On-line battery testing: a reliable method for determining battery health?

Pb-acid battery testing is based upon measuring the electrical response of a cell in a battery to an applied test signal. The intended result is to determine the cell's state of health from the measured response. Such testing represents a faster, less expensive alternative to load-cycle testing, which is the most accurate way to measure the capacity of a battery. Recognizing this, many companies have begun marketing various types of cell "impedance" or conductance/resistance testers (categorized as 'ZSR' testers). The main advantage of ZSR testing over load-cycle testing is the fast turn-around time and low personnel requirements, because the battery does not have to be disconnected from operating equipment for the test. However, as supported by presentations at battery-oriented technical conferences, ZSR testers do not always perform as well as expected, because: the ZSR/battery capacity relationship is poorly understood; the accuracy of the technique remains controversial. To clarify these observations, the following questions must be considered: What battery parameters are we actually measuring with a ZSR tester? Are the measured parameters relevant to battery performance? Using a simple, "black box" equivalent circuit model of a battery and well-established electrochemical principles, we can answer the above questions. An examination of the electrochemical of a ZSR measurement is presented. With this knowledge, users of ZSR battery testing equipment will gain an appreciation of the principles governing the capabilities (and limitations) of their testing equipment.