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During an IPO Edge webinar last week, Joby Aviation founder and CEO JoeBen Bevirt made a bold claim: that the company expects to get at least 10,000 flight cycles out of the lithium-ion batteries in its eVTOL air taxis, making the cost of replacing batteries “essentially insignificant” in the context of Joby’s overall operating economics.
According to Bevirt, maximizing cycle life is important to the company not only from an economic standpoint — since Joby plans to operate as well as manufacture its aircraft — but also from an environmental one. “And so we’ve done a lot of work on the overall aircraft design: the size of our propellers, which drives our disc loading, which drives our hover power requirement, which drives our battery life. . . . We’ve been very thoughtful there to develop an aircraft where we can get more than 10,000 flight cycles out of a battery pack,” he said.
Joby expects its average flight length in urban air mobility operations to be just 26 miles (42 kilometers) — well short of its vehicle’s anticipated 150-mile (241-km) maximum range. That means that each flight cycle represents only a partial battery discharge, rather than the full discharge that is typically used to measure battery cycles.
But the 10,000 flight cycle target is still ambitious. As recently as 2019, Uber Elevate — the Uber business unit that was acquired by Joby last year — was estimating an amortized battery cost for eVTOL air taxis of $76 to $90 per flight hour, based on similarly short journeys but assuming a realistic flight cycle life of just 3,500 (and taking into account some salvage value).
If those figures were applied to Joby’s target of 2,555 flight hours per aircraft per year, the annual battery cost per aircraft would be in the range of $194,000 to $230,000 — perhaps not the largest share of the company’s costs, but hardly insignificant.
Nevertheless, Joby is confident it will be able to reach its 10,000 flight cycle goal, an assumption that is baked into the financial estimates in the form S-4 registration statement filed for its proposed merger with the blank check company Reinvent Technology Partners. Moreover, Joby claims to have laboratory data to back this up.
“We have run extensive testing on the lifetime of our batteries and have successfully demonstrated more than 10,000 flight cycles with fast charge in the lab environment,” a company spokesperson told eVTOL.com. “The testing replicates our expected operational flight profile.”
Venkat Viswanathan, an associate professor of mechanical engineering at Carnegie Mellon University who last year analyzed Joby’s maximum range estimate for Forbes, finds Joby’s flight cycle claim to be credible.
“Batteries today can go somewhere between 1,000 to 2,000 cycles with full charge,” he said. “Basically that 10,000 flight cycles simply means somewhere between 1,500 to 2,000 cycles on the battery pack, which is certainly doable with today’s battery technology.”
According to Viswanathan, “battery cycle life has been improving tremendously over the past few years,” so it’s reasonable to expect that cycle life estimates would have increased since Uber Elevate shared its cost calculations two years ago.
Joby has performed just over 1,000 test flights across its various eVTOL prototypes, so it still has a long way to go to demonstrate a 10,000 flight cycle battery in the real world. Electric Power Systems (EPS), a leading provider of lithium-ion battery solutions for electric aviation, said that a variety of factors will determine the actual life of a battery in operational service.
“Batteries are capable of over 10,000 cycles under certain conditions. But this is only one data point that does not present a complete picture of the problem. The challenge is to accomplish this over time when exposed to a variety of operating conditions,” the EPS technical leadership team told eVTOL.com via email.
“For example, EPS collaborated with NASA to collect data and model the life of an aviation propulsion battery. We found that exposure to shock, vibration, and especially high temperature can have far more impact on battery life than cycling. Aircraft usage spectrums vary, among other things, in range and speeds flown. Longer range and higher speed flights affect depth of discharge (DoD), which affects battery life as well.”
Joby appears to be developing multiple technologies that should assist with optimizing battery life. A 2020 article in Aviation Week indicated that the battery packs in its eVTOL will have onboard liquid cooling, which should help keep them at a constant desired temperature during discharge. For recharging, Bevirt said during the IPO Edge webinar, the company will be using proprietary chargers “that do both the charging and the thermal management of our battery system while we’re charging, and that allows for a very rapid charge rate and a very rapid turnaround time that’s on the order of the time that it takes for us to unload one set of passengers and load the next.”
Last year’s Forbes article called attention to a Joby patent application for a battery thermal management system that includes an offboard cooling system, which could help manage the heat generated by such rapid recharging without adding extra weight to the aircraft.
Thanks to the U.S. Air Force’s Agility Prime program, Joby should also have opportunities for field testing its batteries well in advance of commercial launch. During the IPO Edge webinar, Joby executive chairman Paul Sciarra said that the company last year finalized a Small Business Innovation Research (SBIR) Phase II Plus contract that gives the company an opportunity to put two vehicles into service with the Department of Defense this year, eight vehicles in 2022, and as many as 10 to 30 in 2023 under a Phase III extension of the contract.
According to Sciarra, Joby aims to certify its aircraft with the U.S. Federal Aviation Administration by the end of 2023, and launch commercial air taxi services in “one or two cities” in 2024. By 2026, the company expects to have 850 aircraft in commercial service, each spending an average of seven hours in flight per day. By then, it should be abundantly clear whether Joby is hitting or missing its battery cycle life targets — and the profit estimates that are partially based on them.