Ultrasonic Welding of (EVs) Connecting wire Busbars

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 of 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 electric vehicle technology. This article mainly introduces the ultrasonic welding of automobile wiring harness busbars and busbars.

Busbars welded to LEONI connecting bolts

1. Current electric vehicle manufacturing landscape
In electric vehicles, large battery packs combined in sealed packages are used to achieve the operating voltage and current required to power the vehicle’s electric motor. Currently, the two main issues in the EV/HEV field are energy storage and driving range. OEMs are addressing these issues in two ways: making larger batteries for greater range, and making more powerful batteries for faster charging. Both approaches have challenges. Yes, batteries can get bigger, but they can only reach a certain size before they become too expensive and heavy to be a viable solution.
Traditional wiring isn’t usually the first place people look for EV innovation, but recent advancements are having a major impact on the EV story. Because they give OEMs two things they desperately need in EV architecture: less mass and more space. One way to free up space and reduce mass is to switch from circular wiring to flat conductors. That’s what the busbar is for.

Flat braided cable solidified and welded by ultrasonic welding

2. What is an electrical busbar?
Derived from the Latin word “omnibus,” which translates as “all” (as in “all currents in a given system”), busbars are flat conductors that are becoming part of the electric vehicle architecture. Busbars are usually installed in switchgear, switchboards and busway enclosures for localized high current distribution. They are also used to connect high-voltage equipment in electrical switchyards and low-voltage equipment in battery banks. Busbars are metal bars or rods 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 many shapes and sizes, and these shapes and sizes will determine the maximum amount of current the conductor can carry before deteriorating.
Today, there are up to 20+ busbars in a battery pack, and this number will increase as battery packs get larger and/or more powerful, while the space inside the battery pack remains very tight. Ultrasonic welding is the preferred joining process for busbars in electric vehicle applications. But since these more powerful batteries are only capable of fast charging, we may soon see more busbar innovation beyond battery packs. Transferring high power from the charging inlet to the battery and on to other high-power motors and equipment increases the need for innovative ultrasonic welding applications.

Solid busbars connected to flat braided wires

3. Why do companies prefer busbars?
In the long term, it is thought that busbars may be preferred over standard cables for some wiring harnesses in the automotive industry. The increasing popularity of electric vehicles, cost-effectiveness, ease of installation, low maintenance and service costs of automobile bus bars, and development of electric vehicle charging infrastructure are some of the key factors driving the growth in demand for automobile bus bars. Furthermore, technological developments in electric vehicle manufacturing and charging infrastructure are expected to benefit the global automotive busbar market. According to market research, due to these factors, the market is expected to generate more than $170 million in revenue in 2030, growing at a CAGR of 24.6% from 2021-2030.

Solid busbars welded to cables

Advantages of using busbars:
• Reduce facility costs and speed installation
• Ability to add, remove or relocate power supplies easily and quickly without downtime
• Future-proof and highly flexible as some plug-ins can be disconnected and reconnected without power interruption
• No routine maintenance required
• Expansion or renovation is faster and cheaper
• More environmentally friendly as it generally requires less installation material and plug-in sockets are reusable and easily repositioned
• 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) cables are an innovative and alternative solution to 200 mm² round aluminum cables.
• Fastening with bolts, the most reliable process available today and less expensive. But it adds extra parts (bolts) and requires specific torque values
• Efficient heat dissipation – more effective than stranded cable
• Various constructions – copper and aluminum, rigid or flexible, laminated. see picture 1
• Internal battery does not require electromagnetic compatibility
• Promote automation, improve safety and quality
Figure 1 – Examples of various busbars – rigid, flexible, custom designs

Cables welded at both ends of solid busbar

4. The importance of busbar material and size
Busbars are usually made of corrosion-resistant copper, brass or aluminum and housed in solid or hollow tubes. The shape and size of the bus bars, whether flat strips, solid rods or rods, allow for more efficient heat dissipation due to a high surface area to cross-sectional area ratio.
Although copper oxidizes over time, it remains conductive, but that usually means more power can push electricity along the surface. Although it does not completely prevent prolonged oxidation, it greatly reduces the effects. Coating the busbar surface will help prevent oxidation.
Busbar coatings typically serve three main purposes:
1. Inhibit corrosion
2. Improve electrical conductivity
3. For cosmetic purposes
Laminated busbars are used to avoid circulating currents in parallel switching devices in power electronic circuits. In addition to its important applications in electric vehicles, it also has extensive applications in solar and wind energy collection and distribution due to its low inductance properties. A more effective and cost-effective method is to use an insulating epoxy coating powder. Epoxy coating powders have extremely high dielectric strength and can be bonded directly to busbar copper, aluminum or silver plating layers.
The size of the busbar depends on its specific use. The most common commercial and industrial busbar sizes are 40–60 amps, 100 amps, 225 amps, 250 amps, 400 amps, and 800 amps.
Current sizes of busbars used in automotive applications are 35, 50 or 90 mm².

Flexible busbars and solid busbars welded to solid busbars

Busbars are available in two materials: copper and aluminum. The main differences to consider when choosing materials are:
• tensile strength
• Current carrying capacity
• Resistance
• weight
• cost
Aluminum busbars are lower cost and work well in high humidity conditions. But aluminum has lower current capabilities and lower resistivity than copper. Copper has better thermal properties than aluminum.
Busbar manufacturers can review the minimum requirements for busbars for EV/HEV or other power distribution applications, detailing cost and material selection trade-offs with performance. Of course, for EV/HEV power distribution applications, driver safety is an additional concern. When selecting busbar materials, the highest possible reliability should be achieved, not only to meet vehicle warranty requirements, but also for driver and passenger safety.
Calculation of conductor sizes is particularly important for the electrical and mechanical performance of busbars. 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 conductors, creating potential hot spots on the bus bars. The maximum current per slice or termination must be considered to avoid hot spots.

Ultrasonic welding of busbars in electric vehicle applications

5. Solid and flexible busbars
Another key difference that must be considered is solid busbars vs. flexible busbars. For automotive applications within EV batteries, solid busbars are used (see Figure 2). Flexible busbars are used in short sections when a specific area needs to be moved for assembly or application. It serves 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 and are designed to efficiently distribute power in AC or DC systems. Solder the copper foil stack in the assembly area so that the ends are rigidly connected while the middle remains flexible. Examples of applications requiring flexible busbars include:
• Electric, hybrid and fuel cell vehicles
• Switchgear and transformers for the energy and offshore industries
• Application of generators in the shipbuilding industry
• Transformers and charging stations
• Switchgear and substations in railway applications, chemical plants and high voltage power distribution
• Generator power link
• Electrical connections in the switch cabinet

Wiring harness copper busbars

Figure 2 – Internal EV battery pack

Customized flexible busbar

Figure 3 – Flexible busbar example

Application of automobile busbars in the future
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 equipment (see Figure 4).
There is an increasing interest in busbars from all OEMs and Tier 1 suppliers, mainly for high voltage applications. Today, battery packs have approximately 15-20 busbars. For the outside of the packaging, an automated masking process is required, which does not exist today. For now, the focus is on the battery pack.
As future innovations increase the utilization of battery pack external busbars, these new applications will create significant opportunities for ultrasonic welding to improve the overall quality of future connection designs in busbar structures. Ultrasonic welding, specifically the twist welding technique, allows welding of larger sizes, gentle vibration, and the ability to join harder to reach areas. As the industry evolves, these capabilities will allow for further busbar implementation outside of EV battery packs. Figure 5 provides several examples of how ultrasonic welding may be implemented in future electric vehicle applications.
Companies such as Tesla, BMW and Ford are pushing the use of busbars outside the battery pack. Recently, global technology company APTIV acquired Italian company Intercable for approximately $600 million, actively seeking to use busbars for high-power distribution outside the battery pack. BMW, one of its top three customers, is showing strong signs of pursuing this new way of distributing electricity. A handful of other companies are developing shielded busbars in the United States and Europe.

Customized various busbars – rigid, flexible

Figure 4 – Inlet Harness with Busbars

Figure 5 – Future implementation of busbar ultrasonic welding in electric vehicle applications

7 Challenges faced by applications outside the battery pack:
1. The busbar outside the battery pack needs to be shielded, which is not currently available – the battery pack has a shell that is sealed and shielded from electromagnetic interference.
2. There is a problem when busbars need to be bent around – they may be too stiff or may be damaged in the corners of the bends
3. The bolting process requires additional parts and specific torque values. Busbars with bolt holes can be replaced for busbar applications other than battery packs
4. Due to corrosion, aluminum busbars require plated bolt holes
5. The terminals are connected to the solid busbar for easy automation
6. Automation has not yet been fully implemented due to shielding
7. Welds and assemblies may require new standards and verification

8 Current applications of bus bars in ultrasonic welding
Ultrasonic welding technology is a proven joining process that automakers are increasingly using for cable-to-terminal connections in electric vehicles, busbars, battery manufacturing and power electronics. Linear soldering is the more traditional and well-known technique 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 smaller areas and in specific geometries, welding orientation issues, and vibration effects on peripheral components.
Telsonic Twist SONIQTWIST® and PowerWheel® technologies provide innovative solutions for electric vehicle connectivity applications that were previously impossible. These innovative technologies allow many connection designs related to busbar applications where linear welding would not be possible. There are already smaller busbar applications using ultrasonic welding for connections. 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 utilizing ultrasonic welding in the busbar implementation of wiring harnesses. Some of the existing uses of ultrasonic welding in busbar applications are described below.

9 Curing of flexible busbars
Flexible busbars require curing at the connecting portion in order to attach (attach) them to standard cables or connectors. In some cases, the connection and solidification of cables or terminals can be accomplished in one step of soldering. Depending on the overall dimensions of the flexible busbar, ultrasonic metal welding can be a high-quality, economical solution. Using the twist welding process, material cross-sections up to 200 mm² can be welded. This welding technique prevents the joining material from hardening, which can lead to material brittleness and noticeable changes in material properties. Additionally, coagulation can be automated with Telsonic devices such as the TT7 PowerWheel®, as shown in the applications in Figures 6 and 7.

Figure 6 – TT7-Tonic Power Wheel®

Figure 7 – Welding solidified flexible busbars and solid busbars to solid busbars using TT7 Telsonic PowerWheel®

10 busbars welded to standard cable
In some applications, the bus bars are soldered to the orange cable, which will be soldered to the current connector. Figure 8 shows an example of a short cable welded to stranded cable. Welding short cables at both ends can result in inconsistent weld quality because the first weld may become weaker due to vibrations caused by the second weld. USCAR-38 requires testing of cables less than 500 mm in length. Using torsion welding provides such gentle vibration that studies have shown that, depending on the terminal design, the impact on stranded cables and flexible busbars is much less (see Figure 9). This allows for shorter cables and appropriate connectors to be soldered together.

Figure 8 – Solid busbar welded to standard cable

Figure 9 – Short cable (200 mm) soldered at both ends

11 Flat braided cable welds
In some cases, manufacturers use flat braided cables instead of orange cables. Flat braided cables are welded and automatically cut into parts with specific lengths and welds at both ends (see Figure 10). Braided cables with welds on both ends are also called shunts. The advantage of using ultrasonic welding to manufacture the shunt is that minimal heat is required when manufacturing the shunt and welding the shunt to the busbar (see Figure 11). This prevents brittle strands and scoring of unusually thin strands caused by the heat generated by resistance welding (another technique that can be used).

Figure 10 – Solid busbars connected to flat braided jumpers

Figure 11 – Flat braided cable cured and welded by ultrasonic welding

12- Torsion welding application capability of busbars
Flexible busbar foils are laminated/plated with materials such as copper to prevent oxidation issues. For solid busbars, the bolt hole connections must be plated. For aluminum solid busbars, the connecting contacts must be copper. Therefore, copper washers are used and connected to the bus bars by twist soldering (see Figure 12). The proven SONIQTWIST® technology as well as the Telsonic TSP welding machine (Fig. 13) can be used for this application.

Figure 12 – Copper nut welded to Al busbar using Telsonic twist welder SONIQTWIST®

Figure 13 – Telsonic SONIQTWIST® TSP
British car manufacturer Jaguar is currently utilizing the twist welding capabilities of SONIQTWIST® and PowerWheel® for power distribution busbar assemblies. The company used busbars instead of copper cables to significantly reduce the weight and cost of the F-TYPE sports car (see Figure 14). Each busbar conducts power from the battery in the vehicle’s trunk to electrical equipment in the engine compartment. Because the relative density of aluminum is significantly lower than copper, the weight of aluminum rods is only 40% to 60% of traditional copper cables. This can save up to 3kg in terms of battery connections alone.
Figure 14 – LEONI connecting bolts welded to busbars

13 Conclusion
The innovative and rapidly growing electric vehicle market requires new and evolving solutions to meet future challenges. Soon, the use of high-voltage busbars will replace some current applications of high-voltage cable terminations. As the industry moves toward using busbars outside the battery pack, new challenges will arise before busbar harness standardization is established in the automotive industry. 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. Torsion welding has become an important joining process in the industry. In addition to battery cable termination solutions for a variety of connectors, the technology also provides welding solutions for electric vehicle weight control, battery packaging, busbars, battery manufacturing and power electronics. Application functionality has expanded beyond previously thought.
As product designers and process engineers become familiar with the twist welding process and its capabilities, the technology will help propel the electric vehicle industry to the next level. Closer working relationships between OEMs, Tier 1 suppliers and equipment suppliers are necessary to drive busbar utilization. We’ll definitely learn more and introduce innovative ideas in time. But ultrasonic welding will undoubtedly be part of the solution to the goals of lower material costs, weight and space reduction, and labor-intensive manufacturing processes.