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13.5 Effects of Float Voltage
Float voltage has a direct effect on the service
life of your battery and can be the cause of
thermal instability.
A float voltage above the recommended values reduces
service life. Table D shows the effects of float voltage
(temperature corrected) on battery life.
TABLE D
FLOAT VOLTAGE EFFECTS ON LIFE
Temperature corrected 25°C (77°F) Percent
Float voltage per cell Reduction
Minimum Maximum in Battery Life
2.23 2.25 0%
2.28 2.30 50%
2.33 2.35 75%
Voltage records must be maintained by the user in accor-
dance with the maintenance schedule published in this
manual. To obtain the optimum service life from the bat-
tery, it is important to make sure the batteryʼs float voltage
is within the recommended range.
13.6 Float Current and Thermal Management
Increased float current can portend a condition known as
thermal runaway, where the battery produces more heat
than it can dissipate. VRLA batteries are more prone to
thermal runaway because the recombination reaction that
occurs at the negative plate, and reduces water loss, also
produces heat. High room temperature, improper applica-
tions, improper voltage settings, and incorrect installation
practices can increase the chances of thermal runaway.
As with good record-keeping practices, monitoring float
current can prevent a minor excursion from becoming a
major issue.
13.7 AC Ripple
AC ripple is noise or leftover AC waveform riding on the
DC charge current to the battery that the rectifier did not
remove. It is usually more pronounced in UPS than tele-
com systems. Proper maintenance of the UPS capacitors
will reduce the amount of ripple going into the battery.
Establishment of absolute limits for AC ripple has always
been problematic because the degree of damage it caus-
es depends on the wave shape, peak-to-peak magnitude
and frequency. Accurate characterization of AC ripple
requires an oscilloscope and even then, only represents a
picture of the ripple at that moment in time.
Whatever its exact characteristics,AC ripple is always harm-
ful to batteries. Depending on its particular properties, ripple
can result in overcharge, undercharge and micro-cycling
that can prematurely age the battery. The most common and
damaging result of AC ripple is battery heating which can
lead to thermal runaway. AC ripple will decrease battery life
and should be reduced as much as possible.
13.8 Ohmic Measurements
Impedance, resistance and conductance testing is collectively
known in the industry as ohmic measurements. Each mea-
surement is derived using a manufacturer-specific and propri-
etary algorithm and / or frequency. This means that one type of
measurement cannot be converted or related easily to another.
“Reference” ohmic values are of dubious value because
so many factors can affect the way the readings are made
and displayed by the devices. Connector configuration
and AC ripple as well as differences between readings of
temperature and probe placement will prevent the ohmic
devices from generating consistent and meaningful data.
The meters work better with monoblocs and small capac-
ity VRLA products and less well with large (>800-Ah)
VRLA and flooded battery designs. Users should be par-
ticularly skeptical of data taken on series-parallel VRLA
battery configurations as the feedback signal to the device
may follow unforeseen paths that can overwhelm it.
It is best for users to establish their own baseline values
for their battery as specifically configured. Do not rely on
reference values.
If users wish to enhance normal maintenance and record-
keeping with ohmic measurements, GNB recommends
the trending of this data over time. Use a first set of read-
ings taken 6 months after initial charge and installation as
the baseline data. Because cell positioning within the
string (connector configuration to a particular cell) can
affect the reading, always compare each cell at baseline
to itself in the new data. Standalone ohmic data is not suf-
ficient to justify warranty cell replacement.
Responsible ohmic device manufacturers acknowledge
that there is no direct relationship between percent ohmic
change from baseline and battery capacity. A change from
baseline of 25% or less is in the normal noise or variability
range. Changes between 25% and 50% may call for addi-
tional scrutiny of the system. An IEEE compliant discharge
test is usually warranted on systems exhibiting more than
a 50% change from baseline. Consult a GNB representa-
tive for specific questions about ohmic data.
SECTION 14: EQUALIZING CHARGE
14.0 General
Under normal operating conditions an equalizing charge
is not required. An equalizing charge is a special charge
given a battery when non-uniformity in voltage has devel-
oped between cells. It is given to restore all cells to a fully
charged condition. Use a charging voltage higher than the
normal float voltage and for a specified number of hours,
as determined by the voltage used.
Non-uniformity of cells may result from low float voltage due
to improper adjustment of the charger or a panel voltmeter
which reads an incorrect (higher) output voltage. Also, vari-