Batteries Become Do-or-Die for Air-Taxi Startups

"The dream of electric air taxis has always depended on the reality of batteries.

With money for moonshots drying up, investors may be wise to focus on today's technology rather than tomorrow's.

Vertical Aerospace, a British startup focused on electric vertical takeoff and landing aircraft, or eVTOL, will source batteries from cell supplier Molicel for its VX4 model -- a move that wasn't previously reported. The Taiwan-owned manufacturer has made the prototype lithium-ion cells that Vertical will use. It will open a new gigafactory to fulfill the orders, and said it has secured key raw materials such as lithium and graphite.

Investors are increasingly aware that batteries are the key hurdle for eVTOL projects, with shortages in the supply chain threatening companies' ambitious post-2025 production targets.

Startups may be more likely to survive if they use commercially available batteries, rather than imagined future ones that capital markets are no longer willing to finance.

For air taxis and electric cars, a key metric is "energy density": How much energy batteries provide per unit of weight they add.

California-based eVTOL leader Joby Aviation, which uses big rotors on rotating shafts -- a design embraced by Vertical and Archer Aviation -- needs battery packs that deliver roughly 240 Wh/Kg, according to a paper published last year in the Proceedings of the National Academy of Sciences. This is more than the 186 Wh/Kg of a Tesla Model S Long Range, but achievable by lithium-ion technology now in the prototype stage.

By contrast, Germany's Lilium would need, as an average estimate, 330 Wh/Kg to carry six passengers 172 miles.

So would Vermont-based Beta Technologies, which aims to use a "lift plus cruise" design to carry four passengers 288 miles.

And eVTOL vehicles have a crucial extra requirement that a Tesla doesn't: They spend a disproportionate share of energy in a quick burst in the minute or so of vertical liftoff.

The more weight relative to the disc area of the rotors, the higher the "specific power requirement."

 Joby and Beta likely fall within the 1.5 kW/Kg provided by today's technology, but Lilium's design of 30 small ducted fans placed in the canard and wings pushes the requirement to 2.5 kW/Kg according to the company, and at least 3 kW/Kg according to the PNAS paper.

Of course, sacrificing range and seat capacity can offset power issues. Archer said it would use existing batteries and not fly beyond 60 miles.

But this trade-off risks undermining the business model. Because of takeoff burn, eVTOL is less efficient than electric cars under 70 miles. Given the hassle of getting to a "vertiport" by car, the promise of an "Uber of the skies" urban market all but vanishes.

This is why Lilium's idea of shuttle services between underserved cities makes the most sense. Unfortunately, it hasn't yet completed test flights longer than a few minutes. It needs a new battery chemistry that the company is working to develop, at a much higher risk for investors.

Is there anywhere left to go for air taxis? Perhaps: Joby has traveled 150 miles in test flights. Likewise, Vertical said its target for Molicel's batteries is only 200 Wh/Kg in energy density and a specific power of 1.1 kW/Kg. That will allow the VX4 to travel 100 miles, serving its ambitions to offer short intercity trips such as Belfast-Glasgow and to challenge the helicopter market.

Both Joby and Vertical would carry two fewer passengers than Lilium, and short city-to-city routes are a far cry from the $1 trillion opportunity predicted by some eVTOL proponents -- the helicopter market is worth just $50 billion world-wide. Still, in the current market a battery in the hand is worth two in the bush." [1]

1. Batteries Become Do-or-Die for Air-Taxi Startups
Sindreu, Jon. 
Wall Street Journal, Eastern edition; New York, N.Y. [New York, N.Y]. 07 July 2022: B.12.