te. When this cell is fully
charged, or in a condition to produce a current of electricity, the
positive plate is made up of peroxide of lead (PbO2), the negative
plate of pure lead (Pb), and the electrolyte of dilute sulphuric acid
(H 2SO4). This is shown diagrammatically in Fig. 19. The chemical
changes that take place when the cell is discharging and the final
result of the changes are as follows:

(a). At the Positive Plate: Lead peroxide and sulphuric acid produce
lead sulphate, water, and oxygen, or:

[Image] Formula (a). PbO2 + H2SO4 = PbSO4 + H20 + 0

(b). At the Negative Plate: Lead and sulphuric acid produce lead
sulphate and Hydrogen, or:

[Image] Formula (b). Pb + H2SO4 = PbSO4 + H2

[Fig. 20 Chemical Reaction in a Storage Cell during Discharge]

The oxygen of equation (a) and the hydrogen of equation (b) combine to
form water, as may be shown by adding these two equations, giving one
equation for the entire discharge action:

[Image] Formula (c). PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O

In this equation we start with the active materials and electrolyte in
their original condition, and finish with the lead sulphate and water,
which are the final products of a discharge. Examining this equation,
we see that the sulphuric acid of the electrolyte is used up in
forming lead sulphate on both positive and negative plates, and is
therefore removed from the electrolyte. This gives us the easily
remembered rule for remembering discharge actions, which, though open
to question from a strictly scientific viewpoint, is nevertheless
convenient:

During discharge the acid goes into the plates.

The chemical changes described in (a), (b), and (c) are not
instantaneous. That is, the lead, lead peroxide, and sulphuric acid of
the fully charged cell are not changed into lead sulphate and water as
soon as a current begins to pass through the cell. This action is a
gradual one, small portions of these substances being changed at a
time. The greater the current that flows through the cell, the faster
will the changes occur. Theoretically, the changes will continue to
take place as long as any lead, lead peroxide, and sulphuric acid
remain. The faster these are changed into lead sulphate and water, the
shorter will be the time that the storage cell can furnish a current,
or the sooner it will be discharged.

Taking the cell in its discharged condition, let us now connect the
cell to a generator and send current