INTERNAL RESISTANCE.
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The resistance offered by a storage battery to the flow of a current
through it results in a loss of voltage, and in heating. Its value
should be as low as possible, and, in fact, it is almost negligible
even I in small batteries, seldom rising above 0.05 ohm. On charge, it
causes the charging voltage to be higher and on discharge causes a
loss of voltage. Fig. 26 shows the variation in resistance.
[Fig. 26 Graph: Changes in internal resistance during charge
and discharge]
The resistance as measured between the terminals of a cell is made up
of several factors as follows:
1. Grids. This includes the resistance of the terminals, connecting
links, and the framework upon which the active materials are pasted.
This is but a small part of the total resistance, and does not
undergo any considerable change during charge and discharge. It
increases slightly as the temperature of the grids rises.
2. Electrolyte. This refers to the electrolyte between the plates, and
varies with the amount of acid and with temperature. As mentioned in
the preceding chapter, a mixture of acid and water in which the acid
composes thirty per cent of the electrolyte has the minimum
resistance. Diluting or increasing the concentration of the
electrolyte will both cause an increase in resistance from the minimum
I value. The explanation probably lies in the degree to which the acid
is split up into "ions" of hydrogen (H), and sulphate (SO4). These
"ions" carry the current through t he electrolyte. Starting with a
certain amount of acid, let us see how the ionization progresses. With
very concentrated acid, ionization does not take place, and hence,
there are no ions to carry current. As we mix the acid with water,
ionization occurs. The more water used, the more ions, and hence, the
less the resistance, because the number of ions available to carry the
current increases. The ionization in creases to a certain maximum
degree, beyond which no more ions are formed. It is probable that an
electrolyte containing thirty per cent of acid is at its maximum
degree of ionization and hence its lowest resistance. If more water is
now added, no more ions are formed. Furthermore, the number of ions
per unit volume of electrolyte will now decrease on account of the
increased amount of water. There Will therefore be fewer ions per unit
volume to carry the current, and the resistance of the electrolyte
increases.
With an electrolyte of a given concentration, an increase of
temperature will cause a decrease in resistance. A decrease in
temperature will, of course, cause an increase in resistance. It is
true, in general, that the resistance of the electrolyte is about half
of the total resistance of the cell. The losses due to this resistance
generally form only one per cent of the total losses, and area
practically negligible factor.
3. Active Material. This includes the resistance of the active
materials and the electrolyte in the pores of the active materials.
This varies considerably during charge and discharge. It has been
found that the resistance of the peroxide plate changes much more than
that of the lead plate. The change in resistance of the positive plate
is especially marked near the end of a discharge. The composition of
the active material, and the contact between it and the grid affect
the resistance considerably.
During charge, the current is sent into the cell from an external
source. The girds therefore carry most of the current. The active
material which first reacts with the acid is that near the surface of
the plate, and the acid formed by the charging current mixes readily
with the main body of electrolyte. Gradually, the charging action
takes place in the inner portions of the plate, and concentrated acid
is formed in the pores of the plate. As the sulphate is removed,
however, the acid has little difficulty in mixing with the main body
of electrolyte. The change in resistance on the charge is therefore
not considerable.
During discharge, the chemical action also begins at the surface of
the plates and gradually moves inward. In this case, however, sulphate
is formed on the surface first, and it becomes increasingly difficult
for the fresh acid from the electrolyte to diffuse into the plates so
as to replace the acid which has been greatly diluted there by the
discharge actions. There is therefore an increase in resistance
because of the dilution of the acid at the point of activity. Unless a
cell is discharged too far, however, the increase in resistance is
small.
If a battery is allowed to stand idle for a long time it gradually
discharges itself, as explained in Chapter 10. This is due to the
formation of a tough coating of crystallized lead sulphate, which is
practically an insulator. These crystals gradually cover and enclose
the active material. The percentage change is not high, and generally
amounts to a few per cent only. The chief damage caused by the
excessive sulphation is therefore not an increase in resistance, but
consists chiefly of making a poor contact between active material and
grid, and of removing much of the active material from action by
covering it.
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