Increase in voltage in electric vehicles' architecture
What are the issues linked to the increase in voltage?
Electric vehicles are key assets in combatting greenhouse gas emissions. As they represent the future of mobility, their development has to be quick in order to offer users the same service level as thermal vehicles.
One ﬁeld of innovation is cabling: These vehicles necessarily carry larger electric cables (for power transmission) which are expensive, hot and make integration more complex.
What is the main constraint of power transmission in on-board electric architectures?
Power transmission is governed by an inescapable physical rule:
Power = Current x Voltage (P=V.I).
We can therefore transfer a given power at high current and low voltage, or at low current and high voltage. The difference between these two 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-RI²).
At constant power, a solution using three times more current will therefore cause nine times more losses through the cables.
One way to avoid this is to reduce cable resistivity (ideally by nine) by increasing its diameter. This approach has several consequences:
- Increase in the price of the cable,
- Increase in the mass of the cable,
- Decrease in the ﬂexibility of the cable (more difﬁcult to integrate into cable trays).
The other way is to increase the voltage: a solution using three times higher voltage will cause nine times less losses in the cables. Consequences are reversed compared to the high current approach.
Why didn't the increase in on-board voltages occur earlier?
The first car was created in 1886. These first automobiles did not have any batteries as they barely integrated any electrical equipment at the time.
It is only in 1918 that the American car manufacturer Hudson Motor Company became the first to use standard batteries. Batteries thus began to be massively used starting in the 1920s.
The first starter-generator system was designed for a 6V voltage and a positive mass. Cars were equipped with 6V systems until the mid-1950s. The change from 6V to 12V occurred when cars became bigger: engine compression ratio was higher and needed more power to start, allowing to divide the cable section by two at the same level of output power. Some cars are still nonetheless equipped with 6V batteries till mid-1960s, such as the Volkswagen Beetle, or till 1970, such as the Citroën 2 CV.
The adoption of high voltage needed time because it is influenced by two major technological limitations:
- Batteries composition : the higher their voltage, the more cells they contain and the more necessary it is to ensure a good cell balance (intrinsic quality of cells chemistry, performance of the associated Battery Management System)
- Power electronics component's availability in high voltage: transistors, diodes, capacitor and insulating material.
The electric mobility market continues to evolve towards high voltage levels:
- In the high voltage power sector (linked to the drive train) 400VDC solutions move to 600V, 800V, 1200V and more.
- In the low voltage power sector (ECU and on-board accessories accessible to the user): traditional 12VDC is completed by a 48VDC line.
In practice, what are the advantages of this evolution?
The advantages of the increase in voltage levels are:
- Decrease in resistive losses (Joule effect) on electric power supplies.
The limitation of losses makes it possible to increase the global return and enhances autonomy.
- Cables become lighter (less copper)
- Better mechanical integration of these cables, which are more flexible.
These improvements apply to both the drive chain and the 48V parts.
Are there any drawbacks of increasing the voltage level in the architecture of electric vehicles?
At this stage of maturity of the electric vehicles market, the process of increasing voltage values is neither yet stabilized nor normalized.
This instability penalizes the emergence of standard solutions.
Tame-Power DC/DC converters have a wide range of operating voltages in both low side and high side (30V … 450V / 30V … 950V), so they can be adapted to a wide variety of architectures.
This characteristic allows electric designers and architects to consider several solutions in order to optimize the global energy performances of their system.