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.[3]
Fig. 36
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.