Electrolyte. In a starting battery the specific gravity of the
electrolyte of a fully charged cell is 1.280-1.300, no matter what the
make of the battery may be. In farm lighting batteries, the different
types have different values of specific gravity when fully charged.
The usual values are as follows:

(a) Batteries with sealed glass jars 1.210 to 1.250

(b) Batteries with open glass jars 1.200 to 1.250

(c) Batteries with sealed rubber jars 1.260 to 1.280

A brief discussion of specific gravity might be helpful at this point.
In any lead acid battery current is produced by a chemical action
between the active material in the plates and the water and sulphuric
acid in the electrolyte. The amount of energy which can be delivered
by the battery depends on the amount of active material, sulphuric
acid, and water which enter into the chemical actions of the cell. As
these chemical actions take place, sulphuric acid is used up, and
hence there must be enough acid contained in the electrolyte to enter
into the chemical actions. The amount of water and acid in the
electrolyte may be varied, as long as there is enough of each present
to combine with the active material of the plates so as to enable the
cell to deliver its full capacity. Increasing the amount of acid will
result in the plates and separators being attacked and injured by the
acid. Increasing the amount of water dilutes the acid, giving a lower
gravity, and preventing the Acid from injuring plates and separators.
This results in a longer life for the battery, and is a desirable
condition. In starter batteries, there is not enough space in the jars
for the increased amount of water. In farm lighting batteries, where
the space occupied by the battery is not so important, the jars are
made large enough to hold a greater amount of water, thus giving an
electrolyte which has a lower specific gravity than in starting
batteries.

Take a fully charged cell of any starting battery. It contains a set
of plates and the electrolyte which is composed of a certain necessary
amount of acid and a certain amount of water. If we put the plates of
this cell in a larger jar, add the same amount of acid as before, but
add a greater amount of water than was contained in the smaller jar,
we will still have a fully charged cell of the same capacity as
before, but the specific gravity of the electrolyte will be lower.

Charging Equipment. Automobile batteries are being charged