2. Destructive Peel Testing

A destructive peel test provides direct evidence of how the welded interface behaves when mechanically loaded. The tab is pulled away from the cell terminal in a controlled direction, often at approximately 90 degrees to the original joint plane.

The purpose is not only to determine whether the tab separates. Engineers should also inspect where the joint fails and whether material is pulled from one of the welded surfaces.

What Failure Mode Should Be Recorded?

A stronger weld may remove a small portion of material from one side of the joint, sometimes described as material pullout or button formation. A clean separation directly along the original interface can indicate insufficient fusion or another process problem.

However, the exact acceptable failure mode depends on the joint design, tab material, cell terminal and required mechanical load. One visual failure pattern should not be treated as a universal specification for every battery weld.

Make the Peel Test Repeatable

1. Use representative production cells and tabs.
2. Use the same weld pattern planned for production.
3. Secure the battery so it does not move during testing.
4. Grip the tab without damaging the weld beforehand.
5. Maintain a consistent pulling direction and approximate angle.
6. Use a repeatable test speed if a force gauge or tensile tester is available.
7. Record the peak force.
8. Photograph the failure surface.
9. Record the failure mode as well as the force result.

A single successful peel test shows that one sample survived. Process validation must demonstrate repeatability across multiple samples and normal manufacturing variation.

3. Pull, Shear and Mechanical Load Testing

A peel test concentrates stress near the edge of the weld. The finished battery pack may experience different forces during assembly, vibration, shock or long-term use.

Depending on the application, additional mechanical tests may be appropriate.

• Axial pull test: applies force away from the terminal.
• Shear test: loads the welded tab parallel to the connection surface.
• Cross-tension test: applies an opening-type load across the weld.
• Flex test: evaluates repeated movement of the tab or wire connection.
• Pack vibration test: evaluates whether welds remain intact in the final battery structure.
• Mechanical shock test: evaluates sudden loads relevant to transport or equipment use.

A mechanical-force requirement should always specify the test method. A statement such as “weld strength must exceed X newtons” is incomplete unless the pull direction, fixture and test speed are also defined.

4. Measure Electrical Resistance

A mechanically strong weld can still create unwanted electrical resistance. Measuring the resistance of the welded connection provides another important quality indicator.

Use Four-Wire Kelvin Measurement

Battery tab weld resistance is very low. A conventional two-wire multimeter can include test-lead resistance, probe contact resistance and wire resistance in the result.

A four-wire Kelvin method separates the current path from the voltage-sensing path and is more suitable for low-resistance measurements.

The measurement fixture should control probe location and contact pressure. If the probes are positioned at different distances from the weld from one sample to another, the reading may include different lengths of nickel strip or cell terminal and become difficult to compare.

Use a Qualified Baseline

Instead of relying on one generic milliohm value, establish a baseline from validated samples. Compare future production measurements with this approved distribution.

Resistance trend monitoring can help identify gradual process changes caused by electrode wear, surface contamination, altered contact pressure or material variation before a severe defect becomes obvious.

5. Metallographic Cross-Section Inspection

Cross-section inspection reveals details that cannot be seen from the outside. The welded sample is cut through the joint, mounted and polished so the interface can be inspected under magnification.

Features to Evaluate

• Fusion between the tab and terminal
• Weld nugget formation
• Penetration into the joined materials
• Incomplete or intermittent fusion
• Voids or porosity
• Cracks
• Excessive material thinning
• Excessive electrode indentation
• Consistency between repeated weld samples

Cross-sectioning is particularly valuable during initial process qualification, root-cause analysis, cell or tab material changes and investigation of unusual field failures.

It should not replace other testing. One cross-section examines only one plane through one weld, so mechanical and electrical testing remain important.

How to Establish a Stable Welding Process Window

Finding one setting that produces one acceptable sample is not enough. The objective of process development is to identify a stable range of settings that consistently creates acceptable welds despite normal production variation.

Step 1: Fix the Joint Configuration

Define the exact cell model, terminal side, tab material, tab thickness, electrode geometry, fixture and weld pattern before comparing welding parameters.

Step 2: Test a Parameter Matrix

Evaluate combinations of welding current or energy, pulse duration, pulse sequence and electrode force. Development samples should cover under-welded, acceptable and over-welded conditions so the practical process boundaries become visible.

Step 3: Apply Multiple Validation Methods

Perform visual inspection, destructive testing and resistance measurement on representative samples. Cross-section selected conditions around the proposed production window.

Step 4: Challenge Normal Manufacturing Variation

Repeat the process using different representative material lots, normal electrode wear and expected equipment variation. Positive-terminal and negative-terminal welds should be considered separately when their structures differ.

Step 5: Select a Production Setting Inside the Stable Region

The preferred production condition should normally be located inside the demonstrated process window rather than directly beside an under-weld or over-weld boundary.

Production Controls After Welding Qualification

Initial qualification shows that the selected process can produce acceptable welds. Production control verifies that the same process remains stable during actual manufacturing.

What In-Process Weld Monitoring Can Do

Modern resistance welders can record parameters such as current, voltage, energy, power and electrode displacement. These signals can help detect abnormal weld conditions in real time.

For example, a sudden change in resistance or energy profile may indicate poor tab contact, contamination, electrode wear or a missing component.

However, weld-monitor data becomes useful only after it has been correlated with actual weld quality. The approved machine signature should be established using samples that have already passed mechanical, electrical and, where required, metallographic testing.

In-process monitoring is most valuable for controlling a previously validated process. It should not be treated as a complete replacement for destructive verification.

18650 vs. 21700: Can the Same Weld Settings Be Used?

Not automatically. The validation methods are similar, but the welding schedule can change between 18650 and 21700 cells.

Different cells may use different terminal geometry, surface coatings, material thicknesses and internal structures. Even cells with the same nominal size can behave differently if they come from different manufacturers or product series.

The welding process should be reviewed when any of the following changes:

• 18650 cells are replaced by 21700 cells.
• The cell manufacturer changes.
• The exact cell model changes.
• The tab material changes.
• The tab thickness changes.
• The tab width changes significantly.
• The weld-point location or pattern changes.
• The welder or power supply changes.
• The electrode material or tip geometry changes.
• A busbar, fuse wire or multilayer conductor is introduced.

OEM teams considering larger cylindrical cells can review PKCELL’s 21700 rechargeable battery options together with available 18650 rechargeable battery models before finalizing the pack structure.

Spot Weld Quality and Battery Pack Current

Spot weld requirements should also reflect the electrical load of the finished battery pack. A connection that appears adequate in a low-current device may become a heating point in a high-current pack if its resistance is excessive.

The weld should therefore be considered together with the conductor cross-section, series and parallel arrangement, continuous current, peak current and thermal environment.

For customized packs, PKCELL can evaluate cell configuration, conductor arrangement, BMS, connector and pack structure through its custom battery pack service.

Common Battery Spot Weld Validation Mistakes

• Approving weld quality based only on appearance.
• Testing only one weld sample.
• Performing an uncontrolled hand-pull test with no defined direction.
• Recording pull force without documenting the failure mode.
• Using a normal two-wire multimeter for very low joint resistance.
• Using inconsistent resistance probe positions.
• Transferring welding settings from one cell model to another without validation.
• Ignoring electrode wear.
• Changing the tab thickness without repeating process checks.
• Inspecting welds only after insulation or the housing has hidden them.
• Assuming a repeatable machine automatically produces an acceptable weld.
• Failing to document cell and material lot traceability.

Recommended Spot Weld Validation Record

A complete weld-validation report should provide enough information for the process to be repeated and audited.

When Should Spot Welding Be Revalidated?

Revalidation should be considered whenever a change could alter current flow, pressure, heat generation or joint geometry.

Typical triggers include a new cell supplier, different tab alloy, different tab thickness, welding-machine replacement, electrode redesign, fixture changes, major equipment maintenance or repeated production failures.

Routine verification should also be performed after significant electrode replacement or dressing because the electrode contact area directly affects current density at the weld.

Information to Send for a Custom 18650 or 21700 Pack

If you are developing a custom cylindrical battery pack, providing the following information helps the battery supplier evaluate both the electrical architecture and the manufacturing process.

• Required pack voltage
• Required capacity
• Preferred 18650 or 21700 cell
• Continuous discharge current
• Peak discharge current and duration
• Charging current
• Series and parallel configuration if defined
• Available pack dimensions
• BMS protection requirements
• Connector and wire specifications
• Operating temperature range
• Vibration or mechanical shock requirements
• Required certifications
• Target sales markets
• Estimated annual quantity

You can submit these specifications through the PKCELL contact page for project review.

Frequently Asked Questions

Conclusion

Spot weld quality in 18650 and 21700 battery packs should be validated as a manufacturing process rather than judged by appearance alone. Visual inspection can identify obvious defects, peel and pull testing provide mechanical evidence, four-wire resistance measurement evaluates the electrical path, and cross-section analysis can reveal hidden fusion quality.

Once a stable welding window has been qualified, production controls should monitor machine settings, electrode condition, fixtures, material lots and periodic destructive test results.

Revalidation is particularly important when changing cell models, tab materials, electrode geometry or welding equipment. A welding schedule proven for one battery configuration should not automatically be assumed suitable for another.