How this instrument works
Model a real battery as an ideal, resistance-free EMF source wired in series with one small resistor, r, buried inside the case. Nothing flows across r until current is drawn, so an idle voltmeter reads the full EMF and hides the resistor completely. Close a circuit and current I forces that hidden resistor to drop I·r volts before the rest ever reaches the terminals, leaving a terminal voltage of V = EMF − I·r. Rearranged for the one unknown you usually cannot measure directly, that becomes r = (EMF − V) ⁄ I — a Thevenin equivalent applied to a single cell, with Kirchhoff's voltage law doing all the real work.
That resistor is not a manufacturing defect; it is physics you cannot design away. Ions have to migrate through electrolyte, electrode surfaces have finite reaction rate and area, separators impose their own drag, and every welded tab and terminal post adds a sliver of ohmic loss. Add them in series and you get r — typically a few milliohms in a fresh automotive battery, tens of milliohms in a AA alkaline cell, and climbing steadily as either one ages, cools, or nears full discharge. A standard ohmmeter cannot read it directly, because an ohmmeter assumes a passive part with no EMF of its own; here the part being probed is the very thing pushing current through the probe.
Treat any single r you calculate as a reading at one operating point, not a fixed spec. Internal resistance climbs sharply in the cold, since chilled electrolyte conducts ions more sluggishly, and it rises permanently as electrode surfaces degrade through cycling or sulfation. It also depends mildly on how hard the cell is being pushed — the r measured at a light load and the r measured at a heavy load on the same battery need not match exactly, since some of what's lost under heavy current is electrochemical polarization rather than plain ohmic drop. Dedicated AC milliohm testers sidestep some of this by exciting the cell with a small alternating signal instead of a DC load, which is why their figures sometimes differ from the two-point method used here.
- With no load connected, let the voltmeter settle and enter that steady figure as EMF (open-circuit voltage).
- Connect the real load — a starter motor, a bulb, a discharge tester — and once the reading has settled, enter it as Terminal voltage under load.
- Read the current that same load is drawing, from an ammeter or a clamp meter, and enter it as Current drawn.
- Read Internal resistance in ohms; switch its unit to kΩ for a nearly dead cell, where the value can climb into the kilohm range.
Worked example — a 9 V battery under a 0.5 A load
A fresh 9 V battery reads a clean 9.00 V on an idle voltmeter, since with no circuit closed, no current crosses its internal resistance and none of that voltage is lost to it. Clip on a load that draws exactly 0.5 A and the terminal reading sags to 8.5 V. Enter EMF = 9, Terminal voltage under load = 8.5, and Current drawn = 0.5, and the instrument returns r = (9 − 8.5) ⁄ 0.5 = 1 Ω. That one ohm lives entirely inside the cell, invisible to any no-load test, and it is exactly why a battery reading full voltage on a shelf can still fail to turn over a cold engine.
Run the same 0.5 A draw through an older cell of the same nominal voltage and the sag can be far worse — terminal voltage down to 7 V instead of 8.5 V, with EMF and current unchanged. Internal resistance then works out to (9 − 7) ⁄ 0.5 = 4 Ω, four times the fresh reading. That jump is the electrochemical signature of age or heavy prior discharge: thinned electrolyte and corroded electrode surfaces make every amp drawn cost several times as many lost volts as it did when the cell was new, which is exactly the contrast a load tester is built to catch.
Questions
Why can't I just measure this with an ohmmeter?
Because a standard ohmmeter assumes it is probing a passive, unpowered part — it pushes its own small test current through the component and reads the resulting voltage. A battery already has its own EMF pushing back, so that assumption fails and the meter either protects itself or returns nonsense. Internal resistance has to be inferred instead, either from the voltage sag between two load conditions, as this calculator does, or with a dedicated AC milliohm tester that injects a small alternating signal and filters the battery's own DC voltage back out.
Is internal resistance a fixed property of a battery?
No — treat any single value as a reading at one moment, not a permanent spec. It rises noticeably as a cell nears full discharge, climbs sharply in cold weather as chilled electrolyte conducts ions more sluggishly, and increases permanently with age and cycle count as electrode surfaces degrade. A fresh AA cell might sit near a few tenths of an ohm; the same cell nearly dead can read several ohms, which is why load testers repeat the measurement rather than trusting one figure for a battery's whole life.
Why does terminal voltage sag under load at all?
Because current flowing through the battery's own internal resistance drops voltage exactly the way current through any resistor does, and that drop subtracts from the EMF before it ever reaches the terminals. Model the cell as an ideal, resistance-free EMF source in series with r and V = EMF − I·r falls straight out of Kirchhoff's voltage law. Draw more current and more of the EMF is consumed inside the cell itself, which is why the reading keeps falling as a discharge test ramps its load higher.
What does a high internal resistance actually cost you?
Current, right when you need it most. A starter motor demanding 200 A through even a modest 0.05 Ω of internal resistance loses 10 V inside the battery alone, often enough to stall a cold-morning crank even though the same battery reads a healthy 12.6 V on an idle voltmeter. High internal resistance turns heavy current demands into disproportionate voltage loss, which is why cold-cranking-amp ratings, not open-circuit voltage, are what starter and inverter specifications actually depend on.
Can internal resistance come out negative or zero?
Zero is possible only for an ideal battery, entered here as a Terminal voltage under load equal to EMF; no real cell manages it, since some series resistance from electrolyte and electrodes is unavoidable. A negative result means the terminal reading entered exceeded the EMF, which usually points to a measurement error — the loaded and unloaded readings swapped between fields, or the EMF recorded after the load had already pulled it down.
What counts as a healthy internal resistance for a car battery?
Roughly 3 to 8 milliohms, or 0.003 to 0.008 Ω, for a healthy automotive lead-acid battery at room temperature, though the exact baseline depends on capacity and chemistry. A reading several times that figure, especially alongside a normal-looking open-circuit voltage, is the classic signature of a battery that will crank weakly or fail outright in cold weather — precisely the gap between EMF and loaded voltage this calculator turns into one number.