CHEMICAL CHANGES.
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Before explaining what happens within one storage cell, let us look
into the early history of the storage battery, and see what a modest
beginning the modern heavy duty battery had. Between 1850 and 1860 a
man named Plante began his work on the storage battery. His original
cell consisted of two plates of metallic lead immersed in dilute
sulphuric acid. The acid formed a thin layer of lead sulphate on each
plate which soon stopped further action on the lead. If a current was
passed through the cell, the lead sulphate on the "anode" or lead
plate at which the current entered the cell was changed into peroxide
of lead, while the sulphate on the other lead plate or "cathode" was
changed into pure lead in a spongy form. This cell was allowed to
stand for several days and was then "discharged," lead sulphate being
again formed on each plate. Each time this cell was charged, more
"spongy" lead and peroxide of lead were formed. These are called the
"active" materials, because it is by the chemical action between them
and the sulphuric acid that the electricity is produced. Evidently,
the more active materials the plates contained, the longer the
chemical action between the acid and active materials could take
place, and hence the greater the "capacity," or amount of electricity
furnished by the cell. The process of charging and discharging the
battery so as to increase the amount of active material, is called
"forming" the plates.
[Fig. 19 Illustration of chemical action in a storage cell
during charge]
Plante's method of forming plates was very slow, tedious, and
expensive. If the spongy lead, and peroxide of lead could be made
quickly from materials which could be spread over the plates, much
time and expense could be saved. It was Faure who first suggested such
a plan, and gave us the "pasted" plate of today, which consists of a
skeleton framework of lead, with the sponge lead and peroxide of lead
filling the spaces between the "ribs" of the framework. Such plates
are known as "pasted" plates, and are much lighter and more
satisfactory, for automobile work than the heavy solid lead plates of
Plante's. Chapter 3 describes more fully the processes of
manufacturing and pasting the plates.
We know now what constitutes a storage battery, and what the parts are
that "generate" the electricity. How is the electricity produced?
Theoretically, if we take a battery which has been entirely
discharged, so that it is no longer able to cause a flow of current,
and examine and test the electrolyte and the materials on the plates,
we shall find that the electrolyte is pure water, and both sets of
plates composed of white lead sulphate. On the other hand, if we make
a similar test and examination of the plates and electrolyte of a
battery through which a current has been sent from some outside
source, such as a generator, until the current can no longer cause
chemical reactions between the plates and electrolyte, we will find
that the electrolyte is now composed of water and Sulphuric acid, the
acid comprising about 30%, and the water 70% of the electrolyte. The
negative set of plates will be composed of pure lead in a spongy form,
while the positive will consist of peroxide of lead.
The foregoing description gives the final products of the chemical
changes that take place in the storage battery. To understand the
changes themselves requires a more detailed investigation. The
substances to be considered in the chemical actions are sulphuric
acid, water, pure lead, lead sulphate, and lead peroxide. With the
exception of pure lead, each of these substances is a chemical
compound, or composed of several elements. Thus sulphuric acid is made
up of two parts of hydrogen, which is a gas; one part of sulphur, a
solid, and four parts of oxygen, which is also a gas; these combine to
form the acid, which is liquid, and which is for convenience written
as H2SO 4, H2 representing two parts of hydrogen, S one part of
sulphur, and 04, four parts oxygen. Similarly, water a liquid, is made
up of two parts of hydrogen and one part of oxygen, represented by the
symbol H2O. Lead is not a compound, but an element whose chemical
symbol is Pb, taken from the Latin name for lead. Lead sulphate is a
solid, and consists of one part of lead, a solid substance, one part
of sulphur, another solid substance, and four parts of oxygen, a gas.
It is represented chemically by Pb SO4. Lead peroxide is also a solid,
and is made up of one part of lead, and two parts of oxygen. In the
chemical changes that take place, the compounds just described are to
a certain extent split up into the substances of which they are
composed. We thus have lead (Pb), hydrogen (H), oxygen (0), and
sulphur (S), four elementary substances, two of which are solids, and
two gases. The sulphur does not separate itself entirely from the
substances with which it forms the compounds H2SO4 and Pb SO4. These
compounds are split into H2 and SO4 and Pb and SO4 respectively. That
is, the sulphur always remains combined with four parts of oxygen.
Let us now consider a single storage cell made up of electrolyte, one
positive plate, and one negative plate. When this cell is fully
charged, or in a condition to produce a current of electricity, the
positive plate is made up of peroxide of lead (PbO2), the negative
plate of pure lead (Pb), and the electrolyte of dilute sulphuric acid
(H 2SO4). This is shown diagrammatically in Fig. 19. The chemical
changes that take place when the cell is discharging and the final
result of the changes are as follows:
(a). At the Positive Plate: Lead peroxide and sulphuric acid produce
lead sulphate, water, and oxygen, or:
[Image] Formula (a). PbO2 + H2SO4 = PbSO4 + H20 + 0
(b). At the Negative Plate: Lead and sulphuric acid produce lead
sulphate and Hydrogen, or:
[Image] Formula (b). Pb + H2SO4 = PbSO4 + H2
[Fig. 20 Chemical Reaction in a Storage Cell during Discharge]
The oxygen of equation (a) and the hydrogen of equation (b) combine to
form water, as may be shown by adding these two equations, giving one
equation for the entire discharge action:
[Image] Formula (c). PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O
In this equation we start with the active materials and electrolyte in
their original condition, and finish with the lead sulphate and water,
which are the final products of a discharge. Examining this equation,
we see that the sulphuric acid of the electrolyte is used up in
forming lead sulphate on both positive and negative plates, and is
therefore removed from the electrolyte. This gives us the easily
remembered rule for remembering discharge actions, which, though open
to question from a strictly scientific viewpoint, is nevertheless
convenient:
During discharge the acid goes into the plates.
The chemical changes described in (a), (b), and (c) are not
instantaneous. That is, the lead, lead peroxide, and sulphuric acid of
the fully charged cell are not changed into lead sulphate and water as
soon as a current begins to pass through the cell. This action is a
gradual one, small portions of these substances being changed at a
time. The greater the current that flows through the cell, the faster
will the changes occur. Theoretically, the changes will continue to
take place as long as any lead, lead peroxide, and sulphuric acid
remain. The faster these are changed into lead sulphate and water, the
shorter will be the time that the storage cell can furnish a current,
or the sooner it will be discharged.
Taking the cell in its discharged condition, let us now connect the
cell to a generator and send current through the cell from the
positive to the negative plates. This is called "charging" the cell.
The lead sulphate and water will now gradually be changed back into
lead, lead peroxide, and sulphuric acid. The lead sulphate which is on
the negative plate is changed to pure lead; the lead sulphate on the
positive plate is changed to lead peroxide, and sulphuric acid will be
added to the water. The changes at the positive plate may be
represented as follows:
Lead sulphate and water produce sulphuric acid, hydrogen and lead
peroxide, or:
[Image] Formula (d). PbSO4 + 2H2O = PbO2 + H2SO4 + H2
The changes at the negative plate may be expressed as follows: Lead
sulphate and water produced sulphuric acid, oxygen, and lead, or:
[Image] Formula (e). PbSO4 + H2o = Pb + H2SO4 + O
The hydrogen (H2) produced at the positive plate, and the oxygen (0)
produced at the negative plate unite to form water, as may be shown by
the equation:
[Image] Formula (f). 2PbSO4 + 2H2O = PbO2 + Pb + 2H2SO4
Equation (f) starts with lead sulphate and water, which, as shown in
equation (c), are produced when a battery is discharged. It will be
observed that we start with lead sulphate and water. Discharged plates
may therefore be charged in water. In fact, badly discharged negatives
may be charged better in water than in electrolyte. The electrolyte is
poured out of the battery and distilled water poured in. The acid
remaining on the separators and plates is sufficient to make the water
conduct the charging current.
In equation (f), the sulphate on the plates combines with water to
form sulphuric acid. This gives us the rule:
During charge, acid is driven out of the plates.
This rule is a convenient one, but, of course, is not a strictly
correct statement.
The changes produced by sending a current through the cell are also
gradual, and will take place faster as the current is made greater.
When all the lead sulphate has been used up by the chemical changes
caused by the current, no further charging can take place. If we
continue to send a current through the cell after it is fully charged,
the water will continue to be split up into hydrogen and oxygen.
Since, however, there is no more lead sulphate left with which the
hydrogen and oxygen can combine to form lead, lead peroxide, and
sulphuric acid, the hydrogen and oxygen rise to the surface of the
electrolyte and escape from the cell. This is known as "gassing", and
is an indication that the cell is fully charged.
Relations Between Chemical Actions and Electricity.
We know now that chemical actions in the battery produce electricity
and that, on the other hand, an electric current, sent through the
battery from an outside source, such as a generator, produces chemical
changes in the battery. How are chemical changes and electricity
related? The various chemical elements which we have in a battery are
supposed to carry small charges of electricity, which, however,
ordinarily neutralize one another. When a cell is discharging,
however, the electrolyte, water, and active materials are separated
into parts carrying negative and positive charges, and these "charges"
cause what we call an electric current to flow in the apparatus
attached to the battery.
Similarly, when a battery is charged, the charging current produces
electrical "charges" which cause the substances in the battery to
unite, due to the attraction of position and negative charges for one
another. This is a brief, rough statement of the relations between
chemical reactions and electricity in a battery. A more thorough study
of the subject would be out of place in this book. It is sufficient
for the repairman to remember that the substances in a battery carry
charges of electricity which become available as an electric current
when a battery discharges, and that a charging current causes electric
charges to form, thereby "charging" the battery.
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