ily the voltage begins to rise
immediately and uniformly. If, however, the battery has been left in a
discharged condition for some time, or has been "over discharged," the
voltage rises very rapidly for a fraction of the first minute of
charge and then drops rapidly to the normal value and thereafter
begins to rise steadily to the end of the charge. This rise at the
beginning of the charge is due to the fact that the density of the
acid in the pores of the plates rises rapidly at first, the acid thus
formed being prevented from diffusing into the surrounding electrolyte
by the coating of sulphate. As soon as this sulphate is broken
through, diffusion takes place and the voltage drops.

[Fig. 24 Graph: voltage changes during charge]

As shown in Fig. 24, the voltage remains almost constant between the
points M and N. At N the voltage begins to rise because the charging
chemical reactions are taking place farther and farther in the inside
parts of the plate, and the concentrated acid formed by the chemical
actions in the plates is diffusing into the main electrolyte. This
increases the battery voltage and requires a higher charging voltage.

At the point marked 0, the voltage begins to rise very rapidly. This
is due to the fact that the amount of lead sulphate in the plates is
decreasing very rapidly, allowing the battery voltage to rise and thus
increasing the charging voltage. Bubbles of gas are now rising through
the electrolyte.

At P, the last portions of lead sulphate are removed, acid is no
longer being formed, and hydrogen and oxygen gas are formed rapidly.
The gas forces the last of the concentrated acid out of the plates and
in fact, equalizes the acid concentration throughout the whole cell.
Thus no further changes can take place, and the voltage becomes
constant at R at a voltage of 2.5 to 2.7.

Density of Electrolyte. Discharge should be stopped when the density
of the electrolyte, as measured with a hydrometer, is 1.150. When we
pass a charging current through the battery, acid is produced by the
chemical actions which take place in the plates. This gradually
diffuses with the main electrolyte and causes the hydrometer to show a
higher density than before. This increase in density continues
steadily until the battery begins to "gas" freely.

The progress of the charge is generally determined by the density of
the electrolyte. For this purpose in automobile batteries, a
hydrometer is placed in a gla