OCV (Open Circuit Voltage) Testing in Lithium-Ion Pouch Cell Manufacturing

Open Circuit Voltage (OCV) testing measures the voltage between the two tabs of a pouch cell while no load or charger is connected. It takes only a second or two per cell, yet it tells the operator whether the cell is sitting at a sensible state of charge and whether its voltage is stable.

Original EngineerX process map: the 11 main steps of pouch cell manufacturing in the order used on this site; the highlighted step is the subject of this article.

In the sequence described in this series, the OCV test follows First Edge Folding. Many plants also repeat it after the aging or rest period that comes with formation, so the same cell may be measured more than once. The exact positions of these checks are set by each plant's process plan.

Why this stage matters

A pouch cell is not accessible from inside, so electrical measurements at the tabs are the quickest way to look at it. Voltage depends on how much lithium sits in the anode and cathode, so OCV is a simple indicator of state of charge. A cell far from the expected value either was charged incorrectly or has lost charge through an internal leak.

Repeated OCV readings are even more useful. If one cell loses voltage faster than its neighbours over the same rest time, it is self-discharging more than normal. This is often the first sign of a micro short caused by a burr, metal particle or separator fault. Finding such cells early prevents them from reaching a customer's battery pack, where one weak cell can drag down a whole series string.

How the process works

Prepare the cell. Confirm the cell has rested for the time required by the process, so that voltage has settled after the previous step.
Position in the fixture. Place the cell so that the positive and negative tabs line up with the probes. Many stations use a guide to prevent the cell being placed backwards.
Make contact. Spring-loaded probes press on the tabs. At this point no current is drawn apart from the tiny measuring current of the meter.
Measure. A high-impedance meter reads the voltage. Some stations take several readings and average them to avoid a noisy value.
Compare with limits. The reading is checked against the lower and upper limits in the specification.
Record and sort. The value is saved with the cell barcode. Out-of-range cells are routed to a hold area for review.
Calculate drift, if used. When a cell is measured twice with a known time gap, the voltage difference is compared against an allowed limit.

Key process parameters

Limits depend on cell chemistry and on the state of charge at which the cell is held. The cell maker's specification gives the actual values; nothing here should be used as a pass limit.

Parameter	What it controls	How it is checked
Voltage window (low and high limit)	Whether the cell is at the expected state of charge	Automatic comparison in the tester software
Voltage drift limit between two readings	Detection of self-discharge	Difference between first and second reading over a fixed time
Rest time before measurement	Settling of the cell after charging or handling	Time stamps in the traceability record
Meter accuracy and input impedance	Trueness of the reading and absence of loading	Calibration against a reference meter
Contact quality	Stable readings, no false failures	Probe inspection, repeat measurement test
Temperature	Small voltage shift with temperature	Room control and station sensor

Typical defects, causes and corrective actions

Voltage below the lower limit. Causes: micro short, damaged separator, incomplete charge in the previous step. Action: re-measure after rest, send persistent low cells to analysis and check upstream stacking and cutting records.
Voltage above the upper limit. Causes: wrong charge setting, incorrect lot routing. Action: verify the recipe used and the lot identity.
Unexpected voltage drift. Causes: internal leak path, moisture, particles. Action: hold the cell, measure again after a longer rest and compare with its lot.
Large cell-to-cell spread in one lot. Causes: uneven formation, temperature differences, material variation. Action: review formation data and the lot's coating records.
Unstable or jumping reading. Causes: dirty or worn probes, oxide on tab surface, loose cell position. Action: clean or replace probes, check the fixture alignment and repeat the test.
Reverse polarity reading. Causes: cell loaded the wrong way. Action: improve fixture guides and add a polarity check to the station.

Quality checks and measurements

Daily verification of the meter with a known reference source or master cell, as the plant procedure requires.
Control charts of average OCV per lot, which show slow drift in upstream processes long before cells fail the limit.
Repeat-measurement checks on a few cells to confirm the station gives the same value each time.
Probe inspection and replacement schedule.
Review of rejected cells in groups. If many failures come from one tray or one lot, the cause is probably in the process and not the individual cells.

Safety and handling notes

Although OCV is a no-load test, the cells are charged. Do not allow probes, tweezers or tools to bridge both tabs, because that creates a short circuit.
Handle cells by the body edges and avoid bending the tabs, which can damage the weld and the seal area.
Swollen, dented or leaking cells should not be tested. Set them aside in the designated container and inform the supervisor.
Electrolyte traces on the tabs should be treated as a hazard: wear the required gloves and eye protection and use the plant's clean-up method.
Keep the station tidy, and follow lockout rules when maintenance staff work on moving probe mechanisms.

Interview questions

What does OCV mean? It is the terminal voltage of a cell when no current flows in or out of it.
Why is OCV related to state of charge? The voltage of each electrode depends on how much lithium it holds, so the cell voltage rises and falls with its charge level.
Why must the meter have high input impedance? A low-impedance meter would draw current and change the reading, so it would no longer be an open-circuit measurement.
Why take two OCV readings with a time gap? To see the voltage drop, which reveals self-discharge from internal leakage.
A cell shows a jumping reading. What do you check first? Probe contact, tab surface and cell position, before blaming the cell.
Can OCV alone prove a cell is good? No. A cell can show a normal voltage and still have capacity or resistance problems, so OCV is read together with other tests.

FAQs

Does OCV measure capacity?

No. It shows the voltage only. Capacity is measured by a full discharge in the grading or formation step.

Why do plants wait before measuring?

Right after charging or handling the voltage is still settling, so a reading taken too soon can vary from the true value.

What happens to a cell that fails OCV?

It is marked and held. Depending on the plant rules it is re-tested, analysed for the cause, or scrapped.