ng 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 fre