at the chemical and
electrical activity of a battery are greatest at the surface of the
plates since the acid and active material are in intimate contact
here, and a supply of fresh acid is more readily available to replace
that which is depleted as the battery is discharged. This is
especially true with high rates of discharge, such as are caused in
starting automobile engines. Therefore, the capacity of a battery will
be greater if the surface area of its plates is increased. With large
plate areas a greater amount of acid and active materials is
available, and an increase in capacity results.

(b) Quantity, Arrangement, and Porosity of Active Materials. Since the
lead and lead peroxide are changed to lead sulphate on discharge, it
is evident that the greater the amount of these materials, the longer
can the discharge continue, and hence the greater the capacity.

The arrangement of the active materials is also important, since the
acid and active materials must be in contact in order to produce
electricity. Consequently the capacity will be greater in a battery,
all of whose active materials are in contact with the acid, than in
one in which the acid reaches only a portion of the active materials.
It is also important that all parts of the plates carry the same
amount of current, in order that the active materials may be used
evenly. As a result of these considerations, we find that the active
materials are supported on grids of lead, that the plates are made
thin, and that they have large surface areas. For heavy discharge
currents, such as starting motor currents, it is essential that there
be large surface areas. Thick plates with smaller surface areas are
more suitable for low discharge rates.

Since the inner portions of the active materials must have a plentiful
and an easily renewable supply of acid, the active materials must be
porous in order that diffusion may be easy and rapid.

(c) Quantity and Strength of Electrolyte. It is important that there
be enough electrolyte in order that the acid may not become exhausted
while there is still considerable active material left. An
insufficient supply of electrolyte makes it impossible to obtain the
full capacity from a battery. On the other hand, too much electrolyte,
due either to filling the battery too full, or to having the plates in
a jar that holds too much electrolyte, results in an increase in
capacity up to the limit of the plate capacity. There is a