Impedance testing-is it a substitute for capacity tests?

Capacity tests are used to reliably determine lead acid battery capacity. The results of the capacity tests are then used to evaluate whether the battery can continue in service or needs to be replaced. Unfortunately, many batteries do not receive regular capacity tests. This occurs because of perceptions that: the test is difficult to perform; takes too much time; the battery is too important to be removed from service for a test; is too costly, and so forth. Few consider the consequences of a failure that causes the loss of a facility where down time lost revenue is measured in millions of $US per day or even millions of $US per minute. The measurement of battery impedance (or conductance, or internal resistance) has come into vogue. At this point in time, many in the industry have agreed that these tests are useful in detecting problem cells. However efforts are also underway to convince battery users that these tests can replace capacity tests. This paper reviews the internal battery impedance components and shows that internal resistance rather than impedance is the parameter that is important to measure. It also discusses the change in battery impedance as the cell ages. The paper describes how the measurements are made and provides an analysis of the impedance measurements. This analysis includes a discussion of published results of impedance testing to date. Finally, a comparison is made between capacity and impedance tests