Flying cars were already supposed to be here, according to those who predict the future (not to mention The Jetsons). Instead, we got the Roomba. A group of researchers at Penn State looked at the requirements for electric vertical takeoff and landing (eVTOL) vehicles. Then they tried their hand at designing and testing different batteries to see what might work.

“I think flying cars have the potential to eliminate a lot of time and increase productivity and open the sky corridors to transportation,” said Chao-Yang Wang, William E. Diefender Chair of Mechanical Engineering and director of the Electrochemical Engine Center at Penn State. “But electric vertical takeoff and landing vehicles are very challenging technology for the batteries.”

The team of researchers reported on their findings in Joule, defining the exact requirements for and describing their own flying car battery prototype.

Wang says that batteries for flying cars require “very high energy density” in order for the cars to stay in the air. The other issue, says Wang, is that eVTOL vehicles need an extra burst of high power on takeoff and landing. “It requires a lot of power to go vertically up and down.”

One of the issues with generating so much power is that users will need to have a method for rapid, almost immediate recharge of the batteries, if flying cars are to be viable, useful, and affordable.

“Commercially, I would expect these vehicles to make 15 trips, twice a day during rush hour to justify the cost of the vehicles,” said Wang. “The first use will probably be from a city to an airport carrying three to four people about 50 miles.”

The Penn State researchers tested two energy-dense lithium-ion batteries that were capable of recharging in five-ten minutes—enough energy for a 50-mile trip in a flying car. The batteries that were tested are capable of over 2,000 quick charges in the course of their lifetime.

Cellphone batteries work best when you let them completely discharge before recharging. A flying car is different. For safety reasons, a flying car battery will always need to maintain some power in order for the car to stay alight. Wang’s team found that heating the battery makes it recharge more quickly, within the five-ten-minute necessary range for a flying car.

“I hope that the work we have done in this paper will give people a solid idea that we don’t need another 20 years to finally get these vehicles,” said Wang. “I believe we have demonstrated that the eVTOL is commercially viable.”