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Summary:
"Checking
batteries isn't as easy as it used to be"
For
many years, users evaluated the condition of a storage battery by
making two measurements. overall or individual open-circuit cell
voltage, and electrolyte specific gravity using a hydrometer. Two
conditions have brought adoption of more sophisticated measurements.
First, liquid electrolyte level and chemistry cannot be monitored in
sealed, "maintenance free" batteries. Individual cell
terminals may or many not be available
Second,
large batteries are being increasingly used in uninterruptible power
supply (UPS) installations. In an emergency calling for sudden
application of heavy load, the battery may be unable to sustain that
load as required, despite exhibiting satisfactory open-circuit
voltage.
Load
or capacity testing has therefore become common practice. One test
method involves taking the battery out of service, substituting a
spare unit to maintain power availability, and connecting a resistive
load bank to draw controlled current and power from the battery being
tested. The test is continued until either a specified time has
elapsed or the battery terminal voltage has fallen to a specified
limit.
Besides
the need for a spare battery (plus the disconnection and reconnection
labor), the disadvantages of such "active" testing are. it
does shorten battery life to some extent, depending upon how severe
the discharge during test; it requires a full recharge afterward; and
(depending upon the test conditions) it may not adequately simulate
actual in-service power demand.
Routine
battery surveillance therefore relies more heavily on
"passive" tests that take much less time and involve much
less disturbance to battery circuits. They directly measure internal
resistance or impedance of the battery, or indirectly measure cell
conductance. Those properties determine how much load current can
flow at any given cell voltage, and are a good measure of battery
capacity--the amount of stored energy available.
Resistance
measurement, using a four-terminal Kelvin double bridge circuit, has
the disadvantage of requiring the battery to be taken off-line for
testing. In contrast, impedance or conductance is readily measured by
passing an alternating current through the battery while it remains
connected to its normal load and charging circuits. The internal
battery circuit involves both resistance and capacitance, each of
which will pass alternating current.
Modern
test equipment involves microprocessor circuitry, allowing automatic
correction for cell temperature; data storage; and downloading
information to a computer.
Copyright 2002, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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