Exide Technologies Section 92.80 - Manual (Page 19)

Do not use float voltages higher or lower than those recom-
mended. Reduced capacity or battery life will result.
Check and record battery terminal voltage on a regular
basis. Monthly checks are recommended. See Section 15.0,
Records. If battery float voltage is above or below the correct
value, adjust charger to provide proper voltage as measured at
the battery terminals.
13.3 Recharge
All batteries should be recharged as soon as possible following
a discharge with constant voltage chargers. To recharge in the
shortest period of time, raise the charger output voltage to the
highest value which the connected system will permit. Do not
exceed the voltages and times listed in Table E in Section 14.2.
13.4 Determining State-of-Charge
If the normal connected load is constant (no emergency load
connected), the following method can be used to determine the
approximate state-of-charge of the battery. The state-of-charge
can be identified to some degree by the amount of charging cur-
rent going to the battery. When initially placed on charge or
recharge following a discharge, the charging current, read at the
charger ammeter, will be a combination of the load current plus
the current necessary to charge the battery. The current to the
battery will start to decrease and will finally stabilize when the
battery becomes fully charged. If the current level remains con-
stant for three consecutive hours, then this reflects a state-of-
charge of approximately 95 to 98%. For most requirements, the
battery is ready for use.
If the normal connected load is variable (i.e. telecommunications),
the following method may be used to check the state-of-charge of
the battery. Measure the voltage across a pilot cell (See Section
15 for definition of pilot cell). If the voltage is stable for 24 con-
secutive hours, the battery reflects a state of charge of approxi-
mately 95%.
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 cor-
rected) 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 accordance
with the maintenance schedule published in this manual. To
obtain the optimum service life from the battery, it is important to
make sure the battery’s 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 dissi-
pate.
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 applications, 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 telecom 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 causes 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 harmful 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 measurement is
derived using a manufacturer-specific and proprietary 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 place-
ment will prevent the ohmic devices from generating consistent
and meaningful data. The meters work better with monoblocs
and small capacity VRLA products and less well with large
(>800-Ah) VRLA and flooded battery designs. Users should be
particularly skeptical of data taken on series-parallel VRLA bat-
tery configurations as the feedback signal to the device may fol-
low unforeseen paths that can overwhelm it.
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