WHAT TAKES PLACE DURING CHARGE.
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Voltage. Starting with a battery which has been discharged until its
voltage has decreased to 1.7 per cell, we pass a current through it
and cause the voltage to rise steadily. Fig. 24 shows the changes in
voltage during charge. Ordinarily 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 glass syringe having a short length of
rubber tubing at one end, and a large rubber bulb at the other. The
rubber tube is inserted in the cell and enough electrolyte drawn up
into the syringe to float the hydrometer so as to be able to obtain a
reading. This subject will be treated more fully in a later chapter.
Changes at Negative Plate. The charging current changes lead sulphate
into spongy lead, and acid is formed. The acid is mixed with the
diluted electrolyte outside of the plates. As the charging proceeds
the active material shrinks or contracts, and the weight of the plate
actually decreases on account of the difference between the weight and
volume of the lead sulphate and spongy lead. If the cell has had only
a normal discharge and the charge is begun soon after the discharge
ended, the charge will proceed quickly and without an excessive rise
in temperature. If, however, the cell has been discharged too far, or
has been in a discharged condition for some time, the lead sulphate
will not be in a finely divided state as it should be, but will be
hard and tough and will have formed an insulating coating over the
active material, causing the charging voltage to be high, and the
charge will proceed slowly. When most of the lead sulphate has been
reduced to spongy lead, the charging current will be greater than is
needed to carry on the chemical actions, and will simply decompose the
water into hydrogen and oxygen, and the cell "gasses." Spongy lead is
rather tough and coherent, it, and the bubbles of gas which form in
the pores of the negative plate near the end of the charge force their
way to the surface without dislodging any of the active material.
Changes at the Positive Plate. When a cell has been discharged, a
portion of the lead peroxide has been changed to lead sulphate, which
has lodged in the pores of the active material and on its surface.
During charge, the lead combines with oxygen from the water to form
lead peroxide, and acid is formed. This acid diffuses into the
electrolyte as fast as the amount of sulphate will permit. If the
discharge has been carried so far that a considerable amount of
sulphate has formed in the pores and on the surface of the plate, the
action proceeds very slowly, and unless a moderate charging current is
used, gassing begins before the charge is complete, simply because the
sulphate cannot absorb the current. The gas bubbles which originate in
the interior of the plate force their way to the surface, and in so
doing cause numerous fine particles of active material to break off
and fall to the bottom of the jar. This happens because the lead
peroxide is a granular, non-coherent substance, with the particles
held together very loosely, and the gas breaks off a considerable
amount of active material.
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