Internal Resistance vs Conductance: The Same Health Signal in Two Units
Two battery testers can look like they measure different things — one reads milliohms, the other reads siemens. They are reading the same thing from opposite directions.
The inverse relationship
| Battery state | Internal resistance | Conductance |
|---|---|---|
| Healthy | Low (a few mΩ) | High |
| Aging / sulfated | Rising | Falling |
| Failed | High | Low |
Aging raises resistance (sulfation, corrosion) and therefore lowers conductance — the same physical change, reported in two units. A conductance number falling over time is the same warning as a resistance number rising.
Why testers chose conductance
The handheld tester works by passing a small AC signal and measuring how well the battery conducts. That measurement is fast, non-destructive, and works even on a partially-charged battery, and it correlates with cranking ability — a battery that conducts well can deliver current. The tester then converts the conductance into a CCA estimate, which is the number users actually want (see conductance testing).
What the units do not tell you
Neither resistance nor conductance is capacity. A battery can read healthy on both while holding little energy, because the measurement reflects the current path, not the stored amp-hours. The reading is a health proxy — excellent for triage, not a substitute for a capacity test when the exact energy matters.
The Author's Take
I would argue: The resistance-vs-conductance confusion is a unit problem, not a physics problem — and the useful habit is reading both as one signal, while remembering that neither one measures capacity.
Why: The inverse relationship means every conductance test is a resistance test in disguise. Users who see that stop being confused by the units — and avoid the real error, which is treating a healthy conductance reading as proof of full capacity.
This is my analysis, not a verified fact or purchasing guarantee.
Frequently asked questions
What is the difference between internal resistance and conductance?
They measure the same thing from opposite directions. Internal resistance (mΩ) is how much the battery resists current; conductance (S) is how easily it passes current — conductance is the reciprocal of resistance. A low resistance means high conductance, and both signal a healthy battery.
Why do battery testers report conductance instead of resistance?
Because a small AC conductance measurement is fast, non-destructive and works on a partially-charged battery, and it correlates well with CCA — a healthy battery conducts well. The tester then converts conductance into a CCA estimate.
Sources
Cross-verified from battery engineering references and tester-manufacturer documentation.
- Battery engineering references — resistance and conductance relationship.
- Tester-manufacturer documentation — conductance measurement and CCA estimation.
Last reviewed: 2026-09-16