WHAT TAKES PLACE DURING DISCHARGE.
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Considered chemically, the discharge of a storage battery consists of
the changing of the spongy lead and lead peroxide into lead sulphate,
and the abstraction of the acid from the electrolyte. Considered
electrically, the changes are more complex, and require further
investigation. The voltage, internal resistance, rate of discharge,
capacity, and other features must be considered, and the effects of
changes in one upon the others must be studied. This proceeding is
simplified considerably if we consider each point separately. The
abstraction of the acid from the electrolyte gives us a method of
determining the condition of charge or discharge in the battery, and
must also be studied.
[Fig. 21 Graph: voltage changes at end and after charge]
Voltage Changes During Discharge. At the end of a charge, and before
opening the charging circuit, the voltage of each cell is about 2.5 to
2.7 volts. As soon as the charging circuit is opened, the cell voltage
drops rapidly to about 2.1 volts, within three or four minutes. This
is due to the formation of a thin layer of lead sulphate on the
surface of the negative plate and between the lead peroxide and the
metal of the positive plate. Fig. 21 shows how the voltage changes
during the last eight minutes of charge, and how it drops rapidly as
soon as the charging circuit is opened. The final value of the voltage
after the charging circuit is opened is about 2.15-2.18 volts. This is
more fully explained in Chapter 6. If a current is drawn from the
battery at the instant the charge is stopped, this drop is more rapid.
At the beginning of the discharge the voltage has already had a rapid
drop from the final voltage on charge, due to the formation of
sulphate as explained above. When a current is being drawn from the
battery, the sudden drop is due to the internal resistance of the
cell, the formation of more sulphate, and the abstracting of the acid
from the electrolyte which fills the pores of the plate. The density
of this acid is high just before the discharge is begun. It is diluted
rapidly at first, but a balanced condition is reached between the
density of the acid in the plates and in the main body of the
electrolyte, the acid supply in the plates being maintained at a
lowered density by fresh acid flowing into them from the main body of
electrolyte. After the initial drop, the voltage decreases more
slowly, the rate of decrease depending on the amount of current drawn
from the battery. The entire process is shown in Fig. 22.
[Fig. 22 Graph: voltage changes during discharge]
Lead sulphate is being formed on the surfaces, and in the body of the
plates. This sulphate has a higher resistance than the lead or lead
peroxide, and the internal resistance of the cell rises, and
contributes to the drop in voltage. As this sulphate forms in the body
of the plates, the acid is used up. At first this acid is easily
replaced from the main body of the electrolyte by diffusion. The acid
in the main body of the electrolyte is at first comparatively strong,
or concentrated, causing a fresh supply of acid to flow into the
plates as fast as it is used up in the plates. This results in the
acid in the electrolyte growing weaker, and this, in turn, leads to a
constant decrease in the rate at which the fresh acid flows, or
diffuses into the plates. Furthermore, the sulphate, which is more
bulky than the lead or lead peroxide fills the pores in the plate,
making it more and more difficult for acid to reach the interior of
the plate. This increases the rate at which the voltage drops.
The sulphate has another effect. It forms a cover over the active
material which has not been acted upon, and makes it practically
useless, since the acid is almost unable to penetrate the coating of
sulphate. We thus have quantities of active material which are
entirely enclosed in sulphate, thereby cutting down the amount of
energy which can be taken from the battery. Thus the formation of
sulphate throughout each plate and the abstraction of acid from the
electrolyte cause the voltage to drop at a constantly increasing rate.
Theoretically, the discharge may be continued until the voltage drops
to zero, but practically, the discharge should be stopped when the
voltage of each cell has dropped to 1.7 (on low discharge rates). If
the discharge is carried on beyond this point much of the spongy lead
and lead peroxide have either been changed into lead sulphate, or have
been covered up by the sulphate so effectively that they are almost
useless. Plates in this condition require a very long charge in order
to remove all the sulphate.
The limiting value of 1.7 volts per cell applies to a continuous
discharge at a moderate rate. At a very high current flowing for only
a very short time, it is not only safe, but advisable to allow a
battery to discharge to a lower voltage, the increased drop being due
to the rapid dilution of the acid in the plates.
The cell voltage will rise somewhat every time the discharge is
stopped. This is due to the diffusion of the acid from the main body
of electrolyte into the plates, resulting in an increased
concentration in the plates. If the discharge has been continuous,
especially if at a high rate, this rise in voltage will bring the cell
up to its normal voltage very quickly on account of the more rapid
diffusion of acid which will then take place.
The voltage does not depend upon the area of the plate surface but
upon the nature of the active materials and the electrolyte. Hence,
although the plates of a cell are gradually being covered with
sulphate, the voltage, measured when no current is flowing, will fall
slowly and not in proportion to the amount of energy taken out of the
cell. It is not until the plates are pretty thoroughly covered with
sulphate, thus making it difficult for the acid to reach the active
material, that the voltage begins to drop rapidly. This is shown
clearly in Fig. 22, which shows that the cell voltage has dropped only
a very small amount when the cell is 50% discharged. With current
flowing through the cell, however, the increased internal resistance
causes a marked drop in the voltage. Open circuit voltage is not
useful, therefore to determine how much energy has been taken from the
battery.
Acid Density. The electrolyte of a lead storage battery is a mixture of
chemically pure sulphuric acid, and chemically pure water, the acid
forming about 30 per cent of the volume of electrolyte when the
battery is fully charged. The pure acid has a "specific gravity" of
1.835, that is, it is 1.835 times as heavy as an equal volume of
water. The mixture of acid and water has a specific gravity of about
1.300. As the cell discharges, acid is abstracted from the
electrolyte, and the weight of the latter must therefore grow less,
since there will be less acid in it. The change in the weight, or
specific gravity of the electrolyte is the best means of determining
the state of discharge of a cell, provided that the cell has been used
properly. In order that the value of the specific gravity may be used
as an indication of the amount of energy in a battery, the history of
the battery must be known. Suppose, for instance, that in refilling
the battery to replace the water lost by the natural evaporation which
occurs in the use of a battery, acid, or a mixture of acid and water
has been used. This will result in the specific gravity being too
high, and the amount of energy in the battery will be less than that
indicated by the specific gravity. Again, if pure water is used to
replace electrolyte which has been spilled, the specific gravity will
be lower than it should be. In a battery which has been discharged to
such an extent that much of the active material has been covered by a
layer of tough sulphate, or if a considerable amount of sulphate and
active material has been loosened from the plates and has dropped to
the bottom of the cells, it will be impossible to bring the specific
gravity of the electrolyte up to 1.300, even though a long charge is
given. There must, therefore, be a reasonable degree of certainty
that a battery has been properly handled if the specific gravity
readings are to be taken as a true indication of the condition of a
battery. Where a battery does not give satisfactory service even
though the specific gravity readings are satisfactory, the latter are
not reliable as indicating the amount of charge in the battery.
As long as a discharge current is flowing from the battery, the acid
within the plates is used up and becomes very much diluted. Diffusion
between the surrounding electrolyte and the acid in the plates keeps
up the supply needed in the plates in order to, carry on the chemical
changes. When the discharge is first begun, the diffusion of acid into
the plates takes place rapidly because there is little sulphate
clogging the pores in the active material, and because there is a
greater difference between the concentration of acid in the
electrolyte and in the plates than will exist as the discharge
progresses. As the sulphate begins to form and fill up the pores of
the plates, and as more and more acid is abstracted from the
electrolyte, diffusion takes place more slowly.
If a battery is allowed to stand idle for a short time after a partial
discharge, the specific gravity of the electrolyte will decrease
because some, of the acid in the electrolyte will gradually flow into
the pores of the plates to replace the acid used up while the battery
was discharging. Theoretically the discharge can be continued until
all the acid has been used up, and the electrolyte is composed of pure
water. Experience has shown, however, that the discharge of the
battery should not be continued after the specific gravity of the
electrolyte has fallen to 1.150. As far as the electrolyte is
concerned, the discharge may be carried farther with safety. The
plates determine the point at which the discharge should be stopped.
When the specific gravity has dropped from 1.300 to 1.150, so much
sulphate has been formed that it fills the pores in the active
material on the plates. Fig. 23 shows the change in the density of the
acid during discharge.
[Fig. 23: Variation of Capacity with Specific Gravity]
Changes at the Negative Plate. Chemically, the action at the negative
plate consists only of the formation of lead sulphate from the spongy
lead. The lead sulphate is only slightly soluble in the electrolyte
and is precipitated as soon as it is formed, leaving hydrogen ions,
which then go to the lead peroxide plate to form water with oxygen
ions released at the peroxide plate. The sulphate forms more quickly
on the surface of the plate than in the inner portions because there
is a constant supply of acid available at the surface, whereas the
formation of sulphate in the interior of the plate requires that acid
diffuse into the pores of the active materials to replace that already
used up in the formation of sulphate. In the negative plate, however,
the sulphate tends to form more uniformly throughout the mass of the
lead, because the spongy lead is more porous than the lead peroxide,
and because the acid is not diluted by the formation of water as in
the positive plate.
Changes at the Positive Plate. In a fully charged positive plate we
have lead peroxide as the active material. This is composed of lead
and oxygen. From this fact it is plainly evident that during discharge
there is a greater chemical activity at this plate than at the
negative plate, since we must find something to combine with the
oxygen in order that the lead may form lead sulphate with the acid.
In an ideal cell, therefore, the material which undergoes the greater
change should be more porous than the material which does not involve
as great a chemical reaction. In reality, however, the peroxide is not
as porous as the spongy lead, and does not hold together as well.
The final products of the discharge of a positive plate are lead
sulphate and water. The lead peroxide must first be reduced to lead,
which then combines with the sulphate from the acid to form lead
sulphate, while the oxygen from the peroxide combines with the
hydrogen of the acid to form water. There is, therefore, a greater
activity at this plate than at the lead plate, and the formation of
the water dilutes the acid in and around the plate so that the
tendency is for the chemical actions to be retarded.
The sulphate which forms on discharge causes the active material to
bulge out because it occupies more space than the peroxide. This
causes the lead peroxide at the surface to begin falling, to the
bottom of the jar in fine dust-like particles, since the peroxide here
holds together very poorly.
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