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Wire Compacting for EV Wire Harnesses using Resistance Welding

Robust solid cuboid shape lessens contact resistance and reduces overall weight

By now, we all know that electric vehicles are powered by batteries and that wires and cables – several kilometers thereof – are used to distribute available current to all of the vehicular components. And as the “central nervous system” of the vehicle, those wire connections must be fault free and robust enough to ensure that the vehicle starts, that headlights come on, tire pressure sensors are read, and battery power level is displayed on the on-board computer. Designing and assembling these essential cables poses numerous challenges, including thermal management, contact resistance, connector dimensions, mechanical stress load, and increased product lifespan. In this article, we will review how resistance weld wire compacting can help design and manufacturing engineers achieve these goals.

Wire compacting solutions

Electric vehicle manufacturers rely on robust electrical connections to distribute power efficiently from the source throughout the vehicle. As the technology has developed, the requirement for higher current carrying capacity has increased the size of the cables to be connected.

To do so, they often turn to stranded wires, known for their flexibility and durability over a solid single element wire, however, the multiple strands are often difficult to manage and often result in frayed ends that can lead to short circuits and lost current. There are a few solutions that can be used:

  • Crimping: One solution is to put a cable lug/connector over the wires and mechanically crimp them in place. This is a relatively easy, straightforward process. This mechanical joint, however, adds significant weight and bulk to the cable harness. In addition, these may loosen over time with vibration and aging of the material.
  • Hot crimping: A second solution is hot crimping, which in addition to the mechanical joint, supplies heat to the crimp location, which can join the elements together and increases the reliability of the joined wires.
  • Ultrasonic welding: A third solution is ultrasonic
  • Resistance Wire Compacting: Another solution  is resistance weld wire compacting, which forms stranded wires to a solid, cuboid shape that is robust, possesses less contact resistance, and, because sleeves are no longer required, can reduce overall weight. Using resistance welding or other joining processes, manufacturers can compact stranded wire and weld it into a desired shape to better facilitate attachment to other components.

In the rest of this blog, we will explore the benefits of resistance wire compacting.

Resistance wire compacting process

Resistance wire compacting (welding or brazing) is suitable for both stranded and solid wire applications. A weld head fitted with tungsten electrodes delivers current and applies vertical force to “compact” the wires, while ceramic tooling inserts apply a sideward force to assist in controlling the final shape. Figure 1 illustrates the compacting process.

Wire compacting process
Figure 1: Six steps of the wire compacting process

Figure 2 provides a few examples of typical compacted wire applications for EVs. In addition to automotive wire harness cabling, wire compacting is used in the production of safety belt sensors, electrical distribution from batteries, and temperature sensor connections, as well as airbag initiators, and Y and T connections.

Typical compacted wire applications
Figure 2: typical wire compacting applications

Compacting wires solves stranded wire challenges

Wire compacting helps EV manufacturers with five key challenges associated with stranded wires, including contact resistance, connection footprint, reduced product lifespan, and mechanical stress load. The table below reviews these challenges and explains how compacting helps solve them.

Wire compacting challenges and solutions

Consider restrictions and limits to determine if compacting is a good fit

Compacting is an ideal solution for copper wire, with or without tin or silver coating. Depending on the parameters chosen, the degree of compacting (compacting strength) can vary from light adhesion of the individual wires to a solid copper end.

Key considerations include:

  • Electrode width must be greater than 0.6 mm to ensure maintaining a stable process.
  • The height of the compacting must not be greater than the width. The smallest possible cross section is 0.33 mm² (0.6 mm x 0.55 mm).
  • To achieve a long electrode lifetime, the electrode cross section should be double the size of the welding area cross section after compacting.
  • The electrodes must create a metallic path.
  • To avoid negative heat generation on the electrode contacting surface and potential damage to electrodes, the electrode force must ensure sufficient contact force on all contacting points during current flow.
  • Higher contacting force is required in case of higher welding currents and worse surface condition of the parts.

The following table provides a brief summary of wire compacting basics.


How to select the right equipment

Selecting the right resistance welding compacting system requires consideration of the power supply, weld head, compacting mechanism, weld controls, and monitoring unit.

It also depends upon the size of the wires to be compacted, and weld force needed. Before choosing equipment, it is important to carefully calculate compacting dimensions. The width times height must be the same or slightly smaller than the nominal wire cross section.

The future looks compacted

We are seeing more and more requests for large compacted cables; especially as electric vehicle technology requires numerous interconnections that carry high currents. Compacting is also being used for connections required in EV battery modules. To ensure success, carefully consider the wire size to be compacted, follow the rules of thumb for establishing the necessary compacting force, and be mindful of the other compacting restrictions and limits. And always test the application first in an established technical laboratory.

Category: Resistance Welding