danger
present, however, because with an excess of electrolyte the plates
will be discharged before the specific gravity of the electrolyte
falls to 1.150. This results in over discharge of the battery with its
attendant troubles as will be described more fully in a later chapter.

It is a universal custom to consider a battery discharged when the
specific gravity of the electrolyte has dropped to 1.150, and that it
is fully charged when the specific gravity of the electrolyte has
risen to 1.280-1.300. This is true in temperate climates. In tropical
countries, which may for this purpose be defined as those countries in
which the temperature never falls below the freezing point, the
gravity of a fully charged cell is 1.200 to 1.230. The condition of
the plates is, however, the true indicator of charged or discharged
condition. With the correct amount of electrolyte, its specific
gravity is 1.150 when the plates have been discharged as far as it is
considered safe, and is 1.280-1.300 when the plates are fully charged.
When electrolyte is therefore poured into a battery, it is essential
that it contains the proper proportion of acid and water in order that
its specific gravity readings be a true indicator of the condition of
the plates as to charge or discharge, and hence show accurately how
much energy remains in the cell at any time.

A question which may be considered at this point is why in automobile,
work a specific gravity of 1.280-1.300 is adopted for the electrolyte
of a fully charged cell. There are several reasons. The voltage of a
battery increases as the specific gravity goes up. Hence, with a
higher density, a higher voltage can be obtained. If the density were
increased beyond this point, the acid would attack the lead grids and
the separators, and considerable corrosion would result. Another
danger of high density is that of sulphation, as explained in a later
chapter. Another factor which enters is the resistance of the
electrolyte. It is desirable that this be as low as possible. If we
should make resistance measurements on various mixtures of acid and
water, we should find that with a small percentage of acid, the
resistance is high. As the amount of acid is increased, the resistance
will grow less up to a certain point. Beyond this point, the
resistance will increase again as more acid is added to the mixture.
The resistance is lowest when the acid forms 30% of the electrolyte.
Thus, if the electrolyt