A battery can sit at a perfectly normal open-circuit voltage and still be failing. Voltage alone tells you the battery's state of charge, not its state of health — a degraded battery can rest at a healthy-looking voltage right up until it's asked to deliver real current, at which point it sags badly, overheats, or simply can't supply the load. Internal resistance testing exists specifically to catch this gap.
Why Internal Resistance Rises as a Battery Ages
As a battery ages or degrades — through cycling, calendar aging, corrosion, electrolyte depletion — internal resistance climbs. Higher internal resistance means more voltage is lost inside the battery when current flows, showing up as voltage sag under load, reduced usable capacity, and excess heat — symptoms that only appear once the battery is delivering current, not when sitting open-circuit.
How the Test Actually Works
A DC internal resistance (DC-IR) test applies a brief current pulse and measures the resulting voltage drop, calculating resistance from Ohm's law. Because the test only needs a brief pulse rather than sustained discharge, DC-IR testing is fast and can be performed on batteries still in service.
An alternative method, AC internal resistance (AC-IR), applies a small AC signal (commonly at 1kHz) and calculates resistance from the resulting response. AC-IR is typically used in manufacturing, since it can be performed very quickly without requiring a particular state of charge beforehand.
AC-IR vs DC-IR — Which to Use
| Factor | AC-IR | DC-IR |
|---|---|---|
| Test speed | Very fast | Fast, but typically needs SOC stabilisation first |
| Typical use case | Production line / manufacturing QC | Field service, fleet maintenance |
| Requires specific state of charge | Generally no | Generally yes |
| Correlation with actual load performance | Good | Often considered closer to real-world behaviour |
Four-Terminal Measurement Matters
Battery internal resistance values can be extremely low — fractions of a milliohm for large-format cells. A genuinely accurate instrument uses four-terminal (Kelvin) measurement rather than a simple two-terminal connection, which would otherwise include lead and contact resistance in the reading.
What Internal Resistance Does and Doesn't Tell You
Internal resistance correlates with battery health, but isn't a direct substitute for a full capacity discharge test. Rising internal resistance generally tracks declining capacity, but the relationship isn't perfectly linear. It's best used as a fast screening and trending tool, flagging cells that have shifted from baseline.
Practical Applications
Fleet and stationary battery maintenance. UPS systems and telecom backup power benefit from periodic testing across every cell in a string, since a single weak cell can limit the entire string's performance.
Incoming inspection and production QC. Manufacturers commonly use AC-IR as a fast pass/fail screen on incoming cells.
Field service and warranty assessment. Internal resistance testing gives a fast field diagnostic without needing lab-grade discharge testing equipment on site.