OCV, AC resistance and DC resistance: what each cell test measures

Sorting machines and testers quote OCV, ACIR and DCIR. They answer different questions and cannot be swapped for each other.

By BatteryEquip Editorial Team · 2026-11-04

Cell sorting machines and end-of-line testers usually list three measurements: open-circuit voltage (OCV), AC internal resistance (ACIR, often called IR) and DC internal resistance (DCIR). They are quick to read on a datasheet and easy to confuse. Each answers a different question about the cell, so a buyer needs to know which one the process actually requires.

OCV: the cell at rest

OCV is the voltage across the terminals with no current flowing. For a given chemistry it follows the state of charge, so it confirms that a cell is charged to the expected level. Its more valuable use in production is over time. A cell that loses voltage faster than its neighbours during storage has higher self-discharge, which can point to contamination or a small internal short.

That is why aging lines measure OCV twice, days apart, and calculate the drop per day, often called the K value. The signal is small. Keysight's examples on 18650 cells show good cells losing well under a millivolt per day, and the voltage keeps relaxing for some time after charging, which can hide the self-discharge signal. Temperature and state of charge both change the result. So the OCV meter needs microvolt-level resolution and stable contacts, and the storage room needs controlled temperature.

ACIR: a fast impedance check

ACIR passes a small alternating current, usually at 1 kHz, through the cell and measures the voltage response. Hioki's battery testers work this way and use four-terminal probes, with separate leads for current and voltage, so that lead resistance does not enter the reading. Large automotive cells can have internal resistance below one milliohm, which makes the four-terminal method essential.

At 1 kHz the result mainly reflects the ohmic part of the cell: electrolyte, separator, current collectors, tabs and welds. It takes a fraction of a second and needs no discharge, which is why OCV/IR sorters such as the WinAck H10B and the ACEY-PS8 combine it with OCV. A high ACIR flags a poor weld, a dry cell or a contact problem. It does not tell you how much power the cell can deliver for ten seconds.

DCIR: resistance under a real load

DCIR applies a current pulse and divides the voltage change by the current change. The INL battery test manual defines pulse resistance this way within its hybrid pulse power characterisation (HPPC) test. In the Battery Design summary of that test, a 10-second discharge pulse is followed by a rest and a charge pulse, repeated at several states of charge. A reading taken about one second into the pulse is close to the ohmic resistance; the 10-second reading adds polarisation from charge transfer and diffusion.

DCIR is therefore normally larger than ACIR for the same cell, and it changes with pulse current, pulse length, state of charge and temperature. Two DCIR figures can only be compared when those four conditions match. Dedicated stations such as the Hangke 46XX DCIR test station and the Hangke high-rate pulse equipment exist because the pulse needs high current and precise timing.

Which one to specify

Use OCV for charge level and, with two readings over time, for self-discharge screening. Use ACIR for fast checks of every cell after assembly and before grouping. Use DCIR where power capability matters, such as cells for high-rate packs, and where the customer's specification asks for it. Many lines use all three at different points.

When cells will be connected in parallel inside a module, sorting matters more. An, Zhao and Li (RSC Advances, 2018) compared sorting by capacity, OCV, DCR and self-discharge rate and found self-discharge a worthwhile criterion.

What to ask the supplier

For OCV, ask for the resolution, accuracy and the temperature at which they apply. For ACIR, ask for the test frequency, the excitation current, the ranges and the probe design, since probe wear changes readings. For DCIR, ask for the pulse current, duration, sampling point and the state of charge the machine assumes. Ask how often the machine checks itself against a reference cell or standard resistor, as the Geesun tray-test station does with master-reference positions.

Put these figures in the RFQ guide format, and read the formation and capacity grading page for how these tests fit after formation. For module lines, the Courage KRZ-A10 sorter and the Honsenze 18650 sorter are examples of grouping cells before welding. Ask each maker to confirm the figures for your cells.

Sources

1. Idaho National Laboratory, Battery Test Manual for Electric Vehicles, INL/EXT-15-34184 Rev. 3 (2015): HPPC test and pulse resistance calculation · checked 2026-11-04
2. Hioki, how to measure a battery's internal resistance with a battery tester · checked 2026-11-04
3. Battery Design, HPPC test procedure · checked 2026-11-04
4. Keysight, measure self-discharge using OCV on lithium-ion cells (Electronic Design) · checked 2026-11-04
5. An, Zhao and Li, cell sorting by capacity, OCV, DCR and self-discharge rate, RSC Advances 8 (2018) · checked 2026-11-04