Optimizing Micro-Resistance Spot Welding for 18650 Li-ion Battery Tabs in Electric Vehicle Applications

As electric vehicles (EVs) become increasingly popular, the demand for efficient and reliable battery packs continues to grow. One key challenge in battery pack manufacturing is ensuring strong and durable connections between nickel tabs and cylindrical 18650 lithium-ion cells. In this context, micro-resistance spot welding (micro-RSW) has emerged as a cost-effective and scalable joining technique, especially suitable for low- to medium-volume applications such as electric bicycles and scooters.

Why Micro-Resistance Spot Welding?

- Low investment and maintenance cost
- Localized heating reduces thermal damage
- Provides strong mechanical bonding
- Low electrical contact resistance for effective current conduction

Experimental Design

Researchers welded 0.2 mm thick nickel tabs to 0.3 mm and 0.4 mm thick Hilumin terminals using a series electrode configuration. A combination of Taguchi design and full factorial experiments was employed to systematically analyze the effects of parameters such as welding current, welding time, and electrode force.

Key Findings

- Welding current and welding time were identified as the most critical parameters affecting joint strength.
- Optimal process parameters:
    - For 0.3 mm Hilumin: 1800–2000 A, 8–12 ms
    - For 0.4 mm Hilumin: 1900–2100 A, 8–12 ms
- Four-nugget welding significantly improved mechanical and electrical performance.
- Higher energy input improved joint strength but must be carefully managed to avoid spatter or material degradation.

Data Table: Maximum Load under Different Parameters (0.3 mm Hilumin)

Electrical Resistance and Temperature Rise Data

Process Optimization and Industrial Relevance

Optimizing micro-resistance spot welding (micro-RSW) parameters is not just an academic exercise—it has practical consequences for battery manufacturers. In large-scale EV battery production, even small improvements in weld quality can translate to substantial gains in energy efficiency, lifespan, and safety. Precise tuning of weld current and time ensures consistent joint integrity across thousands of cells, helping avoid internal resistance buildup or thermal hotspots.

Comparison with Other Welding Techniques

While laser welding and ultrasonic welding are also used for cell tab connections, micro-RSW stands out for its simplicity, cost-efficiency, and minimal equipment requirements. Laser welding, for instance, requires higher capital investment and may cause deeper thermal penetration, potentially affecting sensitive battery components. Ultrasonic welding is limited by material thickness and mechanical fixture complexity. In contrast, micro-RSW provides localized heat input, rapid cycle time, and scalable implementation—ideal for applications like e-scooters, power tools, and lightweight EVs.

Material Considerations

The selection of suitable materials is critical for effective welding. Nickel is preferred due to its high electrical conductivity, excellent corrosion resistance, and compatibility with steel terminals like Hilumin. However, differences in thermal conductivity and melting points between dissimilar metals can lead to challenges such as uneven nugget formation or crack propagation. This study’s use of 0.2 mm Ni tabs with 0.3/0.4 mm Hilumin terminals provides a balanced combination of strength, conductivity, and manufacturability.

Thermal Management in Battery Packs

One of the key concerns in EV battery design is heat generation during high-current operation. Poor weld quality may result in increased electrical resistance at the tab-terminal interface, leading to Joule heating. As observed in this study, joint temperature can rise over 60°C under continuous 30 A current, which is beyond the safe operating limit (~45°C) for most Li-ion batteries. Therefore, understanding the thermal behavior of welds is essential for designing effective thermal management systems in battery modules.

Failure Modes and Quality Assessment

Mechanical tests such as lap shear and 90° peel tests provide insights into joint strength and failure mechanisms. Interfacial failure typically occurs in under-welded joints, while well-executed welds tend to exhibit pull-out failure, indicating stronger metallurgical bonding. Cross-sectional microscopy further reveals nugget formation and any deformation in terminal sheets. This non-destructive and destructive testing approach ensures that only robust and reliable welds are used in production.

Implications for Future Research

As battery technology continues to evolve—especially with the emergence of solid-state batteries, tabless designs, and high-voltage platforms—joining techniques will need to adapt. Micro-RSW remains a viable method, but may require integration with intelligent control systems, real-time monitoring, or even machine learning algorithms to dynamically adjust welding parameters. Additionally, exploration into advanced alloys, composite tabs, and alternative coatings can further optimize weld reliability.

Sustainability and Lifecycle Considerations

Reliable welding not only ensures product performance but also supports long-term sustainability. Batteries with consistently strong welds are less likely to fail prematurely, reducing the frequency of replacements and electronic waste. Moreover, a well-joined battery is easier to recycle, as automated disassembly systems can be programmed to target uniform weld zones. Optimized micro-RSW thus aligns with both performance and environmental goals in modern energy systems.

Conclusion Reiterated

This extended study reaffirms the importance of parameter optimization in micro-RSW for cylindrical Li-ion cells. By focusing on process design, material selection, mechanical integrity, and thermal behavior, manufacturers can significantly improve battery pack reliability for electric vehicles. As the EV market grows, adopting robust and scalable welding solutions like micro-RSW will be essential for maintaining quality at scale.