Although the technology behind electric vehicles (EVs) has been around for some time, the past decade has seen a significant increase in sales of EVs and hybrid electric vehicles (HEVs) as personal motor vehicles. With the benefits they offer, EVs are seen by many as the future of the automotive industry.
As the EV and HEV markets grow, manufacturers will seek technological advancements to increase the efficiency, power, and capabilities of these vehicles, and ultrasonic welding will play a role in these technological advancements.
The automotive wiring harness manufacturing industry has been the largest user of ultrasonic welding since the late 1980s, primarily using the technology for wire splicing. However, new applications for the technology are being used as part of future processes that will ultimately provide automakers with solutions to many of the shortcomings of today’s EV technology.
Current state of electric vehicle manufacturing
In electric vehicles, large battery packs combined in a sealed package are used to achieve the operating voltages and currents needed to power the vehicle’s electric motors. The two main issues in the EV/HEV space right now are energy storage and driving range. OEMs are addressing these issues in two ways: making batteries bigger to achieve greater range, and making batteries more powerful to charge faster. Both approaches have challenges. Yes, batteries can get bigger, but they can only get to a certain size before they become too expensive and too heavy to be a viable solution.
Conventional wiring isn’t the first place one usually looks for EV innovations, but recent advances have had a major impact on the EV story because they give OEMs two things they desperately need in EV architectures: less mass and more space. One way to free up space and reduce mass is to move from round wiring to flat conductors. That’s where busbars come in.
What is an electrical busbar?
Busbars are flat conductors that are becoming part of the architecture of electric vehicles. Busbars are typically installed inside switchgear, distribution boards, and busway enclosures for localized high-current power distribution. They are also used to connect high-voltage equipment in electrical switchyards and low-voltage equipment in battery packs. Busbars are metal strips or bars made of copper, brass, or aluminum that are used for grounding and conducting electricity.
Electrical busbars can be coated with various materials, such as copper, to provide different conductivity limits and variations. Busbars come in a variety of shapes and sizes that will determine the maximum amount of current the conductor can carry before deterioration.
Today, there are as many as 20+ busbars in a battery pack, and as battery packs get larger and/or more powerful, that number will increase, while space inside the pack remains very tight. Ultrasonic welding is the preferred joining process for busbars in electric vehicle applications.
However, since these more powerful batteries are only as good as their ability to charge quickly, we will soon see more busbar innovations outside of the battery pack, transferring high power from the charging inlet to the battery and on to other high-power motors and devices, increasing the need for innovative ultrasonic welding applications.
Why companies prefer to choose busbar
In the long run, it is believed that busbars may become preferred over standard cables for some wiring harnesses in the automotive industry. The growing popularity of electric vehicles, cost-effectiveness, ease of installation, low repair and service costs of automotive busbars, and development of charging infrastructure for electric vehicles are some of the key factors for the growth in the demand for automotive busbars. Moreover, technological developments in the manufacturing and charging infrastructure of electric vehicles are expected to benefit the global automotive busbar market. Due to these factors, the market is expected to generate revenues of over USD 170 million by 2030, registering a CAGR of 24.6% during 2021-2030, according to market research.
Benefits of using busbars:
- Reduced facility costs and faster installation
- Ability to easily and quickly add, remove or relocate power without downtime
- Future-proof and highly flexible, as some plug-ins can be disconnected and reconnected without powering down
- No routine maintenance required
- Faster and less expensive to expand or retrofit
- More environmentally friendly, as it generally requires less installation material and the plug-in sockets are reusable and easily repositionable
- Flat conductors take up less space and are 70% shorter in height
- Can support 15% more power than cables with the same cross-sectional area
- Less weight and packaging space, better flexibility. For example, 160 mm² Flexible Flat Aluminum (FF-Al) cable is an innovative and alternative solution to 200 mm² round aluminum cable.
- Fastening with bolts, which is the most reliable process today and less costly. But it adds extra parts (bolts) and requires specific torque values
- Efficient heat dissipation – more efficient than stranded cables
- Multiple structures – copper and aluminum, rigid or flexible, laminated.
- Internal battery without electromagnetic compatibility
- Facilitates automation, improves safety and quality
Importance of busbar material and size
Busbars are typically made of corrosion-resistant copper, brass or aluminum and housed in solid or hollow tubes. The shape and size of the busbar, whether flat bar, solid rod or bar, allows for more efficient heat dissipation due to the high surface area to cross-sectional area ratio.
Although copper will oxidize over time, it is still conductive, but this generally means more power can be pushed along the surface. While it cannot completely prevent oxidation over time, it greatly reduces the effects. Coating the busbar surface will help prevent oxidation. Busbar coatings generally have three main purposes:
- Inhibits corrosion
- Improve electrical conductivity
- For aesthetic purposes
Laminated busbars are used to avoid circulating currents in parallel switchgear in power electronic circuits. In addition to its important application in electric vehicles, it also has wide applications in solar and wind energy collection and distribution due to its low inductance properties. A more efficient and cost-effective method is to use insulating epoxy coating powders. Epoxy coating powders have extremely high dielectric strength and can be directly bonded to the busbar copper, aluminum or silver plating.
The size of the busbar depends on its specific use. The most common commercial and industrial busbar sizes are 40–60A, 100A, 225A, 250A, 400A and 800A.
The current sizes of busbars used in automotive applications are 35, 50 or 90 mm².
Busbars are available in copper and aluminum. The main differences to consider when choosing the material are:
- Tensile strength
- Current carrying capacity
- Resistance
- Weight
- Cost
Aluminum busbars are less expensive and work well in high humidity conditions. However, aluminum has lower current capacity and resistivity than copper. Copper has better thermal properties than aluminum.
Busbar manufacturers can review minimum requirements for busbars used in EV/HEV or other power distribution applications, detailing the tradeoffs between cost and material selection and performance. Of course, for EV/HEV power distribution applications, driver safety is an additional concern, and busbar materials should be selected to achieve the highest reliability possible, not only to meet vehicle warranty requirements, but also for driver and passenger safety.
Calculating conductor size is particularly important for the electrical and mechanical performance of the busbar. Current-carrying requirements determine the minimum width and thickness of the conductor. Mechanical considerations include rigidity, mounting holes, connections, and other subsystem elements. The width of the conductor should be at least three times the thickness of the conductor. Adding lugs and mounting holes changes the cross-sectional area of the conductor, creating potential hot spots on the busbar. The maximum current per tab or termination must be considered to avoid hot spots.
Solid and flexible busbars
Another key distinction that must be considered is solid busbar versus flexible busbar. For automotive applications within EV batteries, solid busbar is used (see Figure 2). Flexible busbar is used in short segments when a specific area needs to be moved for assembly or application. It is used as an electrical “jumper.” An example of a flexible busbar is shown in Figure 3.
Flexible busbars have several thin layers of copper or aluminum designed to efficiently distribute power in AC or DC systems. The copper foil stack is welded in the assembly area, making the ends rigidly connected while the middle remains flexible. Examples of applications that require flexible busbars include:
- Electric, hybrid and fuel cell vehicles
- Switchgear and transformers in the energy and offshore industries
- Generators for marine applications
- Transformers and charging stations
- Switchgear and substations in railway applications, chemical plants and high-voltage power distribution
- Generator power links
- Electrical connections in switchgear
Future automotive busbar applications
Busbar innovation outside the battery pack will be a hot topic in the future, transmitting high power from the charging inlet to the battery and then to other high-power motors and devices (see Figure 4).
There is a growing interest in busbars across all OEMs and Tier 1 suppliers, primarily for high voltage applications. Today, a battery pack has approximately 15-20 busbars. For the outside of the pack, an automated shielding process is required, which does not exist today. Currently, the focus is on the battery pack.
As future innovations increase the utilization of busbars outside the battery pack, these new applications will create significant opportunities for ultrasonic welding to improve the overall quality of future connection designs in busbar construction. Ultrasonic welding, particularly torsional welding technology, allows for larger weld sizes, low vibrations, and the ability to connect harder to reach areas. As the industry evolves, these capabilities will allow for further implementation of busbars outside of the EV battery pack. Figure 5 provides several examples of how ultrasonic welding may be implemented in future EV applications.
Challenges of applications outside the battery pack:
- Busbars outside the battery pack require shielding, which is not currently available – the battery pack has a sealed and EMI-shielded housing
- There is a problem when busbars need to bend around – they may be too stiff or may break at the corners of the bend
- The bolting process requires additional parts and specific torque values. Busbars with bolt holes can be replaced for busbar applications outside of battery packs
- Aluminum busbars require plated bolt holes due to corrosion
- Terminals are attached to solid busbars to facilitate automation
- Automation is not yet fully realized due to shielding
- Welds and assembly may require new standards and validation
Current Application of Busbars in Ultrasonic Welding
Ultrasonic welding technology is a proven joining process that is increasingly being used by automotive manufacturers for cable-to-terminal connections, busbars, battery manufacturing, and power electronics in electric vehicles. Linear welding is the more traditional and well-known technology used by all equipment manufacturers and is the standard process for splicing wires. However, like many other joining processes, linear welding has size limitations, welding difficulties in small areas and specific geometries, welding orientation issues, and vibration effects on peripheral components.
There are already smaller busbar applications that are joined using ultrasonic welding. Ultrasonic welding is the preferred joining process for many busbars, such as flexible flat busbars up to 160 mm². In the future, there will be many new applications that utilize ultrasonic welding in busbar implementations in wiring harnesses. Some existing uses of ultrasonic welding in busbar applications are described below.
Curing of flexible busbar
Flexible busbars require solidification at the connection section to connect (attach) them to standard cables or connectors. In some cases, the connection and solidification of the cable or terminal can be completed in a single welding step. Depending on the overall dimensions of the flexible busbar, ultrasonic metal welding can be a high-quality, economical solution. Using the torsional welding process, cross-sections of up to 200 mm² can be welded. This welding technique prevents hardening of the connected material, which can lead to brittleness and noticeable changes in the material properties.
Busbars welded to standard cables
In some applications, the busbars are welded to the orange cable, which will be welded to the current connector. Figure 8 shows an example of a short cable welded to a stranded cable. Welding a short cable at both ends can result in inconsistent weld quality, as the first weld may be weaker due to vibrations caused by the second weld.
Flat braided cable welds
In some cases, manufacturers use flat braided cable instead of orange cable. Flat braided cable is welded and automatically cut into pieces with specific lengths and welds on both ends (see Figure 10). Braided cable with welds on both ends is also called a shunt. The advantage of using ultrasonic welding to make shunts is that minimal heat is required when making the shunt and welding the shunt to the busbar (see Figure 11). This prevents brittle strands and scoring of unusually thin strands due to the heat generated by resistance welding (another technique that can be used).
Torsional Welding Busbar Applications and Capabilities
Flexible busbar foil is laminated/plated with materials such as copper to prevent oxidation problems. For solid busbars, the bolt hole connection part must be plated. For aluminum solid busbars, the connection contacts must be copper. Therefore, copper washers are used and connected to the busbar by torsional welding (see Figure 12).
Summarize
The innovative and rapidly growing electric vehicle market requires new, evolving solutions to meet future challenges. Soon, the use of high-voltage busbars will replace some current applications for high-voltage cable terminations. As the industry moves toward using busbars outside the battery pack, new challenges will arise before the automotive industry can establish standardization for busbar harnesses. As new applications require more innovative welding solutions, challenges will arise at all levels, including welding equipment manufacturers. But new processes and concepts will provide more efficient and economical solutions for wiring harnesses in the electric vehicle market. Torsional welding has become an important joining process in the industry. In addition to battery cable termination solutions for various connectors, the technology also provides welding solutions for electric vehicle weight control, battery packaging, busbars, battery manufacturing, and power electronics. The application capabilities are expanding beyond what was previously imagined.
As product designers and process engineers continue to become familiar with the torsional welding process and its capabilities, the technology will help drive the electric vehicle industry to greater heights. A closer working relationship between OEMs, Tier 1 suppliers, and equipment suppliers is necessary to drive busbar utilization. We will certainly learn more and introduce innovative ideas in time, but ultrasonic welding will undoubtedly be part of the solution to achieve the goals of material cost, weight and space reduction, and labor-intensive manufacturing processes.