Electric vehicles are a key asset in the fight against greenhouse gas emissions. Given that they represent the future of mobility, their development must be rapid if they are to offer users a level of service comparable to that of combustion-powered vehicles.

One of the areas for innovation concerns wiring: vehicles necessarily carry larger electrical cables (to transport power), which are costly, hot and make integration more complex.

What is the main constraint linked to the transport of power in on-board electrical architectures?

Power transmission obeys an inescapable physical rule:

Power = current x voltage (P=U.I)

A given amount of power can therefore be transferred at high current and low voltage, or at low current and high voltage. The difference between these 2 approaches lies in the losses they cause due to the resistivity of the electrical cables used: these losses are proportional to the square of the current (P = R.I²).

At constant power, a solution using 3 times more current will therefore generate 9 times more losses in the cables.

One way of avoiding this is to reduce the resistivity of the cable (ideally by a factor of 9) by increasing its diameter.

This approach has several consequences:

• Increase in the price of the cable,
• Increased mass,
• Reduced cable flexibility (more difficult to integrate into cable trays).

The other way is to increase the voltage: a solution using a voltage 3 times higher will result in 9 times fewer losses in the cables. The consequences are reversed compared with the high-current approach.

Why didn’t on-board voltages increase before?

A little history…

The first car was built in 1886. The first cars had no batteries, as their electrical equipment was virtually non-existent at the time. It was not until 1918 that the American car manufacturer Hudson Motor Car Company became the first to use standardised batteries. Batteries began to be used on a large scale from the 1920s onwards.

The first starter-recharge system was designed for a voltage of 6 volts with a positive earth. Cars were equipped with 6 V systems until the mid-1950s. The change from 6 to 12 V occurred when vehicles became larger: the compression ratio of the engines was higher and therefore required more energy to start, making it possible, among other things, to halve the cross-section of electrical cables for the same distributed power.

However, some cars continued to be equipped with 6 V batteries, such as the Beetle until the mid-1960s and the Citroën 2 CV until 1970.

Technological barriers

The adoption of high voltages took time, because two major technological obstacles had to be overcome:

• The constitution of batteries: the higher their voltage, the more cells they contain, and the greater the need to ensure good balancing of these cells (intrinsic quality of cell chemistry, performance of the associated Battery Management System).
• Availability of high-voltage power electronic components: transistors, diodes, capacitors, insulators.

Market trends

The electric mobility market is continuing to move towards high voltages:

• In the area of high power (linked to the traction chain), 400VDC solutions are migrating towards 600V, 800V, 1200 V, or even more.
• In the area of ‘low voltage’ power (supplying ECUs and on-board components accessible to users): the traditional 12VDC is being supplemented by a 48VDC line.

In practice, what are the advantages of this development?

The advantages of increasing voltage levels are

• Reduced Joule effect losses on high-power electrical systems.
• Limiting losses increases overall efficiency, and therefore increases autonomy.
• Lighter cables (less copper)
• Improved mechanical integration of these more flexible cables in vehicles.

These improvements apply equally to the powertrain and the 48V part.

Are there any disadvantages to increasing the voltage level in the architecture of an electric vehicle?

At this stage of maturity in the electric vehicle market, the process of increasing voltage levels has not yet been stabilised or fully standardised.

This instability hampers the emergence of standard solutions.

Tame-Power converters have a wide range of operating voltages, both low side and high side (30V … 450V / 30V … 950V), so they can be adapted to a wide variety of architectures.

This feature allows electrical designers and architects to consider several solutions to optimise the overall energy performance of their system.