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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.

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