Why more voltage?
Thinking about electrical drive systems will bring back memories of physics 101 on electricity: electric power (wattage) equals electric potential (voltage) times current (amperage). So, if you want to increase electric power, you’ve got two options: increasing amperage or voltage. Higher amperage, however, requires proportionately higher cable cross sections and therefore more copper, more design space, and more weight. “By contrast, if you increase voltage the lines will remain slim or may even become thinner,” Schaeffler’s expert Rösel explains, continuing that “Plus, 800-volt systems permit a higher permanent load of the propulsion system on the road and especially during charging.”

What are the greatest advantages?
Clearly, for charging, because power is even more important for charging than for driving, at least in fast-charging mode. Modern fast-charging stations are designed for up to 400 kW of charging power. This high charging power is usable only with 800 volts because the permissible current of 500 amperes would clearly be exceeded in the case of 400-volt systems. “Because the higher voltage reduces the generation of heat 800-volt systems can handle such high charging power better,” says Rösel, even though the most effectively charging models currently remain below 350 kW.

On the vehicle side, what is important for being able to use the voltage during the charging process?
Higher charging power increases the charging stress of the batteries. That must be compensated for. “The cell chemistry of the batteries, battery management, and especially thermal management ensuring well-balanced cell temperatures preventing the formation of so-called hot spots must be adjusted to the charging power,” says Rösek, “Otherwise, the speed-related benefits during the charging process will be equalized because the systems will reduce the power output.” If that doesn’t happen the battery will become unstable and be damaged.

Why are vehicle manufacturers only now increasingly opting for 800 volts?
Doubling of voltage requires highly capable power electronics performing two important tasks in power management: converting the battery’s DC voltage into AC voltage for the electric motor and adjusting the power output of the electric motor to the driving situation. “If you doubled voltage in the face of this challenging dual load, conventionally designed current-regulating transistors would have to grow disproportionately. That in turn would clearly reduce efficiency and offset any of the advantages gained,” Rösel explains. “The solution was produced by introducing the clearly more effective silicon carbide material instead of the previously used silicon. That resulted in a key component for doubling the potential from 400 to 800 volts.”

Full potential, full portfolio

Due to the merger with Vitesco Technologies, Schaeffler has extended its electric mobility portfolio specifically in the 800-volt range once again. The technological spectrum ranges from battery management and charging systems to power electronics with efficiencies of more than 99 percent in defined load ranges to highly efficient permanent magnetically excited synchronous and asynchronous motors, e-axles, and thermal management modules. The systems are available for both passenger cars and commercial vehicles.

What are additional challenges?
The electric potential leap to 800 volts entails different insulation and safety requirements. “Air clearances and creepage distances must be considered, which means that you need bigger gaps between the conductors on a PCB,” says Rösel. All that comes at the expense of the previously saved design space and weight.

Are 800-volt systems more expensive?
Rösel says, “For 800-volt systems, you can’t go to a 400-volt shelf, so you need to develop dedicated components, which initially costs money.” What’s more, says the electric drive expert, the silicon carbide power semi-conductors are more expensive than the silicon chips of the 400-volt systems, while costs could be saved with the 800-volt version due to smaller cable cross sections etc. That’s a zero-sum calculation, as Rösel points out, “The bottom line is that the costs for an 800-volt parts package are similar to those of 400-volt systems – at least if the vehicle concept is not strictly geared to performance.” By contrast, due to the thermal loads acting on the motor and the power electronics, the power electronics systems of performance-oriented vehicles are more complex in terms of technology and therefore more cost-intensive.”

Are 800 volts automatically better?
Not necessarily, Rösel says, “That greatly depends on the overall optimization of an automobile concept. Voltage per se isn’t inevitably the key criterion. A very well-designed 400-volt system can achieve very high performance. Drivers regularly traveling long distances and benefiting from fast charging derive the greatest advantages from the higher voltage.”

„Doing a good job of managing the system’s design and acting safely in the thermal domain is a lot more important than the absolute voltage rating.“

What does the future hold?
Aside from technical feasibility, there are standards that establish limits. EU legislation, for instance, currently allows a maximum electric potential of 1,500 volts, whereas China allows “only” 1,000 volts. Increasing charging power by further voltage increases is an attractive idea, “But,” Rösel cautions, “doing a good job of managing the system’s design and acting safely in the thermal domain is a lot more important than the absolute voltage rating.” Charging power ratings of 500, 600 kW, or all the way into the megawatts range call for immensely complex in-vehicle interaction of the current and voltage interface and battery and thermal management. The cell chemistry must be optimized accordingly as well to limit aging of the battery cells during fast-charging processes. The technical complexity of charging stations is considerable too. Due to the better cost-benefit ratio, different design space options, and aspects of charging infrastructure, Rösel tends to see significantly higher nominal potential than 800 volts in the commercial vehicle than the passenger car sector. Even so, Rösel is convinced that, “Any notable voltage leap such as the currently envisioned one to 1,000 volts also presupposes a development leap with the semiconductors of power electronics – entailing correspondingly high costs.” Especially in the passenger car sector, Rösel prefers a different pathway: “Bringing 800-voltage technology to market on a broader scale, especially in lower-priced vehicles, is a lot more important than making another voltage leap. That would clearly boost electric mobility in general.”