BATTERIES IN GENERAL
There are two ways of "generating" electricity on the car: 1.
Magnetically, 2. Chemically. The first method is that used in a
generator, in which wires are rotated in a "field" in which magnetic
forces act. The second method is that of the battery, and the one in
which we are now interested.
If two unlike metals or conducting substances are placed in a liquid
which causes a greater chemical change in one of the substances than
in the other, an electrical pressure, or "electromotive" force is
caused to exist between the two metals or conducting substances. The
greater the difference in the chemical action on the substances, the
greater will be the electrical pressure, and if the substances are
connected together outside of the liquid by a wire or other conductor
of electricity, an electric current will flow through the path or
"circuit" consisting of the liquid, the two substances which are
immersed in the liquid, and the external wire or conductor.
As the current flows through the combination of the liquid, and the
substances immersed in it, which is called a voltaic "cell," one or
both of the substances undergo chemical changes which continue until
one of the substances is entirely changed. These chemical changes
produce the electrical pressure which causes the current to flow, and
the flow will continue until one or both of the substances are changed
entirely. This change due to the chemical action may result in the
formation of gases, or of solid compounds. If gases are formed they
escape and are lost. If solids are formed, no material is actually
lost.
Assuming that one of the conducting substances, or "electrodes," which
are immersed in the liquid has been acted upon by the liquid, or
"electrolyte," until no further chemical action can take place, our
voltaic cell will no longer be capable of causing a flow of
electricity. If none of the substances resulting from the original
chemical action have been lost as gases, it may be possible to reverse
the entire set of operations which have taken place. That is, suppose
we now send a current through the cell from an outside source of
electricity, in a direction opposite to that in which the current
produced by the chemical action between the electrodes and electrolyte
flowed. If this current now produces chemical actions between
electrodes and electrolyte which are the reverse of those which
occurred originally, so that finally we have the electrodes and
electrolyte brought back to their original composition and condition,
we have the cell just as it was before we used it for the production
of an electrical pressure. The cell can now again be used as a source
of electricity as long as the electrolyte acts upon the electrodes, or
until it is "discharged" and incapable of any further production of
electrical pressure. Sending a current through a discharged cell, so
as to reverse the chemical actions which brought about the discharged
conditions, is called "charging" the cell.
[Fig. 2 A complete cell; Negative group; Positive group]
Cells in which an electrical pressure is produced as soon as the
electrodes are immersed in the electrolyte are called it "primary"
Cells. In these cells it is often impossible, and always
unsatisfactory to reverse the chemical action as explained above.
Cells whose chemical actions are reversible are called "storage" or
"secondary" cells. In the "storage" cells used today, a current must
first be sent through the cell in order to cause the chemical changes
which result in putting the electrodes and electrolyte, in such a
condition that they will be capable of producing an electrical
pressure when the chemical changes caused by the current are complete.
The cell now possesses all the characteristics of a primary cell, and
may be used as a source of electricity until "discharged." It may then
be "charged" again, and so on, the chemical action in one case causing
a flow of current, and a reversed flow of current causing reversed
chemical actions.
We see from the above that the "storage" battery does not "store"
electricity at all, but changes chemical into electrical energy when
"discharging," and changes electrical into chemical energy when
"charging," the two actions being entirely reversible. The idea of
"storing" electricity comes from the fact that if we send a current of
electricity through the cell for a certain length of time, we can at a
later time draw a current from the cell for almost the same length of
time.
[Fig. 3 Complete Element]
Fig. 3. A complete element, consisting of a positive and negative
group of plates and separators ready for placing in the hard rubber
jars.
Three things are therefore required in a storage cell, the liquid or
"electrolyte" and two unlike substances or electrodes, through which a
current of electricity can pass and which are acted upon by the
electrolyte with a chemical action that is greater for one substance
than the other. In the storage cell used on the automobile today for
starting and lighting, the electrodes are lead and peroxide of lead,
and the electrolyte is a mixture of sulphuric acid and water. The
peroxide of lead electrode is the one upon which the electrolyte has
the greater chemical effect, and it is called the positive or "+"
electrode, because when the battery is sending a. current through an
external circuit, the current flows from this electrode through the
external circuit, and back to the lead electrode, which is called the
negative, or electrode.
When starting and lighting systems were adopted in 1912, storage
batteries had been used for many years in electric power stations.
These were, however, large and heavy, and many difficult problems of
design had to be solved in order to produce a battery capable of
performing the work of cranking the engine, and yet be portable,
light, and small enough to occupy only a very limited space on the
automobile. As a result of these conditions governing the design, the
starting and lighting battery of today is in reality "the giant that
lives in a box." The Electric Storage Battery Company estimates that
one of its types of batteries, which measures only 12-5/8 inches long,
7-3/8 wide, and 9-1/8 high, and weighs only 63-1/2 pounds, can deliver
enough energy to raise itself to a height of 6 miles straight up in
the air. It must be able to do its work quickly at all times, and in
all sorts of weather, with temperatures ranging from below 0° to 100°
Fahrenheit, or even higher.
The starting and lighting battery has therefore been designed to
withstand severe operating conditions. Looking at such a battery on a
car we see a small wooden box in which are placed three or more
"cells," see Fig. 1. Each "cell" has a hard, black rubber top through
which two posts of lead project. Bars of lead connect the posts of one
cell to those of the next. To one of the posts of each end cell is
connected a cable which leads into the car, and through which the
current leaves or enters the battery. At the center of each cell is a
removable rubber plug covering an opening through which communication
is established with the inside of the cell for the purpose of pouring
in water, removing some of the electrolyte to determine the condition
of the battery, or to allow gases formed within the cell to escape.
Looking down through this opening we can see the things needed to form
a storage battery: the electrolyte, and the electrodes or "plates" as
they are called. If we should remove the lead bars connecting one cell
to another, and take off the black cover, we should find that the
posts which project out of the cells are attached to the plates which
are broad and flat, and separated by thin pieces of wood or rubber.,
If we lift out the plates we find that they are connected alternately
to the two lead posts, and that the two outside ones have a gray
color. If we pull the plates out from each Other, we find that the
plates next to the two outside ones, and all other plates connected to
the same lead post as these have a chocolate-brown color. If we remove
the jar of the cell, we find that it is made of hard rubber. Pouring
out the electrolyte we find several ridges which hold the plates off
the bottom of the jar. The pockets formed by these ridges may contain
some soft, muddy substance. Thus we have exposed all the elements of a
cell, posts, plates, "separators," and electrolyte. The gray colored
plates are attached to the "negative" battery post, while the
chocolate-brown colored ones are connected to the "positive" battery
post. Examination will show that each of the plates consists of a
skeleton metallic framework which is filled with the brown or gray
substances. This construction is used to decrease the weight of the
battery. The gray filler material is pure lead in a condition called
"spongy lead." The chocolate-brown filler substance is peroxide of
lead.
We have found nothing but two sets of plates--one of pure lead, the
other of peroxide of lead, and the electrolyte of sulphuric acid and
water. These produce the heavy current necessary to crank the engine.
How this is done, and what the chemical actions within the cell are,
are described in Chapter 4.
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