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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CHAPTER 9.
CARE OF THE BATTERY ON THE CAR.
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