storage batteries used in: Central lighting and power stations; electric railway power houses; municipal and office building lighting and power; steel mills; electric hoist and elevators;

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Fig. 54.-Connecticut Closed-Circuit Ignition System Uses Storage-Battery Current.

isolated lighting plants for hotels; suburban residences and farms; railway switch and signal service; railway car lighting; interlocking switch service; United States Government submarine and gunfiring service; telephone, telegraph, wireless and fire-alarm service; laboratory and school work; electroplating; automobile enginestarting; gas-engine ignition; automobile lighting; electric trucks and pleasure cars; street railway cars; electric launches and mine and industrial locomotives.

The Storage Battery for Gasoline-Engine Ignition.-Because of the almost universal employment of electricity for lighting and starting ystems, the battery ignition system has been improved materially, inasmuch as the storage battery supplying the current is constantly charged by a generator. A number of systems have been devised, these operating on two different principles, the open circuit and the closed circuit. An example of the closed-circuit system is shown at Fig. 54, and is of Connecticut design, the complete ignition system consisting of a combined timer and hightension distributor, a separate induction coil and a switch. The system is distinctive in that the timer is so constructed that the primary circuit of the coil is permitted to become thoroughly saturated with electricity before the points separate, with a result that a spark of maximum intensity is produced. The action is very much the same as that of a magneto on account of the saturation of the winding. Another feature is the incorporation with the switch of a thermostatically operated electro-magnetic device, which automatically breaks the connection between the battery and the coil should the switch be left on with the motor idle.

The contact-breaker mechanism consists of an arm A carrying one contact, a stationary block B carrying the other contact, a fiber roller R, which is carried by the arm A, and operated by points on the cam C, which is mounted on the driving-shaft. Normally, the contacts are held together under the action of a light spring. As the four cams, which in touching the roller R raise the arm and separate the contacts, are 90 degrees for a fourcylinder motor, the period of saturation of the coil or the length of time the current flows through it to the battery is sufficiently long so that when the points have separated, the current, which

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