There are three types of ohmic battery testers — impedance testers, conductance testers and resistance testers. Each method is recognized by the IEEE and each can be used as a means of determining the State-of-Health (SOH) of a battery. Perhaps one of the greatest values of ohmic testing is that it is performed while the battery is on-line and it fills the gap between discharge tests by providing an indication of internal changes to the battery.
AC Test Methods:
Impedance testers, such as the Megger BITE series, apply an AC test current to the battery under test and provides an ohmic representation of the combined internal impedance of the battery. The measurement theory is based on Ohm’s Law. The Megger BITE products inject an AC test current of either 50 or 60hertz (I) into a battery or cell. The AC current is measured by the BITE receiver and the AC potential drop across the cell (E) is measured using potential probes. The cell’s impedance (Z) is derived from: E/I = Z.
Equivalent Battery Circuit – AC measured components
Conductance testers, such as the Midtronics Celltron Ultra, also measure the flow of AC current through a battery but measure the conductance rather than the impedance.
DC Test Methods:
Resistance testers measure the drop in DC voltage by applying resistance. A resistance tester, such as the Alber Cellcorder CRT-400 Cell Resistance Tester, places the battery cell under momentary load (approx. 3 second load of approx. 72 amps). The instantaneous voltage drop at time zero and voltage recovery when the load is removed will show a voltage drop across the internal resistance.
Equivalent Battery Circuit – DC measured components
Although different techniques are utilized to capture internal ohmic values of a battery cell, the data can be managed in a similar manner. In short term analysis each individual cell’s ohmic reading can be compared to that of the average ohmic value for all of the cells. This analysis will quickly expose possible weak cells that are falling outside the bank average. If a baseline is established for the battery, the readings can be compared against the baseline to determine approximately how much life has been used. Baselines are best established from a known healthy cell or after a capacity test. Finally, by comparing readings over time the aging rate of the battery can be established.
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Editor’s Note: This post was originally published August, 2011 and has been updated freshness, accuracy and comprehensiveness.