The advancement of lithium-ion battery technology has not only introduced a new era of battery-electric vehicles but has also triggered a surge in high-voltage vehicle architectures. The recent progress in eMobility can be attributed to the widespread adoption and commercialization of components designed to function at elevated voltage levels. This encompasses crucial power components such as motors and inverters, whose efficiency stands to benefit from the utilization of a high voltage EV design.
This article explores the significance of voltage levels in electric vehicle (EV) design. Additionally, when a vehicle is equipped with both high and low-voltage supplies, a pertinent question arises: from which voltage source should auxiliary systems (such as pumps, compressors, work functions, etc.) draw power?
What is High Voltage?
In the automotive world, high voltage DC is any voltage above 60VDC [ECE R100:Rev.2]. Common voltage architectures below this threshold are 12V and 48V. Common voltage architectures above this threshold are 400V-800V. The reason for categorizing a “high voltage” is primarily to flag the need for safety measures to prevent electric shock.
If proper safety precautions are not taken during design, high voltage can be extremely dangerous. This raises a question: If high voltage demands extra engineering precautions in system and component design, why opt for high voltage systems in the first place?
The High Voltage Advantage
While the benefits of elevated voltage are often linked to faster charging, this article focuses on how higher voltage enhances operational performance, contributing to improved vehicle range and reliability.
To illustrate the effect of increasing voltage in a system, consider connecting a fixed load, like a light source, to a low voltage battery vs. a higher voltage battery. When the battery switches from a lower voltage to a higher voltage, the bulb shines brighter (more power). This is because the bulb has a fixed resistance, and a fixed resistance draws a higher current at a higher voltage.
However, it’s important to note that power electronics and motors in EVs generally face variable loads, altering the role of voltage. For variable loads, such as a power steering pump, the maximum power is often a predetermined specification. If a low-voltage motor can meet the power requirement, the rationale for increasing the voltage isn’t purely about boosting power output. Rather, it’s about the relationship between voltage and current.
For a given amount of power, raising the voltage level reduces the corresponding current draw. This relationship is demonstrated through Watt’s Law:
P (Power) = V (Voltage) x I (Current)
This relationship points the way toward significant efficiency gains. A key concept in electronics is that higher current translates directly to increased heat. The primary source of heat here is the copper losses, where high current causes heat in the conductors through a resistive load. This necessitates additional measures for heat dissipation, such as thicker wiring and/or active cooling systems. In fact, the relationship is exponential:
P (Electrical Power Loss) ∝ R (Resistance) x I2 (Current)
Essentially, losses increase with current flow. In addition, all that energy that is lost as heat is energy that could have been used for more useful purposes like propelling the vehicle. When the power requirement cannot be compromised, increasing the voltage supply emerges as the solution. In direct terms, doubling the voltage supply cuts the current draw in half for the same power, resulting in a quartering of resistive losses.
As an analogy, raising voltage in BEVs is like boosting water pressure in a hose to wash a car. A low-pressure hose requires a high flow of water because it lacks the force for efficient cleaning. By increasing the pressure, the same task can be accomplished with significantly less water and in less time. The key takeaway is that higher voltage allows the same work to be done with considerably less current.
Applications of High Voltage
Typically, the practical current limitation in low voltage DC cables is around 250A, rendering sub-60VDC systems impractical for power ratings exceeding 15-20kW. Above this power, the high current causes the cable to simply become too large, and the active cooling requirements become impractically complex. While low-voltage systems offer flexibility and inherent safety, they become unwieldy beyond a certain power threshold.
The advantages that high voltage provides has caused most medium/heavy-duty original equipment manufacturers (OEMs) to utilize higher voltage systems in the range of 400-800VDC. This voltage range significantly lowers the current draw for the high power levels that these vehicles operate at.
Higher-voltage systems also offer advantages in broadening the operational spectrum of various power electronics components. For example, a vehicle manufacturer may wish to downsize their inverter to reduce cost. Inverters are typically sized based off the rated current of the motor, as current is the limiting factor in thermal and operational performance. Therefore, increasing the supply voltage can allow for a heavily down-sized package.
High-voltage motors and inverters can channel high power with minimal losses. These components have specific voltage ranges within which they function optimally, with the battery voltage often dictating which components, such as inverters and motors, are suitable for a particular EV architecture.
High voltage serves as a cornerstone in contemporary EV design. It’s not just about faster charging; it’s about improving efficiency, extending range, and bolster overall reliability. As the automotive industry continues its electrification journey, high-voltage systems continues to play an increasingly pivotal role in shaping the future of electric mobility.
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