Battery Module Manufacturers Need Robust High-speed Options for Collector Plate Attachment
Advanced Current Collector Plates Can Be Attached Using Wire Bonding, Laser Welding, or Ultrasonic Welding
The global growth of electric-powered vehicles is raising the bar for creating more robust battery module designs that can be cost-effectively manufactured in large production volumes. The previous Tech Bulletin in this series, “Robust, Customizable Battery Interconnect Systems Streamline Design and Manufacturing of Battery Modules” described in detail how the Interplex Cell-PLX™ interconnect system addresses these issues from an overall assembly standpoint.
This Tech Bulletin goes deeper to address the need for multiple approaches for the attachment of battery collector plates to cells within the battery module. It specifically looks at the tradeoffs and other considerations for using wire bonding, laser welding or ultrasonic welding and detailed design and process issues that are involved with each approach.
Overview of Battery Modules
Battery modules typically consist of a large number of individual cells. Grouped together, they interface with current collectors that gather the specified power output levels. The number of cells and power output of each cell are factors that determine the overall output that the battery module can deliver.
Battery interconnect systems must be able to connect all the discrete cells in series and/or parallel to achieve the total voltage and current requirements of the battery module.
Advanced battery modules for electric drive EVs and public transport vehicles can contain up to thousands of individual cells, increasing the complexity of the overall system.
The Role of Current Collector Plates
As the name implies, current collector plates perform the key function of interfacing all the individual battery cells to aggregate the power into a single output meeting the required performance specification.
Specific design of the collector plates and their attachment approach depends on the number of cells and the particular vehicular application, ranging from small two-wheel scooters to huge trucks and buses.
Collector plates provide positive and negative interconnect terminals for attachment to the battery cells. Typically, the cells are connected to a thicker current collector or two current collectors separated by a dielectric layer.
Customized current collector designs make provisions for these terminals to be optimally positioned for mating with the cells, improving the quality and robustness of the connections.
As discussed in the following sections, the thickness of current collectors can be tailored to match the current density and structural requirements of the battery modules and the individual cell terminals can be of thinner material than the plate itself, thereby adapting for different cell attachment approaches while maintaining strength and structural integrity across the entire current collector.
Comparison of Cell Attachment Methods
As EV battery designs have evolved, the range of preferred attachment methods has expanded to accommodate different applications as well as the need for more cost-effective high-volume production requirements.
Attachment approaches include:
- Wire bonding / Laser Bonding
- Resistance welding
- Laser welding
- Ultrasonic welding
Wire Bonding / Laser Bonding
Wire bonding has been the most widely employed attachment method since the early development of EV battery designs. This is largely due to the flexibility of wire bonding to create connections between the cells and plates without having to maintain tight control over tolerances and to achieve a perfect fit for physical mating interfaces between them.
Because it is inexpensive and flexible, wire bonding became an expedient approach for handling connections as early designs were developed and modified. However, it is a relatively slow process and therefore has hindered the ability of battery manufacturers to ramp up for higher production levels. Laser bonding also is an alternative in this area, with many of the same issues.
Resistance welding is a potential alternative that has not gained much traction in advanced battery applications because of the requirement to pulse current through the parts to be joined. Experience has shown that resistance welding can cause damage to the battery cells and/or to thin battery terminals on the collector plates. Resistance welding is relatively inexpensive compared to laser or ultrasonic welding, but it is inherently slower. Also, it is limited to use with nickel material and is not appropriate for use with copper. Therefore, it may have useful application in relatively simple battery module designs but the material limitations and potential for damage, waste or rework make it less effective for high-volume assembly of more complex battery modules.
Laser Welding & Ultrasonic Welding
These two approaches are similar in that they can deliver robust joining results to attach individual cells in large battery packs while providing the high-speed and consistency needed for volume production requirements.
Laser welding is generally a faster process than ultrasonic, but it also has higher capital costs. Both processes provide vibration-free, fast throughput and consistent welding results.
While these approaches require investment in more sophisticated production systems and application-specific tooling, the higher production throughput and consistent high-quality connections generally can deliver rapid return on investment (ROI).
High-speed welding is rapidly becoming a preferred approach for connecting large cell arrays and collector plates. However, battery manufacturers need to understand the specifics of collector plate design and manufacturing that are necessary to optimize laser or ultrasonic welding processes. These are discussed in the next section.
Optimizing Collector Plates for High-Speed Welding Processes
Collector plate design is critical for success with high-speed laser or ultrasonic welding
Some of the key factors that must be addressed include:
- Achieving a close tolerance fit between the battery cells and collector plate tabs
- Controlling connector tab material thickness to optimize welding results
- Providing overall collector plate thickness needed for mechanical integrity
- Maintaining consistency of collector plate tolerances over high volumes
To achieve these goals, it has become increasingly important for battery manufacturers to work closely from the beginning of their design processes with collector plate vendors who deeply understand overall battery module requirements and can deliver the application-specific collector plate designs required for their unique requirements.
The key challenge often is the need to combine a thicker overall collector plate with thinner tabs that are precisely located to interface with the individual battery cell POS/NEG terminals to assure quality welding results.
Collector plate vendors may use a variety of approaches to achieve the specified material thicknesses for both the plate and connection tabs. These include:
- Skiving – removing slices of material to reduce the thickness
- Coining – compressing the material under pressure to reduce the thickness
- Joining – combining two different materials of different thickness
Each of these approaches entails different trade offs in terms of material characteristics (e.g. coining can increase brittleness) and cost (e.g. secondary joining processes can be more costly).
As Electric Vehicles become a larger share of the overall vehicle market, battery makers are facing the challenge of delivering much higher volumes, while at the same time needing to drive down production costs.
At the heart of this challenge will be the transition from relatively low volume wire bonding attachment methods to high-throughput laser or ultrasonic welding processes.
To achieve these goals, battery makers need to work with experienced and knowledgeable battery interconnect design partners that have the expertise to create application-specific collection plates that are tailored to meet the parameters of high-speed welding processes.
Since there is no “one-size-fits-all” approach and each battery application is unique, collection plate suppliers must understand the big-picture of battery design and be able to provide a range of capabilities to meet each battery maker’s design and production goals.