was 2.25 v/c, the ground fault would be
approximately 60 cells (ten 12 volt units) from
the battery system output terminal.
Battery System
Float Charging Current
If the DC float current can be measured, it can
provide an indication of the proper current
acceptance of the battery system. Depending on
the charging voltage per string and the temperature,
the float current per string should be approximately
that shown in Figure 3. The float current will
approximately double for each 18°F (10°C)
above 77°F (25°C).
If the DC float current is zero there is an open
circuit in the battery string. If the float current is
higher than anticipated, it may be due to elevated
temperature of the battery or shorted cells within
the string. In either case the cause should be
determined and corrected. Elevated temperatures
and shorted cells are both situations which can
lead to thermal runaway.
Individual Battery
Float Charging Voltage
While the battery string may be charged at an
average of between 2.25 and 2.3 volts per cell,
ot all cells will float at the exact average voltage.
Each cell has a somewhat different impedance
and rate of oxygen recombination and will therefore
exhibit a slightly different float voltage at the
same float current. For example, all the 12 volt
batteries in a string charged at 2.3 volts per cell
will not float at 13.8 VDC but may vary from 13.3
to 14.5 and still be normal. If the system is equalized
for 24 hours upon installation, or after with an
extended time in float service, this spread in float
voltage will normally decrease.
Refer to table 1, Electrical Characteristics, for
each series battery as listed in the documents
noted in the table of contents, which lists the
minimum and maximum DC float voltages to be
measured across batteries in a series string. If an
individual battery measures too low, it may be an
indication of a shorted cell.
If an individual unit measures too high, it may be
an indication of increased resistance within the
cell. If one unit measures very high while the
balance of the units in the string indicate near the
open circuit value, the high voltage cell may have
an open circuit.
Shorted cells within the string will lead to
increased voltage applied to the remaining
good cells in the string and higher float current.
For example, a 24 cell string charging at 55.2
VDC (2.3 v/c) which has 2 shorted cells will be
charging the remaining 22 cells at 2.5 v/c
(55.2 VDC/22 cells) and the resulting increase
in float current is sure to result in eventual
thermal runaway.
A battery with a shorted or open cell can usually
be confirmed by comparing the impedance of the
individual units or by comparing the AC ripple
voltage measured across the individual units.
DO NOT perform a high rate load test on batteries
that are suspected of having a shorted or open
cell. This would be hazardous since a spark
internal to the cell could ignite the internal gases.
A battery suspected of having a shorted or open
cell should be removed and replaced immediately.
More information concerning the measurement
and interpretation of individual battery float
voltages is contained in the Technical Bulletin
"Integrity Testing" #41-7264.
0
1
2
3
4
5
2.15
2.2
2.25
2.3
2.35
2.4
2.45
2.5
M
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ia
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pe
re
F
lo
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C
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P
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A
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Figure 3 - Float Current Vs. Voltage
Figure 3-Float Current Vs. Voltage
41-7546/0213/CD 9 www.cdtechno.com
