Part 16

There are two distinct types of storage battery. One is called the "lead" or "acid" storage battery, and the other the "alkaline" or "nickel-iron" storage battery. Each of them simply acts as a reservoir to hold the energy of the electric current which is led into it, and each of them, under proper conditions, will give back that energy. As the lead storage battery is the oldest in point of discovery and invention, we will describe it first.

THE LEAD STORAGE BATTERY

A lead storage battery usually consists of a glass or hard-rubber jar containing lead plates and a solution consisting of water and sulphuric acid. A single unit is usually called a "cell." (Fig. 36.)

There are always at least two lead plates in a storage-battery cell of this kind, although there may be any number above that. For the sake of making a clearer explanation to you, we will take as an illustration a cell containing only two plates.

We have, then, a glass or hard-rubber jar containing two lead plates and a solution consisting of water and sulphuric acid. These plates are called the "elements," and one is called the positive and the other the negative element. The solution is called the "electrolyte."

The positive element is a sheet of lead upon which is spread a paste made of red-lead. The negative element is a similar sheet of lead upon which is spread a paste made of litharge.

Now, when these plates are thus prepared, they are put into the acid solution in the jar, and a wire attached to each plate is connected with the two wires from a dynamo or other source of electric current, just as a lamp would be connected.

The electric current then goes into the storage-battery cell, entering by the positive plate and coming out by the negative. These plates and the paste upon them offer some resistance, or opposition, to the passage of the current, so the electricity must do some work to get from one to the other. The work it does in this case is to so act upon the paste that its chemical nature is changed.

So, after the primary current has been passed from one plate to the other for some time, and after several "discharges," the storage battery may be disconnected, being now "formed."

The paste on the lead plates is now found to have changed its chemical nature, the paste on the positive plate having been transformed into peroxide of lead, and that on the negative plate into spongy lead. On arriving at this condition, the paste on the plates is called "active material."

This process of "formation" is absolutely essential before the lead storage battery is ready to be used for actual work. So, when the plates have been fully "formed," the storage battery may be again connected with a source of electric current which again enters by the positive plate and leaves by the negative. This current so acts on the active material that it combines with the acid solution and, through the energy of the charging current, forms other chemical compounds which may for convenience be called "sulphates." When the charging current has flowed through the battery long enough to produce these changes in the active material the battery is said to be "charged," and is ready for useful work.

If the two wires attached to the plates are now connected with electric lamps, or a motor, or other device, the active material will develop energy in the effort to again change its nature. This energy takes the form of an electric current, which leaves the battery and passes through the conductors and operates the lamps, motors, or other devices in its passage.

In this way the battery is said to be "discharged," and at the end of its discharge it can again be charged and discharged in a similar manner for a long time, until the active material is either used up or drops off the plates.

So far as the actual details of construction are concerned, lead storage batteries are made in a great many different ways, but the materials are, in general, of the same nature as those we have mentioned above.

THE ALKALINE STORAGE BATTERY

We shall now describe an entirely different type of storage battery, which contains neither lead nor acid. It is one of the many inventions of Thomas A. Edison.

In the alkaline storage battery the gas called oxygen plays a very important part, and we will try to make it clear to you what this part is.

You are well aware of the fact that if you leave your pocket-knife out in the air it will get rusty. The reason for this is that iron or steel quickly tends to combine with the oxygen of the air, and this combination of oxygen and iron is rust, otherwise called oxide of iron, or iron oxide.

This iron oxide, or rust, is therefore the result of a chemical action between the iron and the oxygen.

Now as all chemical actions require the expenditure of energy, there has been developed either heat or electricity in the process. The oxygen may be taken away from the iron oxide, chemically; but here again would be another chemical action which would require energy to be once more expended.

Iron oxide may be made chemically in many different ways. It is frequently made in the form of a powder. Therefore, we do not have to depend upon iron rust for a supply of this material.

Before going further we must consider another oxide--namely, nickel oxide. It is characteristic of nickel that when it is combined with oxygen to a certain degree so as to form the compound known as nickel oxide, it will receive still more oxygen.

Now, if under proper conditions we compel iron oxide to give up its oxygen to some other kind of chemical compound, such as nickel oxide, we must expend energy. But, on the other hand, if this nickel oxide gives back the oxygen to the iron--which it will do if opportunity is given--there is energy produced again in receiving the oxygen. In other words, the energy previously expended, or part of it, is now returned.

This action and reaction are practically those that take place in the Edison alkaline storage battery. For simplicity of illustration we will consider a cell containing only two plates, one positive and one negative.

The negative plate is made up of a number of small, flat, perforated pockets containing iron oxide in the form of a fine powder. The positive plate is made up of small, perforated tubes containing nickel oxide mixed with very thin flakes of metallic nickel. (Fig. 37 illustrates these plates, the positive being in front.)

These two elements, positive and negative, having wires or conductors attached, are placed in a nickeled-steel can containing the electrolyte, which consists of a potash solution. You will see that this differs from a lead storage battery, in which the electrolyte is sulphuric acid and water. If we were to put this acid solution into a metallic can (except one made of lead) the can would not last long, as the acid would quickly eat holes through it.

Now let us see what takes place in the Edison alkaline storage battery. If an electric current from a dynamo or other source of electricity is caused to pass through the positive to the negative plate the oxygen present in the iron oxide passes to and remains with the nickel oxide. During all the time this is going on the battery is said to be "charging," and when all the oxygen has been removed from the iron oxide and is taken up by the nickel oxide, then the battery is said to be "charged," and the flow of current into the battery is stopped.

A change has now taken place. The powder in the negative plate is no longer iron oxide, but has been reduced to metallic iron, because the oxygen has been removed. The powder in the positive plate is now raised to a higher or super oxide of nickel, because it has taken the oxygen that was in the iron.