B
BATTERIES AND BATTERY CHARGER
Read all of SAFETY and this section before attempting any procedure. Pay particular attention to Notices, Cautions, Warnings and Dangers.
19
Repair and Service Manual
10002660
Hydrometer
A hydrometer (P/N 50900G1) is used to test the state of
charge of a battery cell (Ref. Fig. 9). This is performed by
measuring the density of the electrolyte, which is accom-
plished by measuring the specific gravity of the electro-
lyte. The greater the concentration of sulfuric acid, the
more dense the electrolyte becomes. The higher the den-
sity, the higher the state of charge.
To prevent a battery explosion that
could result in severe personal
injury or death. Never insert a metal
thermometer into a battery. Use a hydrometer with a
built in thermometer that is designed for testing bat-
teries.
Fig. 9 Hydrometer
Specific gravity is the measurement of a liquid that is
compared to a baseline. The baseline is water which is
assigned a base number of 1.000. The concentration of
sulfuric acid to water in a new vehicle battery is 1.280
which means that the electrolyte weighs 1.280 times the
weight of the same volume of water. A fully charged bat-
tery will test at 1.275 - 1.280. A discharged battery will
read in the 1.140 range.
NOTICE: Do not perform a hydrometer test on a bat-
tery that has just been filled. The battery must go
through at least one charge and discharge cycle in
order to allow the water to adequately mix with the
electrolyte.
The temperature of the electrolyte is important. The
hydrometer reading must be corrected to 80° F (27° C).
High quality hydrometers are equipped with an internal
thermometer that will measure the temperature of the
electrolyte and will include a conversion scale to correct
the float reading. It is important to recognize that the elec-
trolyte temperature is significantly different from the ambi-
ent temperature if the vehicle has been operated.
The electrolyte in a battery is an
acid solution which can cause
severe burns to the skin and eyes.
Treat all electrolyte contact to the body and eyes
with extended flushing with clear water. Contact a
physician immediately.
Always wear a safety shield or
approved safety goggles when
using a hydrometer to test the elec-
trolyte solution.
Any electrolyte spills should be
neutralized with a solution of 1/4
cup (60 ml) sodium bicarbonate (baking soda) dis-
solved in 1-1/2 gallons (6 liters) of water and then
flushed with water (Ref. Fig. 7).
Using Hydrometer
1.
Draw electrolyte into the hydrometer several times to
permit the thermometer to adjust to the electrolyte
temperature and note the reading. Examine the color
of the electrolyte. A brown or gray color indicates a
problem with the battery and indicates the battery is
nearing the end of its life.
2.
Draw the minimum quantity of electrolyte into the
hydrometer to permit the float to float freely without
contacting the top or bottom of the cylinder.
3.
Hold the hydrometer in a vertical position at eye level
and note the reading where the electrolyte meets the
scale on the float.
4.
Add or subtract four points (.004) to the reading for
every 10° F (6° C) the electrolyte temperature is
above or below 80° F (27° C). Adjust the reading to
conform with the electrolyte temperature, e.g., if the
reading indicates a specific gravity of 1.250 and the
electrolyte temperature is 90° F (32° C), add four
points (.004) to the 1.250 which gives a corrected
reading of 1.254. Similarly if the temperature was
70° F (21° C), subtract four points (.004) from the
1.250 to give a corrected reading of 1.246 (Ref. Fig.
5.
Test each cell and note the readings (corrected to
80° F or 27° C). A variation of fifty points between
any two cell readings (example 1.250 - 1.200) indi-
cates a problem with the low reading cell(s). As a
battery ages the specific gravity of the electrolyte will
decrease at full charge. This is not a reason to
replace the battery, providing all cells are within fifty
points of each other.
Since the hydrometer test is in response to a vehicle
showing a performance problem, the vehicle should be
recharged and the test repeated. If the results indicate a
weak cell, the battery or batteries should be removed and
replaced with a good battery of the same brand, type and
approximate age.
Weight
Float
Bulb
Cylinder
Thermometer
Add to Float
Reading
Subtract
from Float
Reading
