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.

The Author's Take

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.

Reasoning: 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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