Power Interconnects from Hybrids to Electric Vehicles

­The ongoing evolution in hybrid and electric vehicles (EV) will involve improving the performance and efficiency of inverters, on-board chargers, batteries, and traction motors. With hybrids and EVs, there is a direct relationship between weight and range, so automotive original equipment manufacturers (OEM) are intent on making every constituent element of their vehicles, including power trains, lighter and more compact.

As hybrids and EVs evolved, their electrical and electronic systems became more extensive, more complicated, and heavier. Auto OEMs have been addressing this issue, and recently have come to favor using power busbars over traditional wiring systems.

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Busbars can be configured in many different ways, and preferences for mechanical designs, conductor materials, and insulation methods vary among OEMs. This article sheds light on the intricacy involved in constructing power busbars, and discusses how ENNOVI’s cutting-edge interconnect solutions are being used by OEMs to reduce size and weight, and enhance both performance and dependability, while also reducing total cost of ownership (TCO).

Busbars for Automotive

The power interconnect products we have available range from very small press-fit pins to busbars up to two meters long. All of these products are customizable solutions, giving OEMs full freedom to configure the electronic/electrical systems in their vehicles as necessary.

Battery pack design is a notable example demonstrating why this flexibility is needed. The battery pack is evolving not only in terms of size but also in terms of the fundamental configuration of the battery. Different battery packs from different OEMs might combine 8-, 10-, or even 12 modules. ENNOVI works closely with carmakers and their battery manufacturers to develop customized cell-to-cell contacting systems, for cylindrical, prismatic and pouch cells.

That already obliged providers of interconnect solutions to be flexible, but now the industry is transitioning from modular battery packs to cell-to-pack (C2P) or cell-to-chassis (C2C) designs – which of course vary from one OEM to the next – and all the interconnections are changing again.

In the past, there were a lot of modules, which translated into lots of small busbars, each a few centimeters in size. Now, as battery designs move to C2P or C2C, these busbar designs have been extended; some have reached two meters long. So ENNOVI is adapting its processes and using its in-house design capabilities to manufacture these larger busbars.

Busbar Variations

The busbar solutions used in hybrid electric vehicles are similar to the fully electric ones, but there are differences. The size of the battery in a hybrid vehicle is much smaller, as is the electric motor, which naturally has ramifications for the busbar, which can be smaller and have few interconnects.

The variations from one OEM to the next extend to material choice, and to features, either of which naturally complicate manufacturing. Some OEMs are adding features to the busbars; one example is the addition of current sensors.

One OEM’s approach could be to use only rigid busbars, another may prefer to use only flexible busbars. This has ramifications for material choice. Some OEMs specify aluminum and others copper, though there are signs the industry might standardize on aluminum because of its weight advantage. There are also reasons to combine copper and aluminum, which we will touch on later.

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There are far more material choices for insulation in busbars to protect against electrical shock, moisture, and vibration – and far less agreement among OEMs on which to use, especially when it comes to the newer C2P and C2C configurations. Some OEMs prefer using epoxy powder coating, others prefer heat shrinkable wrap tape. Some OEMs call for power busbar insulation that is over-molded or extruded.

The choices do not end there. OEMs can select from a wide variety of connectors, starting with the battery junction box. This unit manages the connections from the battery pack inverter and charging system, acting as a gateway, allocating power to the high voltage systems in the vehicle. By the way, the battery junction box uses yet another different type of busbar, though they can be relatively simple, without any insulation.

Busbars and Thermal Management

Separately, OEMs are gradually increasing the voltage of their battery packs, some to 800V, others beyond that, in the service of delivering faster charging for vehicle owners. Moving to a higher voltage impacts the busbar, of course. Increasing the voltage reduces the current, but having a higher voltage also has an impact on the choice of insulation material.

If we look at hybrid vehicles, they also require very efficient electric motors optimized for current flow and thermal performance. In the past, power busbars in hybrids were relatively simple, with only three phases. But now they are becoming more complex because there is an additional phase to manage the inductance.

Hybrid and EV power trains will frequently need to be cooled, just as gas-powered vehicles need to be cooled, but occasionally parts of the EV will actually have to be heated. So thermal management in hybrids and EVs is already complex, but when OEMs reduce the size of inverters, DC-DC converters, and onboard chargers, that complicates thermal management even more, even impacting power interconnection.

The industry is evaluating how various polymers and resin materials are responding to these new thermal pressures. Material testing is ongoing to determine the most durable and reliable materials.

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Busbar Intricacies

Vibration is an existential condition for motor vehicles. To compensate for vibration some OEMs are specifying the use of flexible busbars. The flexibility of these busbars depends on their size and the required tolerances. The middle of the busbar is typically the flexible part, usually made from very thin layers of copper or aluminum, which are then welded on both sides to the terminals.

On some EVs, there is an electric motor on each hub of the wheel, and on others, there are two for driving the front and the back wheels. Typically, OEMs are trying to harmonize the design of these electric motors. So, if the vehicle has two motors, they will try to use the same design and busbars to minimize cost. That said, the diameters of the electric motors differ greatly in size (presently the biggest ones can measure up to 280 millimeters) not only within any given vehicle but also from one auto maker to the next.

ENNOVI Interconnect

Meeting these diverse demands requires customization for size, shape, signal, power, and more. This of course requires close collaboration at the design stage, which as a practical matter means ENNOVI is transitioning from a Tier 2 to a Tier 1. Early engagement to transfer know-how to the OEMs to guide them in the choice of raw materials and to determine the parameters where the busbars or power connectors are installed is essential.

Typically, this collaboration starts with drawings to conduct a visibility study of the design. The back-and-forth continues as we work with manufacturers to help them specify tolerances, materials, and interconnections, and determine the optimum processes to use. Collaboration continues through the prototyping stage and then to volume manufacturing.

ENNOVI (formerly Interplex) has served the automotive industry for more than 60 years, supporting the development of hybrid and fully electric vehicles. The two main application areas are power electronics, which include inverters, DC-DC converters, onboard chargers, and controllers, and the powertrain, which includes charging systems, electric motors, and the battery pack.

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