Next Frontier for Battery EVs: A Race to Efficiency
February 2022
The acceleration in the sales of battery electric vehicles (BEV) observed in 2021 shows the market has passed the tipping point. We are now in the vertical portion of the S curve. BEV sales jumped from 1.0M in 2020 to 2.7M in 2021 in China and from 730k to 1.2M in Europe. Last year, BEVs reached 13% of the light vehicle market in China and 10% in Europe. The USA continues to lag at about 2.5%, yet the momentum is building fast.
This steep acceleration is forcing OEMs to hasten the launch of new BEVs, increase production capacity and shift engineering capabilities away from internal combustion engines The whole industry is also bolstering the entire supply chain to prepare for significant growth going forward.
The battery cell industry is engaged in a marathon to boost capacity, particularly in Europe and the USA. Both regions need to drastically reduce their dependence on Asia – especially China. Since the major players are currently Chinese (CATL, BYD…), Korean (LG, Samsung, SK) and Japanese (Panasonic), a slew of new players has emerged in Europe to achieve independence, such as Northvolt, AAC, Verkor or Britishvolt.
Tension concerning minerals (cobalt, nickel and manganese) are growing in part due to geo-political reasons – i.e., mining conditions and China’s domination in mineral processing. Many OEMs are starting (or planning) to use lithium iron phosphate cells (LFP) for their lower priced vehicles, such as Tesla for the Model 3 RWD, which will relieve the pressure on sensitive minerals – and provide a lower cost base.
In addition, a battery recycling industry is emerging, with players such a Redwood Materials or Li-Cycle, as well as a used battery collection and trading activity, with Cling Systems for instance. These companies will ease pressure in the supply chain and help minimize the quantity of minerals mined.
Other key options to ease the rising tension in the supply chain include boosting both batteries’ energy density and BEVs’ energy efficiency – as measured respectively in kWh/kg and kWh/100 km (or miles/kWh). Not all BEVs are born equal. The analysis I made in my November 2019 article showed Tesla was then leading the pack in energy efficiency by 10 to 30%. The performance race is on!
OEMs are boosting their efforts to enhance their BEVs’ efficiency, addressing the complete vehicle. These efforts focus on the motors, power electronics, aerodynamics, system voltage, thermal management, tires or even solar panels to harvest energy. Let’s have a look at what is happening in these various domains.
Motors
OEMs can choose among several motor technologies depending on their objectives which include system efficiency, cost, power density or independence from critical supply chains.
A majority of BEVs use permanent magnet synchronous motors (PMSM) – over 80% in 2021 according to IDTechEx and rising. These motors provide high efficiency and power density (kW/kg) vs. other technologies. However, PMSMs use rare earth which present sourcing issues – China is by far the main source. Their magnets also generate constant drag, which can result in efficiency losses in case the driver selects low regeneration.
Conversely, the asynchronous motor (ASM or induction motor) offers lower cost, better high-speed performance but lower efficiency then the PMSM. A third alternative is the electrically excited synchronous motor (EESM) which provides better efficiency then ASMs at a cost below that of PMSMs. EESM is the technology used for the future 200-kW motor Renault and Valeo just announced (current product above). Neither ASMs nor EESMs require rare earth, which removes certain sourcing issues vs. PMSMs.
With Model 3 dual motor versions, Tesla introduced a combination of an ASM in the front and a unique solution in the rear. The front motor is only activated when needed and generates no drag otherwise. For the rear motor, Tesla created an Internal Permanent Magnet Synchronous Reluctance Motor (PMSRM) which provides high efficiency at low and high speed, reportedly higher than that of ASMs – this technology is also used on the latest Model S/X (below: Model S Plaid’s front e-drive), replacing ASMs.
On the new Q4 e-tron all-wheel drive, Audi also opted for two different technologies. It uses an ASM in the front (activated only when needed) and a permanently excited synchronous motor, both being able to coast drag-free. By comparison, Ford Mach-E and VW ID.3/ID.4 use PMSMs only.
Weight also impacts efficiency. On the recently introduced Air, Lucid focused on energy efficiency, power density and compacity for its e-drives. Using a permanent magnet technology, the drive assembly (motor, transmission, converter) weighs only 74 kg for 500 kW. By comparison, the equivalent front unit on Tesla Model S Plaid weighs 95 kg for a lesser power.
Overall, permanent magnet-based technologies take the lead – at least for the main motor – in order to improve efficiency. However, the sourcing of rare earth will likely remain a strategic challenge when using magnets. Besides, it will be interesting to see what choice OEMs make for their entry BEV for which ASM’s cost advantage may trump the efficiency gap vs. PMSMs.
Whereas the above motors all operate with radial flux (flux lines running in planes perpendicular to the motor’s axis), there is yet another type that uses axial flux (flux lines running parallel to the axis). These motors feature a very high-power density and are typically used for performance vehicles. Among the suppliers are Yasa, a British company acquired by Mercedes-Benz last year for applications in AMG models, and Whylot, a French company in which Renault bought a minority stake in 2021.
Power Electronics
BEVs require several high-voltage power electronic units. They include the DC-DC converter (from the battery’s high voltage to 12V), the on-board charger (charger-battery interface) and the inverter, which is a critical component to improve efficiency. The inverter converts DC from the battery into AC for the motor in acceleration phases and vice-versa in regeneration phases. Packed with software, this key module also computes and controls the optimal amount of traction and regenerative power as well as the switching between the two modes. (Bosch inverter below)
Strategies to increase efficiency include hardware and software improvements in the inverter. For instance, Silicon carbide (SiC) and gallium nitride (GaN) increasingly replace silicon. Inverter cooling is becoming standard. One of the ways to boost efficiency via software consists in switching at higher frequency between power and regenerative modes. Developed by France-based Silicon Mobility, this solution results in up to 20% energy gain from lower switching losses in the inverter. Pure software-based improvements can also be pushed over-the-air during a vehicle’s life. The inverter is definitely a core component.
Aerodynamics
Aerodynamic drag has a direct impact on a vehicle’s power consumption, thus its range. OEMs have made massive improvements over the years. The revolutionary Citroën DS introduced in 1955 featured a drag coefficient (Cd) of 0.36. Sixty years later, the hybrid BMW i8 reached 0.26 with its aero appendices.
Today, the best is class models have achieved a Cd of 0.21 or even 0.20. These vehicles are BEVs: Mercedes EQS, Tesla Model S, Lucid Air and NIO ET7. Yet OEMs are not stopping here: Mercedes recently presented the Vision EQXX concept car (above) with a Cd of 0.17 – though the esthetics are not for everyone! That is less than half the drag vs. the 1955 Citroën DS, assuming a similar frontal surface area.
Other Efficiency-Enhancing Factors
There is a series of other parameters the industry can leverage to improve energy efficiency, some being more intrusive or impactful than others.
Whereas most BEVs operate at 400V, a higher voltage is pursued to boost efficiency. Eight hundred-volt architectures are emerging (e.g. Porsche Taycan, Hyundai Ioniq5 / Kia EV6) and will become increasingly common as most OEMs have announced they will embrace this solution. The Lucid Air even operates at 924V. Doubling voltage results in a 75% reduction in heat losses.
Tires have always played a key role to reduce fuel consumption by minimizing rolling resistance. Tire manufacturers have developed special tires for EVs as lower resistance triggers a virtuous circle where higher efficiency results in lower battery mass for a given range. Unique EV specifications are also required to handle BEVs’ typically higher torque and weight.
Thermal management is yet another area where we can see an increasing amount of activity in the industry. Reversible heat pumps were first introduced on Tesla Model 3/Y and are now used by Hyundai. They generate more thermal energy (heat or cold) than they use electrical energy, which contribute to higher system efficiency vs. the traditional combination of heating and air conditioning.
Last – though this is probably more marginal – is the appearance of solar panels covering body panels, or at least the roof. The technology is featured on the forthcoming Sono Motors’ Sion and Lightyear’s One (above) as well as on the previously mentioned Mercedes Vision EQXX. On the Lightyear, this tech contributes to enabling a range of 400 km at a steady speed of 130 km/h with a 60 kWh battery, resulting in 141 Wh/km.
These various approaches to boost BEVs’ energy efficiency will certainly translate in a continued technology race which will bring about additional differentiation between players and models.
Marc Amblard
Managing Director, Orsay Consulting
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