A BETA Technologies ALIA aircraft performs a hover test at Burlington International Airport in Vermont. Credit: BETA Technologies
In an electric car, an inaccurate dashboard readout of remaining battery energy could force a driver to pull over to the side of the road. But in an electric aircraft, miscalculating the remaining battery charge could prove disastrous — even fatal.
As a result, the aviation industry is developing new methods to help a pilot determine the state of charge in a battery and remaining flight time.
It’s not a straightforward issue. Matthew Clarke, assistant professor of aerospace engineering at the University of Illinois Urbana-Champaign, said depleting battery cells is a vastly different process than emptying a tank of conventional jet fuel.
“Batteries are unique in that their behavior is nonlinear, unlike gasoline. With traditional aviation fuel, regardless of whether you are at full tank or on the last tick, the fuel will behave the same,” Clarke said. “But batteries supply power that is dependent on the voltage and current.”
And while electric cars offer a historic precedent, an aircraft’s performance is more likely to be impacted by turbulence and other changes in air currents — headwinds versus tailwinds, for instance.
Among those grappling with the best way to display battery charge is charter seaplane company Harbour Air, based in Vancouver, Canada, which since 2019 has been flight testing a conventional de Havilland Beaver retrofitted with a magniX electric powertrain.
Displaying battery charge for aircraft “is a complex issue that is being looked into by the regulators, industry and standards development working groups,” Erika Holtz, Harbour Air’s engineering and quality manager, said in an email. She is also the company’s project manager and lead engineer for its electrification efforts.
“When we dispatch a test flight, the pilot will be given a ‘bingo’ state of charge” to denote when he should return to base while still maintaining a crucial reserve charge in case of delays or emergency,” Holtz said. FAA’s regulations for powered-lift aircraft, a category that encompasses many electric designs, require 20 minutes of flying time as a reserve, taking into account wind or forecast weather conditions. Pilots and dispatchers need to confirm compliance with regulatory reserves, not just a percentage display.
The electric Beaver displays an estimated “time remaining” to the pilot, but Holtz said that hasn’t proven helpful.
“We have found the battery time remaining varies so much with slight changes in engine power,” depending on flight conditions, she said. “We are not yet settled on what will be required for the certification aircraft.” The company aims to have the electric aircraft certified by 2027.
Holtz said electric vertical takeoff and landing designs, or eVTOLs, may require an even more sophisticated prediction method to gauge not only remaining battery energy but also landing conditions.
For its part, Vermont electric air taxi developer BETA Technologies decided years ago that a percentage of charge wasn’t sufficient.
“Batteries are weird, right? Your smartphone battery degrades over time, so 100% charge means one thing when it’s new and a lot less usage time when it’s a few years old,” Tyler Seeholzer, a pilot and training developer at BETA, told me in an interview.
While it’s possible to calculate the remaining energy based on the age and condition of the battery, that requires number-crunching time that could be better spent, he said.
“I want to focus on flying and not on making calculations like that,” Seeholzer said.
BETA’s gauge shows a calculated picture of how much energy remains in five separate battery packs, Seeholzer said. “This is the first time I’m aware that somebody has presented a promise of how many kilowatt hours you can use, as opposed to what you have used, like the power company at home does.”
Meanwhile, researchers at University of Michigan also are evaluating how to measure the useful remaining energy in an aviation battery.
“You need to know battery capacity and resistance as the battery is depleting and aging and heating up during operations,” Anna Stefanopoulou, professor of mechanical engineering, told me by email. “You also need to know how long you might be needing the power reserve. For landing, you need probably 20 to 40 seconds of high-power reserve, and 4 to 5 times that if you want to be sure you can abort a landing, climb up and re-try it.”
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