Internal Resistance & Conductance Testing

Battery Testing Methods · Last reviewed 2026-09-15

Direct answer: A conductance tester sends a small AC signal through the battery and measures how easily current passes — conductance, the inverse of internal resistance. Because resistance rises with age (sulfation, corrosion), the tester uses the reading to estimate CCA and state of health. It is fast and non-destructive, but it is an estimate, not the true load test it replaced.

Conductance vs a true load test

	Conductance test	Load test
Method	Small AC signal, measures conductance	Heavy resistive load (~½ CCA for ~15 s)
What it reads	Internal resistance → estimates CCA	Actual voltage under load
Speed / battery state	Seconds; can run on a discharged battery	Slower; needs a full charge first
Nature of result	Estimate	Direct measurement

Why resistance tracks health

A healthy battery has low internal resistance — a few milliohms. Aging raises it: sulfation coats the plates and blocks current, corrosion adds resistance at the grid, and a low state of charge also raises it. Because a higher resistance means the battery can deliver less current (and therefore less CCA), the conductance reading maps to a CCA estimate. That correlation is strong but not perfect, which is why the number is an estimate, not a measurement.

How to read the result

Conductance near the rated CCA — the battery is likely healthy.
Conductance well below the rated CCA — capacity or cranking ability has degraded.
Requires full charge for a load test — a partially-charged battery reads low on either test.

How to interpret the numbers

Conductance readings map to estimated CCA: a battery reading near its rated CCA is healthy; one reading substantially below — typically under about 70–80% of rating — is nearing end of life. The trend matters as much as the reading: a battery whose conductance keeps falling across visits is failing gradually, which is why fleet programs log the readings instead of testing once (see conductance vs load test).

The Author's Take

My position: The conductance tester won because it is fast and safe, but the skill is knowing it reads resistance and estimates CCA — a proxy that is good enough for triage, not a courtroom-grade measurement.

What drives this: For most decisions — is this battery still good — the estimate is sufficient. But when a warranty or a large purchase turns on the exact number, the load test is the one to trust.

This is the author's editorial view, not a purchasing guarantee.

Sources

Battery Council International (BCI) — battery testing and rating guidance.
Tester-manufacturer documentation — conductance measurement and CCA estimation.
SAE J537 — CCA test method (reference).

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