11
Electrical Systems - 8
2014 Holiday Rambler Travel Trailer
The most accurate method is to test the
electrolyte solution with a battery hydrometer.
Two main styles of battery hydrometers
are available; one type is a cylinder with
graduation marks (preferred), the other uses a
cylinder with floating balls.
Hydrometers can be purchased from most
auto parts stores. The hydrometer tests the
battery’s electrolyte solution measured in
specific gravity. The electrolyte solution is a
mixture of distilled water and sulfuric acid.
Distilled water has a specific gravity reading of
1,000.
The hydrometer is calibrated to this mark at
80º F. Pure sulfuric acid has a specific gravity
of 1,840. The acid is 1.84 times heavier than
water. The electrolyte solution is about 64%
water to 36% acid (fully charged battery). A
fully charged battery at 80º F., has a specific
gravity reading of 1265 per cell.
Hydrometers with cylinder graduation are
graphed and the exact state of specific gravity
can be determined.
Draw the electrolyte solution from a battery
cell into the hydrometer. Note the reading
for that cell. Complete the same test for the
rest of the cells on that battery. Temperature
affects the hydrometer readings, the higher the
electrolyte temperature, the higher the specific
gravity reading.
A lower ambient temperature will have a
lower the specific gravity reading. Add or
subtract four points for each 10° variance from
the 80° F. chart. Readings between cells should
not vary more than 50 points.
Temperature and recent battery activity
(charging or discharging) affect hydrometer
readings. It is best to check the battery
when it has been at rest for three hours,
although readings taken at other times will be
approximate. If one cell in the battery bank
is at a 50% state of charge and the other cells
indicate nearly fully charged, charge only that
battery to see if the low cell will rise in specific
gravity but at the same time do not over-charge
the healthy cells.
If the specific gravity reading of the bad
cell does not rise after charging, the low cell
can drain the rest of the battery bank and that
battery should be replaced. An accurate digital
Volt meter + - .5% will also give an indicator
of the battery’s state of charge.
NOTE:
See the chart for temperature
compensation. Liquid levels should
be even between the cells of the
battery being tested as it will affect the
accuracy of the test.
Placing a load on the Battery:
Another test that can be performed is to
place a specific load on the battery for a
predetermined length of time equal to that
particular battery’s rating. This machine is
usually an adjustable carbon pile that can vary
the load being applied to the batteries while
monitoring voltage to see if they will perform
to their specific rated capacities.
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