through the cell from the
positive to the negative plates. This is called "charging" the cell.
The lead sulphate and water will now gradually be changed back into
lead, lead peroxide, and sulphuric acid. The lead sulphate which is on
the negative plate is changed to pure lead; the lead sulphate on the
positive plate is changed to lead peroxide, and sulphuric acid will be
added to the water. The changes at the positive plate may be
represented as follows:

Lead sulphate and water produce sulphuric acid, hydrogen and lead
peroxide, or:

[Image] Formula (d). PbSO4 + 2H2O = PbO2 + H2SO4 + H2

The changes at the negative plate may be expressed as follows: Lead
sulphate and water produced sulphuric acid, oxygen, and lead, or:

[Image] Formula (e). PbSO4 + H2o = Pb + H2SO4 + O

The hydrogen (H2) produced at the positive plate, and the oxygen (0)
produced at the negative plate unite to form water, as may be shown by
the equation:

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

Equation (f) starts with lead sulphate and water, which, as shown in
equation (c), are produced when a battery is discharged. It will be
observed that we start with lead sulphate and water. Discharged plates
may therefore be charged in water. In fact, badly discharged negatives
may be charged better in water than in electrolyte. The electrolyte is
poured out of the battery and distilled water poured in. The acid
remaining on the separators and plates is sufficient to make the water
conduct the charging current.

In equation (f), the sulphate on the plates combines with water to
form sulphuric acid. This gives us the rule:

During charge, acid is driven out of the plates.

This rule is a convenient one, but, of course, is not a strictly
correct statement.

The changes produced by sending a current through the cell are also
gradual, and will take place faster as the current is made greater.
When all the lead sulphate has been used up by the chemical changes
caused by the current, no further charging can take place. If we
continue to send a current through the cell after it is fully charged,
the water will continue to be split up into hydrogen and oxygen.
Since, however, there is no more lead sulphate left with which the
hydrogen and oxygen can combine to form lead, lead peroxide, and
sulphuric acid, the hydrogen and oxygen rise to the surface of the
electrolyte and escape from the cell. This is known as "gassing", and
is an in