Electricity for the farm Part 14

To determine the ampere hours from this table, multiply the total number of lamp hours by the current in amperes required for each lamp.

As 10, 15, and 20-watt tungsten lamps require .33, .50 and .67 amperes, respectively at 30 volts pressure, the above requirements in ampere hours would be 12, 17-1/2, or 24 ampere hours, according to the size of lamp chosen. This gives the average current consumption for one night. If it is desired to charge the battery twice a week on the average, multiply the number of lamp hours by 4, to get the size of battery required.

The foregoing ill.u.s.tration is not intended to indicate average light requirements for farms, but is given merely to show how a farmer may figure his own requirements. In some instances, it will be necessary to install a battery of 120 or more ampere hours, whereas a battery of 40 or 60 ampere hours would be quite serviceable in other instances.

It all depends on how much light you wish to use and are willing to pay for, because with a storage battery the cost of electric light is directly in proportion to the number of lights used.

As a general rule, a larger generator and engine are required for a larger battery--although it is possible to charge a large battery with a small generator and engine by taking more time for the operation.

_How to Charge a Storage Battery_

Direct current only can be used for charging storage batteries. In the rare instance of alternating current only being available, it must be converted into direct current by any one of the many mechanical, chemical, or electrical devices on the market--that is, the alternating current must be straightened out, to flow always in one direction.

A shunt-wound dynamo must be used; else, when the voltage of the battery rises too high, it may "back up" and turn the dynamo as a motor, causing considerable damage. If a compound dynamo is already installed, or if it is desired to use such a machine for charging storage batteries, it can be done simply by disconnecting the series windings on the field coils, thus turning the machine into a shunt dynamo.

The voltage of the dynamo should be approximately 50 per cent above the working pressure of the battery. For this reason 45-volt machines are usually used for 30 or 32-volt batteries. Higher voltages may be used, if convenient. Thus a 110-volt dynamo may be used to charge a single 2-volt cell if necessary, although it is not advisable.

_Direction of Current_

Electricity flows from the positive to the negative terminal. A charging current must be so connected that the negative wire of the dynamo is always connected to the negative terminal of the battery, and the positive wire to the positive terminal. As the polarity is always marked on the battery, there is little danger of making a mistake in this particular.

When the storage battery is charged, and one begins to use its acc.u.mulation of energy, the current comes out in the opposite direction from which it entered in charging. In this respect, a storage battery is like a clock spring, which is wound up in one direction, and unwinds itself in the other. With all storage battery outfits, an ammeter (or current measure) is supplied with zero at the center. When the battery is being charged, the indicating needle points in one direction in proportion to the strength of the current flowing in; and when the battery is being discharged, the needle points in the opposite direction, in proportion to the strength of the current flowing out.

Sometimes one is at loss, in setting about to connect a battery and generator, to know which is the positive and which the negative wire of the generator. A very simple test is as follows:

Start the generator and bring it up to speed. Connect some form of resistance in "series" with the mains. A lamp in an ordinary lamp socket will do very well for this resistance. Dip the two ends of the wire (one coming from the generator, the other through the lamp) into a cup of water, in which a pinch of salt is dissolved. Bring them almost together and hold them there. Almost instantly, one wire will begin to turn bright, and give off bubbles. The wire which turns bright and gives off bubbles is the _negative_ wire. The other is the positive.

[Ill.u.s.tration: A rough-and-ready farm electric plant, supplying two farms with light, heat and power; and a Ward Leonard-type circuit-breaker for charging storage batteries]

_Care of Battery_

Since specific directions are furnished with all storage batteries, it is not necessary to go into the details of their care here. Storage battery plants are usually shipped with all connections made, or plainly indicated. All that is necessary is to fill the batteries with the acid solution, according to directions, and start the engine. If the engine is fitted with a governor, and the switchboard is of the automatic type, all the care necessary in charging is to start the engine. In fact, many makes utilize the dynamo as a "self-starter" for the engine, so that all that is necessary to start charging is to throw a switch which starts the engine. When the battery is fully charged, the engine is stopped automatically.

The "electrolyte" or solution in which the plates of the lead battery are immersed, is sulphuric acid, diluted with water in the proportion of one part of acid to five of water, by volume.

The specific gravity of ordinary commercial sulphuric acid is 1.835.

Since its strength is apt to vary, however, it is best to mix the electrolyte with the aid of the hydrometer furnished with the battery.

The hydrometer is a sealed gla.s.s tube, with a graduated scale somewhat resembling a thermometer. The height at which it floats in any given solution depends on the density of the solution. It should indicate approximately 1.15 for a storage battery electrolyte before charging.

It should not be over 1.15--or 1,150 if your hydrometer reads in thousandths.

Only pure water should be used. Distilled water is the best, but fresh clean rain water is permissible. Never under any circ.u.mstances use hydrant water, as it contains impurities which will injure the battery, probably put it out of commission before its first charge.

_Pour the acid into the water._ Never under any circ.u.mstances pour the water into the acid, else an explosion may occur from the heat developed. Mix the electrolyte in a stone crock, or gla.s.s container, stirring with a gla.s.s rod, and testing from time to time with a hydrometer. Let it stand until cool and then pour it into the battery jars, filling them to 1/2 inch above the top of the plates.

Then begin charging. The first charge will probably take a longer time than subsequent charges. If the installation is of the automatic type, all that is necessary is to start the engine. If it is not of the automatic type, proceed as follows:

First be sure all connections are right. Then start the engine and bring the dynamo up to its rated speed. Adjust the voltage to the pressure specified. Then throw the switch connecting generator to battery. Watch the ammeter. It should register in amperes, one-eighth of the ampere-hour capacity of the battery, as already explained. If it registers too high, reduce the voltage of the generator slightly, by means of the field rheostat connected to the generator. This will also reduce the amperes flowing. If too low, raise the voltage until the amperes register correctly. Continue the charging operation until the cells begin to give off gas freely; or until the specific gravity of the electrolyte, measured by the hydrometer, stands at 1.24. Your battery is now fully charged. Throw the switch over to the service line, and your acc.u.mulator is ready to furnish light if you turn on your lamps.

Occasionally add distilled water to the cells, to make up for evaporation. It is seldom necessary to add acid, as this does not evaporate. If the battery is kept fully charged, it will not freeze even when the thermometer is well below zero.

A storage battery should be installed as near the house as possible--in the house, if possible. Since its current capacity is small, transmission losses must be reduced to a minimum.

In wiring the house for storage battery service, the same rules apply as with standard voltage. Not more than 6 amperes should be used on any single branch circuit. With low voltage batteries (from 12 volts to 32 volts) it is well to use No. 10 or No. 12 B. & S. gauge rubber-covered wire, instead of the usual No. 14 used with standard voltage. The extra expense will be only a few cents for each circuit, and precious volts will be saved in distribution of the current.

CHAPTER XII

BATTERY CHARGING DEVICES

The automatic plant most desirable--How an automobile lighting and starting system works--How the same results can be achieved in house lighting, by means of automatic devices--Plants without automatic regulation--Care necessary--The use of heating devices on storage battery current--Portable batteries--An electricity "route"--Automobile power for lighting a few lamps.

The water-power electric plants described in preceding chapters are practically automatic in operation. This is very desirable, as such plants require the minimum of care. It is possible to attain this same end with a storage battery plant.

Automatic maintenance approaches a high degree of perfection in the electric starting and lighting device on a modern automobile. In this case, a small dynamo geared to the main shaft is running whenever the engine is running. It is always ready to "pump" electricity into the storage battery when needed. An electric magnet, wound in a peculiar manner, automatically cuts off the charging current from the dynamo, when the battery is "full;" and the same magnet, or "regulator,"

permits the current to flow into the battery when needed. The principle is the same as in the familiar plumbing trap, which constantly maintains a given level of water in a tank, no matter how much water may be drawn from the tank. The result, in the case of the automobile battery, is that the battery is always kept fully charged; for no sooner does the "level" of electricity begin to drop (when used for starting or lighting) than the generator begins to charge. This is very desirable in more ways than one. In the first place, the energy of the battery is always the same; and in the second place, the mere fact that the battery is always kept fully charged gives it a long life.

The same result can be achieved in storage battery plants for house lighting, where the source of power is a gasoline or other engine engaged normally in other work. Then your electric current becomes merely a by-product of some other operation.

Take a typical instance where such a plant would be feasible: Farmer Brown has a five horsepower gasoline engine--an ordinary farm engine for which he paid probably $75 or $100. Electric light furnished direct from such an engine would be intolerable because of its constant flickering. This five horsepower engine is installed in the milk room of the dairy, and is belted to a countershaft. This countershaft is belted to the vacuum pump for the milking machine, and to the separator, and to a water pump, any one of which may be thrown into service by means of a tight-and-loose pulley. This countershaft is also belted to a small dynamo, which runs whenever the engine is running. The milking machine, the separator, and the water pump require that the gasoline engine be run on the average three hours each day.

The dynamo is connected by wires to the house storage battery through a properly designed switchboard. The "brains" of this switchboard is a little automatic device (called a regulator or a circuit breaker), which opens and shuts according to the amount of current stored in the battery and the strength of the current from the generator. When the battery is "full," this regulator is "open" and permits no current to flow. Then the dynamo is running idle, and the amount of power it absorbs from the gasoline engine is negligible. When the "level" of electricity in the battery falls, due to drawing current for light, the regulator is "shut," that is, the dynamo and battery are connected, and current flows into the battery.

These automatic instruments go still farther in their brainy work.

They do not permit the dynamo to charge the battery when the voltage falls below a fixed point, due to the engine slowing down; neither do they permit the dynamo current to flow when the voltage gets too high due to sudden speeding up of the engine.

Necessarily, an instrument which will take care of a battery in this way, is intricate in construction. That is not an argument against it however. A watch is intricate, but so long as we continue to wind it at stated intervals, it keeps time. So with this storage battery plant: so long as Farmer Brown starts his engine to do his farm ch.o.r.es every day, his by-product of electricity is stored automatically.

Such installations are not expensive. A storage battery capable of lighting 8 tungsten lamps, of 16 candlepower each, continuously for 8 hours (or fewer lamps for a longer time); a switchboard containing all the required regulating instruments; and a dynamo of suitable size, can be had for from $250 to $300. All that is necessary to put such a plant in operation, is to belt the dynamo to the gasoline engine so that it will run at proper speed; and to connect the wires from dynamo to switchboard, and thence to the house service. The dynamo required for the above plant delivers 10 amperes at 45 volts pressure, or 10 45 = 450 watts. A gasoline, gas, or oil engine, or a windmill of 1-1/2 horsepower furnishes all the power needed. If the farmer uses his engine daily, or every other day, for other purposes, the cost of power will be practically negligible. With this system electric lights are available at any time day or night; and when the gasoline engine is in service daily for routine farm ch.o.r.es, the battery will never run low.

This system is especially desirable where one uses a windmill for power. The speed of the windmill is constantly fluctuating, so much so in fact that it could not be used for electric light without a storage battery. But when equipped with a regulator on the switchboard which permits the current to flow only when the battery needs it, and then only when the speed of the windmill is correct, the problem of turning wind power into electric light is solved.

If the farmer does not desire to go to the additional expense of automatic regulation, there are cheaper plants, requiring attention for charging. These plants are identical with those described above, except they have no regulators. With these plants, when the battery runs low (as is indicated by dimming of the lights) it is necessary to start the engine, bring it up to speed, adjust the dynamo voltage to the proper pressure, and throw a switch to charge the battery. For such plants it is customary to run the engine to charge the battery twice a week. It is necessary to run the engine from 8 to 10 hours to fully charge the discharged battery. When the battery approaches full charge, the fact is evidenced by so-called "ga.s.sing" or giving off of bubbles. Another way to determine if the battery is fully charged is by means of the voltmeter, as the volts slowly rise to the proper point during the process of charging. A third way, and probably the most reliable is by the use of the hydrometer. The voltage of each cell when fully charged should be 2.5; it should never be discharged below 1.75 volts. Many storage battery electric light plants on the market are provided with a simple and inexpensive circuit breaker, which automatically cuts off the current and stops the engine when the battery is charged. The current is then thrown from the dynamo to the house service by an automatic switch. If such a circuit breaker is not included, it is necessary to throw the switch by hand when charging is begun or ended.

Since the princ.i.p.al item of first cost, as well as depreciation, in a storage battery electric light plant is the storage battery itself, the smallest battery commensurate with needs is selected. Since the amount of current stored by these batteries is relatively small, electric irons and heating devices such as may be used freely on a direct-connected plant without a battery, are rather expensive luxuries. For instance, an electric iron drawing 400 watts an hour while in use, requires as much energy as 20 tungsten lamps of 16 candlepower each burning for the same length of time. Its rate of current consumption would be over 13 amperes, at 30 volts; which would require a larger battery than needed for light in the average farm home.

The use to which electricity from a storage battery is put, however, is wholly a matter of expense involved; and if one is willing to pay for these rather expensive luxuries, there is no reason why he should not have them. Heating, in any form, by electricity, requires a large amount of current proportionally. As a matter of fact, there is less heat to be had in thermal units from a horsepower-hour of electricity than from three ounces of coal. When one is generating current from water-power, or even direct from gasoline or oil, this is not an argument against electric heating devices. But it becomes a very serious consideration when one is installing a storage battery as the source of current, because of the high initial cost, and depreciation of such a battery.

Farmers who limit the use of their storage battery plants to lighting will get the best service.

_Portable Batteries_