he plates are pretty thoroughly covered with
sulphate, thus making it difficult for the acid to reach the active
material, that the voltage begins to drop rapidly. This is shown
clearly in Fig. 22, which shows that the cell voltage has dropped only
a very small amount when the cell is 50% discharged. With current
flowing through the cell, however, the increased internal resistance
causes a marked drop in the voltage. Open circuit voltage is not
useful, therefore to determine how much energy has been taken from the
battery.

Acid Density. The electrolyte of a lead storage battery is a mixture of
chemically pure sulphuric acid, and chemically pure water, the acid
forming about 30 per cent of the volume of electrolyte when the
battery is fully charged. The pure acid has a "specific gravity" of
1.835, that is, it is 1.835 times as heavy as an equal volume of
water. The mixture of acid and water has a specific gravity of about
1.300. As the cell discharges, acid is abstracted from the
electrolyte, and the weight of the latter must therefore grow less,
since there will be less acid in it. The change in the weight, or
specific gravity of the electrolyte is the best means of determining
the state of discharge of a cell, provided that the cell has been used
properly. In order that the value of the specific gravity may be used
as an indication of the amount of energy in a battery, the history of
the battery must be known. Suppose, for instance, that in refilling
the battery to replace the water lost by the natural evaporation which
occurs in the use of a battery, acid, or a mixture of acid and water
has been used. This will result in the specific gravity being too
high, and the amount of energy in the battery will be less than that
indicated by the specific gravity. Again, if pure water is used to
replace electrolyte which has been spilled, the specific gravity will
be lower than it should be. In a battery which has been discharged to
such an extent that much of the active material has been covered by a
layer of tough sulphate, or if a considerable amount of sulphate and
active material has been loosened from the plates and has dropped to
the bottom of the cells, it will be impossible to bring the specific
gravity of the electrolyte up to 1.300, even though a long charge is
given. There must, therefore, be a reasonable degree of certainty
that a battery has been properly handled if the specific gravity
readings are to be taken a