Battery System Float
Charging Voltage
The recommended battery system float
charging voltage for the 10 Year VRLA
Batteries with a specific gravity of 1.280 to 1.300
is equal to the number of cells in the system
multiplied by the range of 2.25 to 2.30 volts per
cell at 77°F (25°C). For example, a string of 30
each 12 volt (6 cell) batteries should be float
charged within the range of 405 to 414 VDC
(180 cells x 2.25 v/c minimum and 180 cells x
2.30 v/c maximum) at 77°F (25°C).
When temperature extremes are encountered
the float charging voltage should be
temperature compensated. The temperature
compensation coefficient is -0.0028 v/c per
degree F (-0.005 v/c per degree C).
For example, if the battery normal temperature
is 90°F (13° above 77°F) the average float
charging voltage range should be reduced
0.036 v/c (13° x 0.0028 v/c per °F) to between
2.21 and 2.26 v/c. For a 180 cell battery this
would be 397.8 to 408.6 VDC. This will help
reduce the potential for thermal runaway at
elevated temperatures.
If the battery operates at cold temperatures,
for example 60°F (17° below 77°F), the charging
voltage can be increased to improve recharge time.
For example, the charging voltage range could
be increased by (17° x -0.0028 v/c per °F) or
0.048 v/c. The float voltage range would be
2.298 to 2.348 v/c. For the 180 cell string this
would be 413.6 to 422.6 VDC.
If the battery is undercharged for a period during
which there have been multiple discharges,
the battery will not fully recharge following each
discharge and it will provide progressively lower
capacity. This condition may be correctable with
an extended equalization charge (eg. 48 to 72
hours). However, if the situation has continued
for too long a time, irreversible sulfation of the
plates may have occurred and the battery may
have to be replaced.
Extended overcharging will cause excessive
float current, corrosion of the plate grids,
gassing and drying of the limited amount of electrolyte.
This constitutes premature aging of the battery
and loss of capacity.
Severe overcharging for extended periods can
induce a thermal runaway condition. This would
also necessitate replacement of the battery system.
While measuring the battery system DC float
charging voltage it may also be convenient to
measure the AC ripple voltage appearing across
the battery system. If the AC ripple voltage is a
sinusoidal waveform the maximum reading should
be less than 0.5% Vrms of the DC float voltage.
In the case of 180 cell string floating at 414 VDC,
this is 2.07 Vrms. When measuring the ripple with
an oscilloscope, the maximum p-p value should
be 1.5% of the float voltage or 6.2 Vp-p when
floating at 414 VDC.
Excessive AC ripple voltage across the battery
could cause gassing and heating of the battery
which would result in reduced life.
Battery System
Ground Fault Detection
If the rectifier used to charge the battery has a
ground fault detection capability, the indicator
should be observed to determine the safety of the
system. If a ground fault is indicated, it should be
isolated and corrected prior to further
maintenance on the battery system.
If the rectifier does not have a ground fault
detection circuit, use the digital voltmeter and
measure the voltage from each polarity of the
battery to ground (the grounded rack or cabinet).
A detected voltage would indicate a short or
leakage current from the battery to ground.
The approximate location of the cell with the
ground fault, from the battery system output terminal,
would be the measured voltage divided by the
average per cell charging voltage. For example,
if the measured voltage to ground was 135 VDC
and the charging voltage.
41-7546/0213/CD 8 www.cdtechno.com
