When taking a hydrometer reading, the base of the hydro-
meter syringe should be pressed firmly against the tube
opening to prevent back splash of electrolyte. Fill and
empty the hydrometer at least once in each cell before read-
ing. This will give a more accurate reading of the average
electrolyte density.
Never inter-mix usage of hydrometers on lead-antimony or
lead-calcium types as cell contamination will result. Assign
hydrometers for exclusive use on one type only.
11.2 Correction for Temperature
When taking specific gravity readings, corrections must be
made for variations in temperature of the electrolyte. For
each 3°F (1.67°C) in temperature of the electrolyte above
77°F (25°C) add one point (.001) in specific gravity to the
observed hydrometer readings; and for each 3°F (1.67°C)
in temperature below 77°F (25°C) subtract one (.001) in
specific gravity from the observed hydrometer reading.
Example:
Reading
Hydrometer
Cell
Corrected to
Reading
Temperature
Correction
77°F (25°C)
1.213 sp. gr 68°F (20°C) -.003 points= 1.210 sp. gr.
1.207 sp. gr. 86°F (30°C) +.003 points= 1.210 sp. gr.
1.204 sp. gr. 95°F (35°C) +.006 points= 1.210 sp. gr.
11.3 Correction for Electrolyte Level
The loss of water from the electrolyte due to evaporation as
well as conversion of the water to hydrogen and oxygen by
charging current also effects the specific gravity value. For
example: A fully charged cell with a correct high level at
77°F (25°C) will have a nominal specific gravity of 1.215.
When the electrolyte level has been reduced from evapora-
tion and charging by 1/4”, the specific gravity will be approx-
imately 6 points (.006) higher or 1.221@ 77°F (25°C).
Therefore when taking hydrometer readings, the electrolyte
level referenced to the high level line should be recorded for
proper evaluation of the specific gravity value. This applies
when taking a pilot cell reading or for 10% of the cells when
taking a quarterly set of readings.
11.4 Specific Gravity Range
The specific gravity may range ± 0.010 points within a bat-
tery for any of the nominal values @ 77°F (25°C) with the
electrolyte level at the high level line and still be considered
satisfactory.
SECTION 12
12.0 Cell Voltage Variation
The tabulation on the following page indicates the normal cell
voltage variation that may exist with the battery on float and
no greater than a 5°F (2.78°C) variation in cell temperature.
NORMAL VOLTAGE RANGE
Average
Type
Float Voltage
Variation
Lead-Calcium
Nominal 1.215 sp. gr. 2.17 to 2.25 V.P.C. ± 0.05 V.P.C.
Nominal 1.250 sp. gr. 2.23 to 2.33 V.P.C. ± 0.05 V.P.C.
Nominal 1.300 sp. gr. 2.28 to 2.37 V.P.C. ± 0.05 V.P.C.
12.1 Cell Voltage Variation
Damp Covers
Cell voltage variation can also be the result of damp cell cover
tops. Spilled electrolyte when taking hydrometer readings can
result in parasitic currents paths across the tops of cell cov-
ers. This reduces the float current through the cell resulting in
voltage variations. See Section 18.0 - Battery Cleaning —to
correct damp cover condition.
12.2 Cell Voltage -
Temperature Correction
To properly analyze cell uniformity within the string, cell volt-
ages should be corrected for cell electrolyte temperature. Cell
voltage variation that may have developed since a previous
quarterly set of readings may be due to cell temperature varia-
tions within the string that may have resulted from a change in
ambient conditions. Therefore, correcting cell voltage for cell
temperature may make it unnecessary to apply an equalizing
charge which otherwise had been believed necessary. See
Section 10.1 - Equalizing Frequency.
12.3 Correction Factor
The temperature correction factor for cell voltage equals 0.003
volts for each degree fahrenheit (0.0055V/C°) using a base
77°F (25°C). The correction is added to the measured cell
voltage above 77°F (25°C) and subtracted below 77°F (25°C).
Example:
Measured cell voltage = 2.19V @ 87°F(30.5°C) cell
temperature. Correction = 10°F x .003V (3.5°C x .0055V) =
.03V. Therefore, corrected cell voltage = 2.19V + .03V = 2.22
volts.
If the cell temperature in the example had been 67°F (19°C),
the correction would be .03 volts which is subtracted from the
measured voltage of 2.19V. The corrected cell voltage = 2.19V
- .03V = 2.16V.
SECTION 13
13.0 Pilot Cell
A pilot cell is selected in the series string to reflect the gen-
eral condition of all cells in the battery regarding specific
gravities, float voltage and temperature. It serves as an
indicator of battery condition between scheduled overall
individual cell readings.
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