Formula E powertrains, fast charging and regen – Virtual Roundtable (part 2 of 2)

Moderated by Jeff Shepard

Moderated by Jeff Shepard, EE World has organized this “virtual roundtable” bringing together experts in Formula E racing technology to share with you their experience and insights into powertrains, fast charging, regenerative braking, and potential technology crossovers between Formula E and conventional electric vehicles. Joining us for this virtual roundtable are: Frank Muehlon (FM), Head of Global E-Mobility Infrastructure Solutions, ABB; Dave Priscak (DP), Vice President, Solutions Engineering with ON Semiconductor; and Niall Lyne (NL), Vice President, Automotive Analog Power & Video Division at Renesas.

JS: Formula E uses a 900V main power bus, how does that compare with consumer and/or commercial battery electric vehicles? What powertrain bus voltage trends do you expect to see in the future?

FM: A high powertrain voltage allows for charging at higher power and, therefore, faster charging times. Premium cars like the Porsche Taycan have an 800V powertrain, and in the future more mass-market EVs will adopt this as well. ABB Formula E allows the car manufacturer teams to test the 900V powertrain and charging fully, and thereby improve their designs for mass-market EVs

DP: Today’s BEV bus is typically 400V, but we already see 800V powertrains in definition. With the introduction of high voltage wide bandgap components, increased power density and smaller motors are possible and speed up the adoption of higher voltage busses. Higher battery voltages also help in the Volt/Amp problem of fast charging. However, changing battery voltages can be problematic for the installed base of EV chargers deployed today and may even change the electrical distribution to accommodate the need for fast charging (480V Mains or even 1200V Mains to the charger is not possible in most installations today). Future chargers will have to be digitally controlled and able to take in multiple voltages, and vehicles will have to be flexible on charging based on the chargers output capabilities.

NL: 900V is two to three times the consumer vehicle main bus voltages used today. However, the higher voltages allow for lower current in a same-sized performance level of vehicle. Thus, the desired trend is to move in that direction in order to reduce electrical wiring mass (less vehicle weight) and thermal losses (plus weight and cost of materials to dissipate those losses) caused by the higher currents.

JS: How long will a 12V battery system remain in Formula E and consumer/commercial BEVs? Will it be eliminated or increased to 24V or 48V?

NL: One of the Renesas team’s efforts focused on reducing the size and weight of the 12V battery in their sponsored Formula E car; this is the trend throughout the industry as a whole. The 12V vehicle bus is prolific, and efforts in the foreseeable future will be to reduce the size and weight of the 12V accessory battery, even augmenting or replacing it with a solid state DC/DC converter (e.g., 48V/12V Bi-directional DC/DC). And while heavier current consuming applications like power steering are moving to higher voltage vehicle buses, the 12V bus in some form is here to stay.

DP: The requirement for a 12V bus will be around for years to come, as this is the voltage used for everything from sensors to infotainment systems to comfort and convenience. However, this does not necessarily mean there will be a need for 12V batteries to provide the 12Volts. We are seeing requirements for high voltage DC/DC converters such as 400V/800V to 12V, 48V to 12V, etc. 48Volts is becoming more of a requirement as many motors need the higher voltage due to increased torque requirements (Park assist, eTurbo). One OEM uses two 12V batteries and uses one to boost to 48V to provide the needed voltage but not create a two-battery voltage architecture. I believe there will be one high voltage battery in the future but multiple voltage rails distributed to support the different needs of the electronic loads.

JS: Formula E charging for Gen3 cars will be 600kW, delivering 4kWh in 30 seconds. That’s much higher than existing fast-chargers for consumer and commercial EVs. How can that rate be sustained beyond 30 seconds? What factors determine the upper limit to the rate of fast charging and how do you expect to see fast charging evolve for consumer, commercial, and fleet BEVs?

FM: Current handling limitations exist in the batteries, which means that charging at such high rates for longer periods cannot be sustained by the battery due to overheating and rapid ageing. The industry is also working on batteries with higher C-rates, and a lot can be learned and gained from ABB Formula E Gen3.

DP: Consumers have the expectations that EV vehicles must have similar use profiles to ICE vehicles in order to fully take them mainstream. This means similar range and time at the pump. Fast charging creates massive power transfers to the vehicle’s battery. Limitations to the speed of charging are the current capacity of the charger/cable, the impedance of the battery, available power source, and the battery’s ability to cell balance are among the many factors involved in fast charging. Higher voltages reduce the charge currents and transmission loss, and I believe we will see higher battery voltages in the future. I envision charging stations (similar to gas stations or collocated with gas stations) connected to 1200V Mains and capable of delivering power to fully charge in minutes.

NL: The answer to the first two questions is ultimately the same answer – the battery, and the battery. As consumer electric vehicle range has reached and exceeded the industry’s original target of 300 miles per charge, the focus on faster charging has intensified. And while building and installing faster-charging systems (the power electronics) is feasible, the final rate of charging is determined by the battery cell chemistry and the thermal and electrochemical stresses that occur at higher rates of charging. So faster charging will come to the electric vehicle market, but only to the extent that it is not at the expense of the useful life of the battery itself.

JS: Regen in Formula E is up to 350kW (from the rear axle), much higher than today’s consumer and commercial BEVs. Do you expect to see regen rates in consumer and commercial BEVs increasing? If so, how high and how soon?

NL: Regen is an energy recovery strategy and is limited primarily by the speed of the vehicle, the rate of deceleration, and the ability of the traction drive system to convert the mechanical energy to electrical energy. Most consumer electric vehicles today are likely capable of much higher levels of regen than they are configured to use. This is really drivability, handling, and ride experience decision. While a race car can be precisely managed on varying road surface conditions, it would be unacceptable if the regen levels on a passenger vehicle caused the tires to lose traction on a wet road, for example.

DP: Drivers in Formula E are constantly accelerating and braking as the cars make their way through the winding courses. This environment is ideal for regeneration as the ratio of brake to run is high. The problem is in how to store the energy from a hard brake as the energy is generated faster than the battery can be recharged. Technologies such as LiIon capacitors or supercaps can be used to temporarily store the energy and transfer to the battery or be consumed upon acceleration. This methodology can be expensive to implement, and the ROI may not justify the expense if the brake to run ratio is low (highway driving). Because regeneration is a critical differentiator in racing, Formula E will continue to invest in researching the best ways to capture and reuse regenerative energy. This will lead to making more effective and cost-optimized systems for consumer use.

JS: Thank you to our Virtual Roundtable participants for sharing their insights and experience! You might also be interested in reading “Formula E Batteries, Software and Technology Transfer” – Virtual Roundtable (part 1 of 2).