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Conductance measurement
Jan 06, 2017

Conductance measurement to evaluate starter batteries was first reported by Keith Champlin in 1975 by demonstrating a linear correlation between load test and conductance. When injecting a frequency of about 90 hertz, capacitive and inductive reactance converge with a 70–90Ah lead acid battery, resulting in a negligible voltage lag that minimizes the reactance. (This frequency rises with a smaller battery and drops with a large pack.) AC conductance meters are commonly used in car garages to measure CCA. The single-frequency method (Figure 5) sees the components of the Randles model as one complex impedance called the modulus of Z.

The 1,000-hertz (Hz) ohm test is another common method. A 1,000Hz signal excites the battery and Ohm’s law calculates the resistance. Note that the AC method shows different values to the DC method when measuring a reactive resistance, and both readings are correct.

For example, Li-ion in an 18650 cell produces about 36mOhm with a 1,000Hz AC signal and roughly 110mOhm with a DC load. Since both readings are valid, yet far apart, the user must consider the application. The pulse DC load method provides valuable readings for a DC application such as a heating element or an incandescent light, while the 1,000Hz method better reflects the performance requirements of a digital load, such as portable computing and mobile phones that rely to a large extent on the capacitive characteristics of a battery. 

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