Between a battery’s two terminals lives a number nobody prints in large type: its internal resistance. Every element of the current’s path fights it a little — grids, welded joints, active material, electrolyte, separators, terminal posts — and the total, for a healthy industrial cell, amounts to mere thousandths of an ohm. Milliohms sound like nothing. They decide almost everything about how a battery BEHAVES under load: how far its voltage sags, how hot it runs, how much power it can actually deliver — and, watched over time, they tell you which cell in a string is quietly dying. This is the quiet number, explained.
What the milliohms do to you
Three effects, all from one line of physics. Sag: the voltage you receive under load is the cell’s open-circuit voltage minus current times resistance — so the harder you pull, the more the terminal voltage droops, which is why a battery that reads beautifully at rest can stumble under a motor start. Heat: the energy lost in that resistance leaves as warmth, and it grows with the SQUARE of current — double the amperes, four times the internal heating. This is the physics behind Peukert’s sprint tax, and behind our obsession with joints: a loose or corroded connection is just internal resistance that moved outside the cell, and under traction currents it becomes a heater — the reason our mining batteries use bolted connectors with guaranteed contact area, and the reason a torque wrench and clean terminals are maintenance tools, not fussiness. The power ceiling: resistance caps how many watts a battery can push regardless of how many ampere-hours it stores. Capacity is the size of the tank; resistance is the width of the tap. They are different questions — hold that thought.
Why the number rises with age
A battery’s resistance is lowest in its healthy youth and climbs as the five killers do their work: grid corrosion thins the conducting framework; hardened sulphate coats active surfaces with a poorly conducting crust; drying concentrates and starves the electrolyte’s ionic path; shedding breaks up the plate’s connected structure. Cold raises effective resistance too — temporarily — because chilled electrolyte thickens and its ions trudge (the cold-weather guide lives on that fact). So a rising resistance trend is age made visible, which is exactly what makes it USEFUL: measure a bank’s per-cell resistance at commissioning as a baseline, re-measure on a schedule, and the cell drifting upward from its siblings has raised its hand years before it fails a duty. In a series string — where the weakest cell commands everyone — that early hand-raise is worth real money.
The honest limit: resistance is not capacity
Now the sentence this article exists to deliver, because an entire industry of handheld testers blurs it: an internal-resistance reading is not a capacity measurement. Resistance answers “how freely does current flow RIGHT NOW”; capacity answers “how many ampere-hours are actually in there” — tap width versus tank contents. The two correlate loosely in late age, which is what makes the confusion seductive, but a battery can hold a respectable resistance reading while its capacity has quietly fallen past retirement — plenty of real banks have passed a milliohm check and then failed the discharge that mattered. Used honestly, resistance measurement is a superb TREND instrument and a brilliant bad-cell FINDER, and a poor fortune-teller of ampere-hours. What actually proves capacity is a measured discharge — and that is the subject of our battery-testing guide, where the quiet number takes its proper place: one instrument in the orchestra, never the whole concert. If a tester’s verdict on your bank doesn’t smell right, ask us before you act on it — the readings usually deserve a second, better-informed look.