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practical on a primary cell where the members attacked by the acid had to be replaced from time to time, but in a storage battery only the electrolyte need be renewed. When the plates are discharged they are regenerated by passing a current of electricity through them. New electrolyte can be easily inserted through vents in which caps are screwed. The cells of which a storage battery is composed are nearly always joined together at the factory with bars of lead, which are burned in place, and only two free terminals are provided by which the battery is coupled to the outer circuit.
The capacity of a storage battery depends upon the area and the number of plates per cell, while the potential or voltage is determined by the number of cells joined in series to form the battery. Each cell has a difference of potential of two and two-tenths volts when fully charged, therefore a two-cell battery will deliver a current of four and four-tenths volts, and a threecell type as shown at Fig. 4, A, will give about six and six-tenths volts between the terminals. While dry cells are often connected in parallel, storage batteries should not be coupled in this manner, as the sets do not divide the load properly unless all the cells are equal in charge, capacity and general condition. A bad cell may so weaken one set that the other set will discharge through it, reducing the charge seriously. When sets are used in parallel it is advisable to test the sets separately. If one set is doing a large proportion of the work, the gravity of the electrolyte will be different in the sets. It is advisable to change parallel sets to series, when practicable, for charging at terminal stations. This insures the same charge being put in each set. For similar reasons all the cells used in a set should be in the same condition and equally well charged to prevent one or more cells reversing in service.
The Edison Storage Battery.—The fundamental principle of the Edison storage battery shown at Fig. 4, B, is the oxidization and reduction of metals in an electrolyte which neither combines with nor dissolves either the active materials or their oxides. Also, an electrolyte which, notwithstanding its decomposition by the action of the electric current, is immediately re-formed in equal quantity, and is, therefore, a practically constant element with
out change of density or conductivity over long periods of time. A storage battery is commonly looked upon as a receptacle in which to store electricity. Electricity is not concrete matter. In fact, nobody knows just what it is. Therefore, in the general apprehension of the term, it is not stored. Electricity simply causes a chemical change to be effected in certain substances, when it is caused to flow through them. These substances in endeavoring return to their original state, produce electricity.
The following elementary explanation of the action of the nonacid battery is given by the Edison Storage Battery Company, and is so simply expressed and easily understood that it is reproduced in full. If the reader grasps the principles expressed, he will have no difficulty in understanding the chemical action that results in current production.
“Suppose we place two pieces of very thin, bright steel out of doors for a few weeks. They become “rusted. The action of the oxygen on the outer layer of the metal has formed it into an oxide commonly known as “rust.' Now let us place these two pieces of steel in a solution composed of potash and water, and connect them by wires to a small dynamo. The electricity, in flowing from the dynamo through the solution, from one of the plates to the other and back to the dynamo, changes the rust to metallic iron on one of the plates, but causes the other plate to become “rusted twice as much as before. Now let us disconnect the plates from the dynamo and connect them, by means of pieces of wire, to an ammeter (an instrument for measuring electricity). Instantly, the excess of oxygen in the rust on the one plate commences to pass back to the bright plate and, by so doing, causes electricity to be generated. Why? Nobody really knows. We have now charged and discharged a primitive storage battery.
“Instead of two thin rusted steel plates, let us mount, say, one hundred such plates, equidistantly spaced, on one rod, and one hundred more on another rod. Now interpose the two groups so the plates of the one group will not touch those of the other and immerse them in a solution of potash. When connected to our dynamo the electricity will flow from one group, through the solution, to the other group, converting the oxide of one group to
metallic iron, and increasing the amount of oxide on the other group. We shall be able to get much more electricity from the battery thus formed, because of the greater plate surface exposed. We have thus determined that large surface is necessary.
“Let us next place a quantity of fine particles of iron rust in two perforated flat steel pockets and, after putting these pockets into potash solution, pass electricity from one to the other, through the solution, as before. All the iron rust in one pocket will be changed to metallic iron, because the oxygen will have passed over to the iron rust in the other pocket, causing this material to possess twice as much oxygen as before.
"Connect the two pockets to your ammeter and you will find that much more electricity is flowing than before, although the two pockets take up much less space than the two hundred steel plates. The reason of this is, the small particles present a very great combined surface to the solution. Suppose, after having made a great number of experiments, you put some iron rust or iron oxide into perforated steel pockets, and mount a number of these pockets in a steel grid or support to form one plate, and place nickel hydrate (a green powder) in perforated steel receptacles, and mount them on another steel grid to form the other plate, then immerse them in a suitable alkaline electrolyte in any kind of container; you have the essential elements of an Edison cell."
The active material of the positive plate of the Edison storage battery is nickel hydrate; that of the negative plate, iron oxide. The electrolyte is a solution of potassium hydrate. The active materials are perfectly insoluble in the electrolyte. When current passes, either on charge or discharge, the electrolyte is broken up into its component parts, which react on the materials with the following results: On charge-Positive oxidized, negative reduced. On discharge—Positive reduced, negative oxidized. The exact chemical changes that go on within the cell are not definitely known, but those occurring during discharge may be approximately represented by the following equations:
Positive: 8K + 6Ni0,= 2Ni,0, + 4K.,O.
The reverse reactions take place on charge. The iron and nickel compounds are probably hydrated, but are here treated as pure oxides, for the sake of simplicity. It will be noted that the same amount of KOH is decomposed, according to the left-hand members of these equations, as is ré-formed, simultaneously, as shown on the right. For this reason, the chemical composition, or specific gravity, of the solution does not change appreciably throughout the cycle of charge and discharge.
How Storage Batteries Differ in Construction-Batteries Using Other Than
Lead Plates—Details of Planté Process—Advantages of Faure Process
How Storage Batteries Differ in Construction. It is not reasonable to expect that any one type of storage battery can be applied universally. Naturally, batteries must be proportioned for the work they are to do. Batteries for stationary work in power plants can be of the open-cell type; those intended for vehicle or boat use should be sealed to prevent danger of splashing out the electrolyte. Various forms of storage batteries are illustrated at Fig. 5. The stationary type in glass or rubber jars is used for isolated lighting plants, telephone, telegraph and signal service and yacht lighting. The larger stationary cells are assembled in leadlined wood tanks, and are used for electric railways, stand-by service in central stations for lighting and power and in large isolated lighting plants. Special types in rubber jars are assembled in trays for electric vehicle service, and still others for trainlighting service. The usual starting, lighting and ignition battery for automobile use is a unit in which three or more cells are imbedded in insulating compound and carried in a substantial wooden case.
The usual practice is to burn all the connecting straps to the plate group terminals and cover the whole with sealing compound. Special gas vents are needed with these batteries. The “couple” type is carried in glass jars and is widely used for signal, fire alarm and private telephone service.
Storage Batteries Using Other Than Lead Plates.—While there is really only one make of cell that is commercially practical that does not use lead plates, this being the Edison, inventors have endeavored to improve storage battery action for some time by try