Advertisement
(291)
ELECTRICAL SYSTEMS
BATTERY
10-15
not only with the percentage of acid in the
liquid, it also varies with temperature. As
temperature increases, the electrolyte expands
so that the specific gravity is reduced. As tem-
perature drops, the electrolyte contracts so that
the specific gravity increases. Unless these
variations in specific gravity are taken into
account, the specific gravity obtained by th~
hydrometer may not give a true indication of
the state of charge of battery.
c. Correction for Temperature
Correction can be made for temperature by
adding .004, usually referred to as 4 "points of
gravity," to the hydrometer reading for every
100 F. that the electrolyte is above 80 0 F. or
subtracting .004 for every 100 F. that elec-
trolyte is below 80 0 F. Figure 10-4 shows the
exact correction figure to use for any tempera-
ture above or below 80
0
F., the three steps used
in obtaining the corrected or true specific grav-
ity, and two examples showing how it is
figured.
If
the electrolyte temperature is not too far
from the 80
0
F. standard, or if only an approx-
-
-
.0 32
EXAMPLE
B,
o
CHANGE HYDROMETER
READING BY AMOUNT
SHOWN ON THIS SIDE
OPPOSITE THE
ELECTROLYTE
TEMPERATURE
o
-
-.016
_
-D20
TEMPERATURE
~
0
0
F.
HYDROMETER
READING
=
1.230
-
-
.024
CORRECTION
@
0
0
F.
=
-
.032
-
-
.02
e
TRUE SPECIFIC
GRAVITY
=
1.198
-
-D12
- - D 0 4
- - . 0 0 8
r>«:
SPECIFIC GRAVITY
-
+
.0
.6
CORRECTION
TAKE TEMPERATURE
OF
ELECTROLYTE
WITH THERMOMETER
EXAMPLE
A,
CD
TAKE
SP.
GR.
OF
ELECTROLYTE
WITH HYDROMETER
ELECTROLYTE
~
TEMPERATURE (F)
120·
TEMPERATURE
=
120
0
F.
30·
HYDROMETER
READING
=
1.230
20.
CORRECTION
@
120
0
F.
=
+
.0 16
TRUE SPECIFIC
10·
GRAVITY
=
1.246
10-19 HYDROMETER TEST OF BATTERY
a. What Hydrometer Measures
The hydrometer measures the percentage of
sulphuric acid in the battery electrolyte in
terms of specific gravity. As a battery drops
from a charged to a discharged condition, the
acid leaves the solution and enters the plates,
causing a decrease in specific gravity of electro-
lyte. By measuring specific gravity of the elec-
trolyte with a hydrometer, an indication of the
approximate state of charge of the battery is
obtained.
.In freezing weather the water should be
added just before using the car or otherwise
charging the battery so that the water will be
mixed with the acid before it is allowed to stand
in freezing temperatures.
If
it is found necessary to add water to the
battery more frequently than about every 1,000
miles and the quantity of water added per cell
is great, check setting of voltage regulator and
adjust, if necessary (par. 10-29). Abnormal
water loss is an indication that the battery is
being overcharged.
Water is the only component of the battery
which is lost as the result of charging and dis-
charging, and it must be replaced before the
electrolyte level falls to the tops of the separa-
tors.
If
the water is not replaced and the plates
and separators become exposed, the acid will
reach a dangerously high concentration that
will char and disintegrate the wood separators
and may permanently sulphate and impair the
performance of the plates. Plates cannot take
full part in the battery action unless they are
completely covered by the electrolyte. Separa-
tors are no longer porous in the area that has
once dried out as a result of exposure, there-
fore the corresponding area of adjoining plates
is rendered inactive and subject to continuous
sulphation.
Pure water for use in automotive batteries
should be either distilled water or a good grade
of chemically analyzed drinking water ap-
proved for battery use. Even slightly impure
water is better than no water at all; however,
such water should be used in emergency only.
Battery water should be kept in a covered glass,
earthenware or hard rubber container;
never
use a metallic container.
b. Effect of Temperature
The specific gravity of the electrolyte varies
Figure 10-4-Speciflc Gravity Temperature Corredion Scale
Advertisement
