CAPACITY OF STORAGE BATTERIES.
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The capacity of a storage battery is the product of the current drawn
from a battery, multiplied by the number of hours this current flows.
The unit in which capacity is measured is the ampere-hour.
Theoretically, a battery has a capacity of 40 ampere hours if it
furnishes ten amperes for four hours, and if it is unable, at the end
of that time, to furnish any more current. If we drew only five
amperes from this battery, it should be able to furnish this current
for eight hours. Thus, theoretically, the capacity of a battery should
be the same, no matter what current is taken from it. That is, the
current in amperes, multiplied by the number of hours the battery,
furnished this current should be constant.
In practice, however, we do not discharge a battery to a lower voltage
than 1.7 per cell, except when the rate of discharge is high, such as
is the case when using the starting motor, on account of the
increasing amount of sulphate and the difficulty with which this is
subsequently removed and changed into lead and lead peroxide. The
capacity of a storage battery is therefore measured by the number of
ampere hours it can furnish before its voltage drops below 1.7 per
cell. This definition assumes that the discharge is a continuous one,
that we start with a fully charged battery and discharge it
continuously until its voltage drops to 1.7 per cell.
The factors upon which the capacity of storage batteries depend may be
grouped in two main classifications:
1. Design and Construction of Battery
2. Conditions of Operation
Design and Construction.
Each classification may be subdivided. Under the Design and
Construction we have:
(a) Area of plate surface.
(b) Quantity, arrangement, and porosity of active materials.
(c) Quantity and strength of electrolyte.
(d) Circulation of electrolyte.
These sub-classifications require further explanation. Taking them in
order:
(a) Area of Plate Surface. It is evident that the chemical and
electrical activity of a battery are greatest at the surface of the
plates since the acid and active material are in intimate contact
here, and a supply of fresh acid is more readily available to replace
that which is depleted as the battery is discharged. This is
especially true with high rates of discharge, such as are caused in
starting automobile engines. Therefore, the capacity of a battery will
be greater if the surface area of its plates is increased. With large
plate areas a greater amount of acid and active materials is
available, and an increase in capacity results.
(b) Quantity, Arrangement, and Porosity of Active Materials. Since the
lead and lead peroxide are changed to lead sulphate on discharge, it
is evident that the greater the amount of these materials, the longer
can the discharge continue, and hence the greater the capacity.
The arrangement of the active materials is also important, since the
acid and active materials must be in contact in order to produce
electricity. Consequently the capacity will be greater in a battery,
all of whose active materials are in contact with the acid, than in
one in which the acid reaches only a portion of the active materials.
It is also important that all parts of the plates carry the same
amount of current, in order that the active materials may be used
evenly. As a result of these considerations, we find that the active
materials are supported on grids of lead, that the plates are made
thin, and that they have large surface areas. For heavy discharge
currents, such as starting motor currents, it is essential that there
be large surface areas. Thick plates with smaller surface areas are
more suitable for low discharge rates.
Since the inner portions of the active materials must have a plentiful
and an easily renewable supply of acid, the active materials must be
porous in order that diffusion may be easy and rapid.
(c) Quantity and Strength of Electrolyte. It is important that there
be enough electrolyte in order that the acid may not become exhausted
while there is still considerable active material left. An
insufficient supply of electrolyte makes it impossible to obtain the
full capacity from a battery. On the other hand, too much electrolyte,
due either to filling the battery too full, or to having the plates in
a jar that holds too much electrolyte, results in an increase in
capacity up to the limit of the plate capacity. There is a danger
present, however, because with an excess of electrolyte the plates
will be discharged before the specific gravity of the electrolyte
falls to 1.150. This results in over discharge of the battery with its
attendant troubles as will be described more fully in a later chapter.
It is a universal custom to consider a battery discharged when the
specific gravity of the electrolyte has dropped to 1.150, and that it
is fully charged when the specific gravity of the electrolyte has
risen to 1.280-1.300. This is true in temperate climates. In tropical
countries, which may for this purpose be defined as those countries in
which the temperature never falls below the freezing point, the
gravity of a fully charged cell is 1.200 to 1.230. The condition of
the plates is, however, the true indicator of charged or discharged
condition. With the correct amount of electrolyte, its specific
gravity is 1.150 when the plates have been discharged as far as it is
considered safe, and is 1.280-1.300 when the plates are fully charged.
When electrolyte is therefore poured into a battery, it is essential
that it contains the proper proportion of acid and water in order that
its specific gravity readings be a true indicator of the condition of
the plates as to charge or discharge, and hence show accurately how
much energy remains in the cell at any time.
A question which may be considered at this point is why in automobile,
work a specific gravity of 1.280-1.300 is adopted for the electrolyte
of a fully charged cell. There are several reasons. The voltage of a
battery increases as the specific gravity goes up. Hence, with a
higher density, a higher voltage can be obtained. If the density were
increased beyond this point, the acid would attack the lead grids and
the separators, and considerable corrosion would result. Another
danger of high density is that of sulphation, as explained in a later
chapter. Another factor which enters is the resistance of the
electrolyte. It is desirable that this be as low as possible. If we
should make resistance measurements on various mixtures of acid and
water, we should find that with a small percentage of acid, the
resistance is high. As the amount of acid is increased, the resistance
will grow less up to a certain point. Beyond this point, the
resistance will increase again as more acid is added to the mixture.
The resistance is lowest when the acid forms 30% of the electrolyte.
Thus, if the electrolyte is made too strong, the plates and also the
separators will be attacked by the acid, and the resistance of the
electrolyte will also increase. The voltage increases as the
proportion of acid is increased, but the other factors limit the
concentration. If the electrolyte is diluted, its resistance rises,
and the amount of acid is insufficient to give much capacity. The
density of 1.280-1.300 is therefore a compromise between the various
factors mentioned above.
(d) Circulation of Electrolyte. This refers to the passing of
electrolyte from one plate to another, and depends upon the ease with
which the acid can pass through the pores of the separators. A porous
separator allows more energy to be drawn from the battery than a
nonporous one.
Operating Conditions.
Considering now the operating conditions, we find several items to be
taken into account. The most important are:
(e) Rate of discharge.
(f) Temperature.
(e) Rate of Discharge. As mentioned above, the ampere hour rating of a
battery is based upon a continuous discharge, starting with a specific
gravity of 1.280-1.300, and finishing with 1.150. The end of the
discharge is also considered to be reached when the voltage per cell
has dropped to 1.7. With moderate rates of discharge the acid is
abstracted slowly enough to permit the acid from outside the plates to
diffuse into the pores of the plates and keep up the supply needed for
the chemical actions. With increased rates of discharge the supply of
acid is used up so rapidly that the diffusion is not fast enough to
hold up the voltage. This fact is shown clearly by tests made to
determine the time required to discharge a 100 Amp. Hr., 6 volt
battery to 4.5 volts. With a discharge rate of 25 amperes, it required
160 minutes. With a discharge rate of 75 amperes, it required 34
minutes. From this we see that making the discharge rate three times
as great caused the battery to be discharged in one fifth the time.
These discharges were continuous, however, and if the battery were
allowed to rest, the voltage would soon rise sufficiently, to burn the
lamps for a number of hours.
The conditions of operation in automobile work are usually considered
severe. In starting the engine, a heavy current is drawn from the
battery for a few seconds. The generator starts charging the battery
immediately afterward, and the starting energy is soon replaced. As
long as the engine runs, there is no load on the battery, as the
generator will furnish the current for the lamps, and also send a
charge into the battery. If the lamps are not used, the entire
generator output is utilized to charge the battery, unless some
current is furnished to the ignition system. Overcharge is quite
possible.
When the engine is not running, the lamps are the only load on the
battery, and there is no charging current. Various drivers have
various driving conditions. Some use their starters frequently, and
make only short runs. Their batteries run down. Other men use the
starter very seldom, and take long tours. Their batteries will be
overcharged. The best thing that can be done is to set the generator
for an output that will keep the battery charged under average
conditions.
From the results of actual tests, it may be said that modem lead-acid
batteries are not injured in any way by the high discharge rate used
when a starting motor cranks the engine. It is the rapidity with which
fresh acid takes the place of that used in the pores of the active
materials that affects the capacity of a battery at high rates, and
not only limitation in the plates themselves. Low rates of discharge
should, in fact, be avoided more than the high rates. Battery capacity
is affected by discharge rates, only when the discharge is continuous,
and the reduction in capacity caused by the high rates of continuous
discharge does not occur if the discharge is an intermittent one, such
as is actually the case in automobile work. The tendency now is to
design batteries to give their rated capacity in very short discharge
periods. If conditions should demand it, these batteries would be sold
to give their rated capacity while operating intermittently at a rate
which would completely discharge them in three or four minutes. The
only change necessary for such high rates of discharge is to provide
extra heavy terminals to carry the heavy current.
The present standard method of rating starting and lighting batteries,
as recommended by the Society of Automotive Engineers, is as follows:
"Batteries for combined lighting and starting service shall have two
ratings. The first shall indicate the lighting ability, and shall be
the capacity in ampere hours of the battery when discharged
continuously at the 5 hour rate to a final voltage of not less than
1.7 per cell, the temperature of the battery beginning such discharge
being 80°F. The second rating shall indicate the starting ability and
shall be the capacity in ampere-hours when the battery is discharged
continuously at the 20-minute rate to a final voltage of not less than
1.5 per cell, the temperature of the battery beginning such discharge
being 80°F."
The discharge rate required under the average starting conditions is
higher than that specified above, and would cause the required drop in
voltage in about fifteen minutes. In winter, when an engine is cold
and stiff, the work required from the battery is even more severe, the
discharge rate being equivalent in amperes to probably four or five
times the ampere-rating of the battery. On account of the rapid
recovery of a battery after a discharge at a very high rate, it seems
advisable to allow a battery to discharge to a voltage of 1.0 per cell
when cranking an engine which is extremely cold and stiff.
(f) Temperature. Chemical reactions take place much more readily at
high temperatures than at low. Furthermore, the active materials are
more porous, the electrolyte lighter, and the internal resistance less
at higher temperatures. Opposed to this is the fact that at high
temperatures, the acid attacks the grids and active materials, and
lead sulphate is formed, even though no current is taken from the
battery. Other injurious effects are the destructive actions of hot
acid on the wooden separators used in most starting and lighting
batteries. Greater expansion of active material will also occur, and
this expansion is not, in general, uniform over the surface of the
plates. This results in unequal strains and the plates are bent out of
shape, or "buckled." The expansion of the active material will also
cause much of it to fall from the plates, and we then have "shedding."
[Fig. 25 Graph: Theoretical temperature changes during charge
and discharge]
When sulphuric acid is poured into water, a marked temperature rise
takes place. When a battery is charged, acid is formed, and when this
mixes with the diluted electrolyte, a temperature rise occurs. In
discharging, acid is taken from the electrolyte, and the temperature
has a tendency to drop. On charging, therefore, there is danger of
overheating, while on discharge, excessive temperatures are not
likely. Fig. 25 shows the theoretical temperature changes on charge
and discharge. The decrease in temperature given-in the curve is not
actually obtained in practice, because the tendency of the temperature
to decrease is balanced by the heat caused by the current passing
through the battery.
Age of Battery.
Another factor which should be considered in connection with capacity
is the age of the battery. New batteries often do not give their rated
capacity when received from the manufacturer. This is due to the
methods of making the plates. The "paste" plates, such as are used in
automobiles, are made by applying oxides of lead, mixed with a liquid,
which generally is dilute sulphuric acid, to the grids. These oxides
must be subjected to a charging current in order to produce the spongy
lead and lead peroxide. After the charge, they must be discharged, and
then again charged. This is necessary because not all of the oxides
are changed to active material on one charge, and repeated charges and
discharges are required to produce the maximum amount of active
materials. Some manufacturers do not charge and discharge a battery a
sufficient number of times before sending it out, and after a battery
is put into use, its capacity will increase for some time, because
more active material is produced during each charge.
Another factor which increases the capacity of a battery after it is
put into use is the tendency of the positive active material to become
more porous after the battery is put through the cycles of charge and
discharge. This results in an increase in capacity for a considerable
time after the battery is put into use.
When, a battery has been in use for some time, a considerable portion
of the active material will have fallen from the positive plates, and,
a decrease in capacity will result. Such a battery will charge faster
than a new one because the amount of sulphate which has formed when
the battery is discharged is less than in a newer battery. Hence, the
time required to reduce this sulphate will be less, and the battery
will "come up" faster on charge, although the specific gravity of the
electrolyte may not rise to 1.280.
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