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Fig. 65.—Plan and Side Views of Electric Truck Chassis, With Battery Box Removed, Showing Sim
plicity of Mechanism.
ally in isolated plants the engine and generator capacity is sufficient for the total number of lamps connected, although they are all seldom in use at one time, consequently the plant operates at but partial load during the total lighting hours. This means low efficiency, poor regulation and high fuel costs. The installation of a storage battery corrects this weakness by permitting the operation of the generator at the full or the most economical load for
a few hours and then shutting down entirely, the battery providing the current for the balance of the time.
In many cases it can be so arranged that the generator need be operated only every second or third day, and then at the most convenient time. No additional labor is required; in fact, this cost is usually lessened, while the fuel cost and maintenance and repair expense are much reduced. By taking current directly from the battery, steady lights are obtained and the noises of engine at night avoided. When on special occasions unusual lighting is required, the battery may be discharged in parallel with generator, and demands equal to the combined capacity of battery and
generator may be supplied. The generating equipment may be stopped at any time for adjustment or repair without interrupting the service, the battery being available for unexpected demands for power. The great advantage of electric current for operating pumps and other machinery at a distance from the engine and generator cannot be overestimated. The employment of electricity for driving fans, heating curling-irons, cooking, etc., is also of great convenience, and by installing batteries current is available at all times.
The following rules for battery selection and methods of installation and operation are reproduced from a bulletin issued by the Gould Storage Battery Company, describing batteries for isolated lighting plant use.
Selection of Battery.—The number of cells is determined by the voltage of the system and is entirely independent of the size of the individual cells.
Isolated plants of the various voltages require batteries of the number of cells given in the following table:
Voltage of System Number of Cells Voltage of System Number of Cells
110 115 125
60 64 70
The size of the individual cells is determined by the number of lamps, their candle-power and efficiency, and the length of time they must be supplied on one discharge. For ordinary purposes it is sufficiently accurate to estimate the energy taken by a 16candle-power carbon filament lamp as 55 watts (110-volt, 16-candle-power lamp taking one-half ampere) and lamps of other candle-power on a proportionate basis. By using tungsten filament or nitrogen-filled bulbs, the current consumption may be materially reduced for a given candle-power, and outfits of lower voltage will give satisfactory light. For example, a recently developed, small-capacity lighting outfit for farm use uses but 16 cells of battery by operating 30-volt tungsten filament lamps. To simplify figuring, we will consider a voltage that is in common use in places where a central station furnishes power or 110 volts.
Storage batteries are rated in "ampere-hours," which defines their capacity and is the product of the number of amperes discharge and the number of hours such discharge can continue. The capacity at the eight-hour rate is considered the normal. As the ampere discharge is increased above the normal or eight-hour rate, the ampere-hour capacity decreases, as will be seen by the following example:
Thus, while 1212 amperes may be obtained for eight hours, or 100 ampere-hours, if the discharge be made at 25 amperes it can be continued for but three hours, or 75 ampere-hours; the remaining capacity of the battery is, however, available at a lower rate. On discharge at less than the eight-hour rate, the capacity of the battery is slightly greater, but the increase is small, and for ordinary calculation it is best to consider the capacity at rates lower than the eight-hour, the same as the eight-hour capacity.
The size of a 110-volt battery can be approximately determined by the method outlined in the following example, the conditions being that the battery will be charged at any time during the day convenient to operate the generator, and that the battery will be able to furnish current for lamps according to the following schedule: