Key takeaways
Three measurements dominate cell testing in battery production, and they are routinely treated as three ways of checking the same thing. They are not. Each answers a different question, and a cell can pass one convincingly while failing another.
Understanding what each measurement actually sees is worth doing properly, because these tests decide which cells are grouped together, and cell grouping decides how the finished pack ages for the next decade.
This article covers what each measurement is, why alternating and direct current resistance are not interchangeable, the practical errors that corrupt them, and how results become sorting decisions.
Three Measurements, Three Different Questions
| Measurement | How it is taken | What it reveals | Practical limit |
|---|---|---|---|
| Open circuit voltage | Voltage measured at rest with no load applied | State of charge, gross defects, and self-discharge when tracked over time | Requires rest before reading, and the voltage curve is very flat for some chemistries |
| DC internal resistance | A current pulse is applied and the voltage drop measured | Overall resistance including electrochemical behavior | Value depends on pulse duration, so results are not comparable across protocols |
| AC internal resistance | A small alternating signal, conventionally at 1 kHz, is applied | The ohmic part: current collectors, tabs, welds and internal connections | Says little about electrochemical performance or power capability |
| Self-discharge | Open circuit voltage tracked over days of rest | Micro internal shorts that no instantaneous measurement detects | Needs time, which is why aging exists |
The distinction that matters most in that table is between the two resistance measurements. They share a name and measure genuinely different properties, and confusing them is the most common technical misunderstanding in production cell testing.
Open Circuit Voltage
Open circuit voltage is the potential across the cell terminals with nothing drawing current. It is the simplest measurement to take and the easiest to take badly.
Its primary use is establishing state of charge. Cells are shipped and stored at partial charge, and a cell arriving well outside the expected range indicates something worth investigating: a mis-shipped batch, extended storage, or a defect.
Two complications matter in practice. First, voltage needs time to settle. Any current flow, including the small draw of a previous measurement, leaves the cell in a polarized state and the reading drifts as it relaxes. A meaningful open circuit voltage requires rest, and rushing that produces readings that look precise and mean little.
Second, the relationship between voltage and state of charge is not equally useful across chemistries. Lithium iron phosphate has a notably flat voltage curve through much of its range, so a small voltage difference corresponds to a large difference in charge. That makes OCV a blunt instrument for state of charge in LFP cells and puts more weight on the other measurements.
DC Internal Resistance
Direct current internal resistance is measured by applying a current pulse and observing the resulting voltage drop. Resistance follows from the change in voltage divided by the change in current.
The subtlety is that the answer depends on how long the pulse lasts. In the first instants the response is dominated by ohmic resistance, the metal, connections and electrolyte conductivity. Over longer periods charge transfer at the electrode surfaces contributes, and over longer periods still, diffusion within the electrode does. A ten second pulse and a one second pulse on the same cell produce different resistance values, and both are correct for what they measured.
That makes DCIR a rich measurement and a difficult one to standardize. Comparing figures between suppliers, laboratories or test protocols is meaningless unless the pulse conditions match, and specifications that quote internal resistance without stating how it was measured are incomplete.
Because it reflects the electrochemical behavior of the cell rather than only its construction, DCIR is the measurement that relates most directly to power capability, which is why it appears in characterization and development work.
AC Internal Resistance
AC internal resistance applies a small alternating signal, conventionally at one kilohertz, and measures the cell impedance at that frequency.
One kilohertz is not an arbitrary choice. At that frequency the signal alternates far too quickly for diffusion or charge transfer to respond meaningfully, so what remains is essentially the ohmic component: current collectors, tabs, internal welds, terminal connections and electrolyte conductivity. The measurement effectively reports on how the cell is built rather than how it behaves electrochemically.
That narrowness is exactly why production lines use it. The measurement takes milliseconds, requires no rest period, moves negligible charge and is highly repeatable. For sorting thousands of cells an hour, nothing else offers that combination.
It is very good at finding construction defects. A poor internal weld, a marginal tab connection or a contact problem shows up clearly as elevated AC resistance, because those are precisely the ohmic elements the measurement sees.
Why ACIR and DCIR Are Not Interchangeable
This deserves stating plainly because it causes real problems in specification and in supplier disputes.
AC resistance at one kilohertz measures ohmic elements. DC resistance over a pulse of a second or ten seconds measures those plus the electrochemical processes that develop over that time. The second figure is typically several times larger than the first, and the ratio between them varies with chemistry, temperature and state of charge.
The practical consequences follow. A cell can show excellent AC resistance and still deliver poor power, because its electrodes rather than its connections are the limitation. Conversely, a cell with sound electrochemistry and a marginal internal weld shows elevated AC resistance while its DC behavior looks acceptable at low current.
Neither measurement is a substitute for the other. Production sorting uses AC resistance because it is fast, repeatable and catches the construction defects that matter for pack building. Characterization uses DC resistance because it relates to how the cell will actually perform. A specification that requires one and reports the other is describing something different from what it intended.
Measurement Practice
Most cell testing problems in production are not about which measurement to take. They are about how it is taken.
Four-wire connection
Cell resistance is small, and the resistance of test leads and contacts is comparable to it or larger. A two-wire measurement includes both, so it reports the fixture as much as the cell. Four-wire measurement, where current is driven through one pair of contacts and voltage sensed through another, removes lead resistance from the reading. It is not optional at these magnitudes.
Contact resistance is the dominant error
Even with four-wire measurement, the contact between probe and terminal matters. Probe force, probe tip condition and surface oxide all affect the reading, and all of them change gradually with use.
This produces a specific and dangerous failure mode. As probes wear and accumulate residue, contact resistance rises, measured values drift upward, and the sorting bands quietly widen. Nothing alarms, every reading looks plausible, and cells that should have been separated are grouped together. The pack still passes end of line testing and ages unevenly in the field. Monitoring the fixture, not only the part, is the defense.
Temperature
Cell resistance falls measurably as temperature rises. Cells measured immediately after arriving from a cold store will read differently from cells that have equilibrated in the plant, and cells warmed by a previous process step will read differently again. Either the measurement is taken at a controlled temperature or it is compensated, and if neither is done then part of the apparent spread in results is thermal rather than real.
Reference cells and calibration
The most practical verification is a set of stable reference cells measured at defined intervals. If the tester reports a different value for the same reference cell than it did last week, the tester has drifted, not the cells. This costs almost nothing and catches the drift that would otherwise be invisible.
From Measurement to Sorting
Measurements become useful when they become decisions. Cells are sorted into bands by voltage and resistance so that groups are electrically similar, and the reason is the one that governs everything in pack building: a group performs at the level of its weakest member. This sits within the wider quality framework covered in the guide to battery quality control.
The mechanism is worth being concrete about. A cell with slightly higher resistance in a parallel group carries less current, which sounds harmless. In a series arrangement it drops more voltage and dissipates more heat, so it runs warmer than its neighbors, and a cell that consistently runs warmer ages faster. Over years the difference compounds until that cell limits the pack.
Band width is a design decision rather than a universal figure. Tighter bands mean better matching and more cells rejected or diverted to lower-grade applications; wider bands mean better utilization and shorter pack life. Where that line sits depends on the application, the warranty and the value of the cells.
Self-Discharge: The Fourth Measurement
The three standard measurements share a limitation. All are instantaneous, and a cell with a microscopic internal short reads entirely normal on every one of them.
Such a cell reveals itself only through time. Tracked over days at rest, its open circuit voltage falls faster than that of healthy cells, and that rate of decay is the signature. This is why cells rest after formation before grading, and why compressing that period to release working capital carries a genuine quality cost.
For pack builders receiving cells from a supplier, this is a reason to treat supplier grading as information rather than as verification. The self-discharge screening happened at the cell plant, under conditions the pack builder did not observe.
“Testing every cell is only the first step; automatically sorting cells by measured performance is what turns test data into consistently matched battery packs.”
See it in action
Why Incoming Cells Are Retested
Cells arrive with certificates. They are tested again anyway, and for good reasons.
Cell Testing in Cybernetik Battery Lines
Cybernetik builds battery pack assembly automation with testing and sorting integrated at the head of the line rather than treated as incoming inspection alongside it.
| Cybernetik cell testing capability | Specification |
|---|---|
| Measurements | OCV, IR and ACIR testing on every incoming cell |
| Sorting | Robotic sorting onto parallel conveyors by test result using SCARA arms |
| Throughput | Up to 6,000 cells per hour through testing and sorting |
| Cell formats | Cylindrical 18650, 21700 and 32140, plus prismatic, pouch and blade |
| Cell diameters | 32, 33, 35, 40, 42 and 46 mm |
| Surface preparation | Plasma cleaning of terminals ahead of testing and welding |
| Data capture | Barcode scanning at entry with results written against the cell identifier |
| Downstream verification | Vision-confirmed insulation, polarity checking and inline weld integrity testing |
| System connectivity | MES connectivity with process logging and predictive maintenance analytics |
The detail that matters most operationally is robotic sorting onto parallel conveyors by test result. Measuring cells produces data; diverting them mechanically by result is what turns that data into matched groups. Where sorting is manual or absent, the measurement exists but the grouping decision is being made by whoever happens to pick up the next cell.
Cybernetik has operated for more than three decades, is headquartered in Pune with additional facilities in Gujarat and Raigad and international offices in the United States and UAE, and has installed over 6,000 systems across 30 plus countries, including more than 400 custom robotic automation solutions. In battery manufacturing that work has been delivered for manufacturers including Hero MotoCorp, TVS Motor, Livguard and Matter. Further background is on the Cybernetik about page.