Battery caps either screw or snap into the openings
in the battery cover. The battery caps (vent plugs)
allow gas to escape and prevent the electrolyte from
splashing outside the battery. They also serve as spark
arresters (keep sparks or flames from igniting the gases
inside the battery). The battery is filled through the
vent plug openings. Maintenance-free batteries have a
large cover that is not removed during normal service.
Hydrogen gas can collect at the top of a
battery. If this gas is exposed to a flame or
spark, it can explode.
provide a means of connecting the battery plates to the
electrical system of the vehicle. Either two round post
or two side terminals can be used.
Battery terminals are round metal posts extending
through the top of the battery cover. They serve as
connections for battery cable ends. Positive post will
be larger than the negative post. It may be marked with
red paint and a positive (+) symbol. Negative post is
smaller, may be marked with black or green paint, and
has a negative (-) symbol on or near it.
Side terminals are electrical connections located
on the side of the battery. They have internal threads
that accept a special bolt on the battery cable end. Side
terminal polarity is identified by positive and negative
symbols marked on the case.
ELECTROLYTE. The electrolyte solution in a
fully charged battery is a solution of concentrated
sulfuric acid in water. This solution is about 60 percent
water and about 40 percent sulfuric acid.
The electrolyte in the lead-acid storage battery has
a specific gravity of 1.28, which means that it is 1.28
times as heavy as water. The amount of sulfuric acid in
the electrolyte changes with the amount of electrical
charge; also the specific gravity of the electrolyte
changes with the amount of electrical charge. A fully
charged battery will have a specific gravity of 1.28 at
80°F. The figure will go higher with a temperature
decrease and lower with a temperature increase.
As a storage battery discharges, the sulfuric acid is
depleted and the electrolyte is gradually converted into
water. This action provides a guide in determining the
state of discharge of the lead-acid cell. The electrolyte
that is placed in a lead-acid battery has a specific
gravity of 1.280.
The specific gravity of an electrolyte is actually the
measure of its density. The electrolyte becomes less
dense as its temperature rises, and a low temperature
means a high specific gravity. The hydrometer that you
use is marked to read specific gravity at 80°F only.
Under normal conditions, the temperature of your
electrolyte will not vary much from this mark.
However, large changes in temperature require a
correction in your reading.
For EVERY 10-degree change in temperature
ABOVE 80°F, you must ADD 0.004 to your specific
gravity reading. For EVERY 10-degree change in
temperature BELOW 80°F, you must SUBTRACT
0.004 from your specific gravity reading. Suppose you
have just taken the gravity reading of a cell. The
hydrometer reads 1.280. A thermometer in the cell
indicates an electrolyte temperature of 60°F. That is a
normal difference of 20 degrees from the normal of
80°F. To get the true gravity reading, you must subtract
0.008 from 1.280. Thus the specific gravity of the cell
is actually 1.272. A hydrometer conversion chart
similar to the one shown in figure 2-4 is usually found
on the hydrometer. From it, you can obtain the specific
gravity correction for temperature changes above or
Figure 2-4.Hydrometer conversion chart.