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]
[Illustration: 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.