• 18 Dec 2023 10:39 PM
• 1

Competing Standards

By Mike Hirschberg, VFS Director of Strategy
Vertiflite, Jan/Feb 2024

As AAM aircraft near production, competing charging standards come to the fore.

This fall, three different electric charging standards were proffered to the advanced air mobility (AAM) community. Each system had been in development for a long time, but came to a nexus as the last issue of Vertiflite was going to press. At the end of October, at Airtec 2023 in Augsburg, Germany (see “VFS Leads the Future of Air Mobility at Airtec,” Vertiflite, Jan/Feb 2024), Electro.Aero presented the latest update on years of SAE International working group meetings in developing electric aircraft charging standards. Two weeks later, Joby and Beta published details of the different charging systems each had been pursuing.

This article provides an overview of these three main approaches. But first, a quick look back at the first comprehensive framework for an air taxi ecosystem, Uber Elevate, and the proposed charging system.

ChargePoint (Uber Elevate)

In January 2018, at the fifth annual VFS workshop on electric vertical takeoff and landing (eVTOL) aircraft, held as part of the Society’s Transformative Vertical Flight (TVF) Technical Meeting (see www.vtol.org/tvf), ChargePoint’s Alan Dowdell cited the divergence of AC and DC charging standards for automobile and other electric charging systems, warning, “Let’s not repeat mistakes of the past.”

Uber had announced a partnership with ChargePoint — which calls itself “the world’s leading and most open electric vehicle (EV) charging networks” — at its first Elevate Summit in April 2017 (see www.vtol.org/uber). A key criterion for Uber’s vision of urban air mobility (UAM) was that air taxis would deplane and load passengers and their luggage, as well as provide a boost in battery reserves, within seven minutes.

These high-power rapid charges would replenish energy used during each flight, with the air taxi supporting hours of operations by continuously “topping off” the battery to a state of charge of around 80% between each hop; a full charge could then be made before each “rush hour” surge in ride demand.

ChargePoint unveiled its concept design for 2-MW high-powered charging of electric semi-trucks and eVTOL aircraft at the second Elevate Summit in May 2018 (see “Electric VTOL News,” Vertiflite, July/Aug 2018). ChargePoint’s press release read, in part: “To meet performance requirements, up to four BMS [battery management system] interfaces and four 500-amp delivery circuits are needed. Each delivery circuit will have a voltage range from 200 to 1000 volts. To facilitate autonomous data and vehicle performance payload offload, a provision for high speed data transfer was also included.” The press release noted that the connector had to be rugged, as well as easy to insert and remove. In addition, the “connector also supports optional auxiliary liquid cooling to the aircraft or semi-truck while it’s being charged.”

The ChargePoint approach was likely not pursued after Uber ended its Elevate initiative.

SAE International (ARP6968 and AIR7357)

SAE International (originally founded as the Society of Automotive Engineers in 1905) is an internationally recognized standards development organization (SDO), with more than 240 technical committees taking advantage of the expertise of nearly 9,000 volunteers. SAE and other SDOs have developed charging standards for EVs around the world.

Best-selling EV manufacturer Tesla developed a proprietary system, so most other cars in North America had been using the Combined Charging System Combo 1 (CCS1) — combining the previous generation of round AC plug with the DC fast-charging port. In the European Union (EU), an incompatible version — Combo 2 (CCS2) — was mandated (even for Teslas), while the GB/T 20234 AC/DC charger is the only system allowed in China, which produces the most electric cars in the world. Japan also has the CHAdeMO standard for DC charging.

In November 2022, however, Telsa opened its proprietary connector design to the public, which it dubbed the North American Charging Standard (NACS) connector; this is now the openly available (draft) SAE J3400 connector. Since then, most major car companies have announced that they would equip cars sold in North America with the NACS charger — with the notable exceptions of Mazda, Mitsubishi, Stellantis (Chrysler, Fiat, etc.), and the Volkswagen Group. Tesla cars in Europe use the CCS2 standard, while Teslas in China use the GB/T standard. Thus, the NACS will not only become an SAE standard, but also the de facto standard that nearly all new cars in North America will use.

Australian electric aircraft charging system developer Electro. Aero (see “Electro.Aero Seeks to Power Electric Aviation,” Vertiflite, Jan/Feb 2024), a frequent presenter at VFS events, set about working with SAE International to develop an industry standard for aircraft charging in February 2019. Electro.Aero co-founder Joshua Portlock chaired the SAE AE-7D Aircraft Energy Storage and Charging Committee’s efforts in developing Aerospace Recommended Practice ARP6968, “Connection Set of Conductive Charging for Light Electric Aircraft,” for aircraft charging requirements of up to 500 kW; this is expected to be finalized in 2024.

Electro.Aero sells mobile charging units that can range from 7 to 240 kW of continuous charging power. The units are available with interconnects for the SAE ARP6968, as well as the automotive CCS1, CCS2 and GB/T charging standards if needed, and the units can be re-configured between standards. At Airtec 2023, Electro noted that automotive CCS chargers don’t work unless plugged in horizontally, have a large and heavy inlet with unutilized AC pins and have no 28-V auxiliary power; in addition, there are the two separate formats, CCS1 and CCS2.

Although the SAE’s ARP6968 connector is based on the Chinese GB/T DC plug, Electro.Aero highlighted the benefits of the new standard for aircraft. Advantages include higher voltage (up to 1,000 V), aviation-standard 28-V auxiliary power that stays on after the charge is finished for better battery health and automation, the lock is activated as soon as it is plugged in and will automatically start charging, and it can charge up to 1,785 battery cells (seven times more than the GB/T limit).

The SAE committee is now working on an aerospace information report (AIR) for aircraft needing more than 500 kW. AIR7357, “MegaWatt and Extreme Fast Charging for Aircraft,” was initiated in November 2020. The SAE website states the rationale: “Current standards (AS6968, J1772, etc.) do not cover the power levels required for extreme fast charge (XFC) for moderate size aircraft applications (150 to 200 kWh batteries to be charged at 5C or greater) and evolving commuter and single aisle concepts (500 kWh to > 1 MWh) to be charged at 2C or greater). Design considerations for such charging need to be understood and use cases for such charging need to be established to aid the industry in developing appropriate equipment standards.”

Similar to how ARP6968 is physically based on the GB/T automotive standard, the AIR7357 standard is currently exploring leveraging the coupler being developed for the automotive Megawatt Charging System (MCS) for heavy-duty electric buses and trucks by Charging Interface Initiative (CharIN) e.V., though this is still a work in progress by SAE as J3271. Notable for this discussion is the dependency of fluid-cooled charging cables to achieve over 35-A, but the fluid does not pass through the triangular-shaped charging plug. With MCS/J3271 not yet finalized, the aerospace-grade AIR7357 is still in an early stage of development; final publication is not expected for several years.

Electro.Aero’s sister company FlyOne has been operating a fleet of two-seat electric Pipistrel Alpha Electro training airplanes commercially for many years. It uses portable external air conditioners ducted into the battery intakes to cool the batteries in the summer heat of Australia — which regularly exceeds 95°F (35°C) — which avoids batteries overheating while charging and reduces capacity degradation. This solution is substantially simpler, lighter, more reliable and lower maintenance than liquid-cooled systems, so offboard air-conditioning for small aircraft should be considered where possible, Portlock told Vertiflite.

Joby Aviation (GEACS)

Uber invested in Joby in 2020 and then divested its Elevate initiative, transferring people and intellectual property to the air taxi company. With the lessons learned from nearly five years of Uber’s UAM research and development, Joby focused on rapid charging — as well as charging approaches that promote best battery conditioning across multi-pack redundant battery systems — to maximize the productivity of its aircraft, aiming to keep them in the air, earning revenue, as much as possible.

In addition, with its four separated, isolated battery packs distributed in the wings and forward motor nacelles for safety and redundancy, Joby iterated its design over the past decade and developed what it had originally planned to be a proprietary system. The company decided late this summer to make its Global Electric Aviation Charging System (GEACS) specifications open to the world; perhaps this was because more companies were nearing certification and seeing Tesla’s example to open up a standard to allow the eVTOL infrastructure to grow, or for other business reasons. Joby began reaching out to other eVTOL original equipment manufacturers (OEMs), and several other companies expressed interest in Joby’s integrated approach.

The company released the overview of the GEACS specifications on Nov. 7. Key to the approach, Joby said, is “the benefit of integrated battery conditioning, secure and high-speed data offload that is expected to meet the safety and cybersecurity requirements outlined by the Federal Aviation Administration (FAA), and the ability to simultaneously charge the multiple independent battery packs.” Joby believes that having independent, isolated battery modules and these other features will result in “a lower certification burden,” though others have questioned the proximity of the cooling fluid and the high-voltage charging lines.

Joby’s GEACS could potentially compete with MCS within the SAE AIR7357 standard, said Portlock, since that it is still in an early information report stage. Off-board liquid cooling for battery conditioning might be required for megawatt-class aircraft, in order to be weight- and charge-time efficient, he observed.

Joby released a flyer of technical details on its charging system that highlights its approach and benefits, saying that an Ethernet connection ensures the large amounts of data generated by the aircraft can be quickly and securely downloaded. The fluid couplings and electrical connectors are designed to minimize connect and disconnect complexity and forces.

In addition, “GEACS includes the exchange of coolant with an external, temperature-controlled source, thereby keeping onboard battery packs at ideal temperatures throughout the charge cycle without the need for full onboard thermal management systems.” This, as well as pre-conditioning before flight, enables rapid charging with minimal impact of battery cell life, the company stated. It’s designed to output 150–1,000 VDC and up to 300 A through each of its two channels; since the Joby S4 has four batteries, it needs two connectors at a time, each charging and cooling two separate battery packs simultaneously.

There is, of course, the weight of the residual liquid (“Deionized 40% ethylene-glycol with corrosion inhibitors”), as well as pumps and controls inside the aircraft. On the other hand, for UAM missions with very fast charge requirement that don’t have the liquid-cooling system integrated in the charge connector, a separate system would have to connect with the aircraft to circulate coolant, potentially flushing the liquid out of the aircraft battery system with air, for example. In addition, small electric conventional or short takeoff and landing (eCTOL or eSTOL) aircraft that don’t need rapid turnarounds may be able to obviate the need for liquid cooling altogether. In a Nov. 10 article in The Air Current, author Elan Head noted that Lilium’s eVTOL aircraft “has been deliberately designed to not require external cooling.”

Beta Technologies (CCS)

Burlington, Vermont-based Beta Technologies has been very vocal about its charging infrastructure plans and progress over the past several years (see “Beta Goes the Distance,” Vertiflite, Nov/Dec 2023). In addition to flight testing prototypes of both its A250 eVTOL and CX300 Alia eCTOL aircraft, the company unveiled its first installed elevated landing pad with crew rest stations, office space and energy storage batteries in 2019. Beta has built a network of 15 charging stations that span from Vermont to Florida or Arkansas, and flew Alia on a 1,700-nm (3,200-km) journey to the US Air Force’s Duke Field in October (see “Beta Flies South for the Winter,” Vertiflite, Jan/Feb 2024). Over the next year, Beta says it will install its chargers at 55 additional locations along the East and Gulf Coasts, and in California, with plans to continue growing its charging network.

Beta has been using the CCS1 standard for its electric aircraft, and the company has been installing multimodal charging stations that could be used for aircraft or automobiles that could drive up to them. Beta’s Charge Cube (described in the last issue) is also designed for up to 1,000 VDC and, it states on its website, “Continuous 320 kW power output will charge an ALIA aircraft in just 50 minutes.”

Beta now also has a Mini Cube, “designed as a self-contained, Level 3 DC Fast Charger for use with all CCS vehicles,” including air and ground vehicles, providing 60 A at 40 kW continuous output power.

There is a critical difference in architecture for the Beta Alia, Archer’s Midnight and many non-VTOL AAM companies. While Joby has four isolated battery packs, other companies opted for parallel battery architectures for charging. Designs like Alia and the Ampaire EEL eSTOL testbed have the battery packs underneath the cabin, reminiscent of the Tesla “skateboard” configuration. This lends itself better towards external cooling system hook-ups. The Air Current article reported that Beta “has a thermal management system that plugs in separately from its charger, which gives OEMs more flexibility in the design of their cooling systems.”

The joint Beta-Archer press release on Nov. 7 was entitled, “Archer Aviation and BETA Technologies Collaborate to Accelerate the Adoption of an Interoperable Charging System Across the Electric Aviation Industry,” and advocates that the CCS-based standard should be used as the single global standard for eVTOL charging.

The press release references the report published by the General Aviation Manufacturers Association (GAMA) in August 2023, titled “Interoperability of Electric Charging Infrastructure,” which 13 AAM companies signed. The signatories noted, “While charging needs will evolve with time, the focus here is on the advantages of shared and compatible, or interoperable, infrastructure,” and pledged to use CCS1 plugs in North America and CCS2 plugs in Europe. Although the two plugs are incompatible with each other without an adapter, chargers will be interoperable with all aircraft using the local CCS standard.

The European Organisation for Civil Aviation Equipment (EUROCAE) Standard ED-308 is entitled, “Guidance on VTOL Charging Infrastructure,” and was issued in February 2023. The document includes minimum requirements for eVTOL charging infrastructure on vertiports. This technical standard confirms that CCS2 (the EU-mandated standard) is suitable for electric aircraft charging as “a readily available solution. As a second step, a future release of this document will extend the scope to MCS as soon as this standard is sufficiently described and matured.”

The Beta-Archer press release quoted Pete Bunce, President and CEO of GAMA, in saying, “The adoption of a unified charging standard will help promote electric aviation’s development at scale.”

Where to from Here?

There are at least 10 different major types of EV automotive plug configurations in active use around the world. In a December interview with FlightGlobal, Wisk Aero CEO Dr. Brian Yutko explained that while its initial aircraft are using the CCS1 standard, the company has also “designed systems that are a lot like what Joby has presented” for potential use on its production eVTOL aircraft.

It’s clear that there is a divergence of options when it comes to chargers for installation in future AAM infrastructure. If other OEMs conclude that having the cooling fluid integrated into the charging connector is better, then something like the GEACS may become more widely used. If not, the CCS1 for North America and CCS2 in Europe may become the ubiquitous aircraft charging plug in the near term, until ARP6968 potentially becomes an internationally accepted standard. Time will tell if they will each peacefully coexist in the short-to-medium term, and if there will be a unified megawatt-scale charging coupler in the future, or if there will be some kind of government-mandated standard(s) in AAM-operating countries, as was the case with the EU and China for EVs.

Electro.Aero’s Portlock continues to advocate for aviation-focused consensus standards. “Initial OEM decisions are always going to be airframe-centric,” he said. “The SAE standards are developed by the charging manufacturers, who have many aircraft manufacturers as their customers to broadly support. The ideal solution would be that the best overall charging standard would be universally accepted internationally — and not country- or airframe-specific.”