Engineers at Penn State published plans Monday for a battery prototype they said is capable of powering flying cars.

"I think flying cars have the potential to eliminate a lot of time and increase productivity and open the sky corridors to transportation," lead author Chao-Yang Wang said in a press release on the study, published in the journal Joule.

"But electric vertical takeoff and landing vehicles are very challenging technology for the batteries," said Wang, director of the Electrochemical Engine Center at Penn State.

In the last couple of years, several prototypes have emerged -- including from companies in South Korea, China, Slovakia and Japan -- which incorporate technology from helicopters and cars to create a hybrid vehicle of sorts.

The prototypes include SkyDrive's SD-03 vehicle that was tested in August, KleinVision's AirCar that could be for sale as soon as this year and Xpeng Motors' Kiwigogo vehicle that debuted at last year's Beijing International Automotive Exhibition.

While some prototypes have included wheels, they all incorporate spinning rotors to facilitate takeoff and landing, including the air taxi shown off last year by Hyundai and Uber, which is basically a small helicopter.

While the AirCar runs on liquid fuel, the others are at least partially powered by electric -- which means they require powerful batteries to fly.

In the new paper, Wang and his research partners established a variety of technical requirements for the batteries of electric vertical takeoff and landing vehicles, or eVTOLs.

In order to get a flying car off the ground, an electric battery must be able to deliver a lot of power and fast.

"Batteries for flying cars need very high energy density so that you can stay in the air," Wang said. "And they also need very high power during take-off and landing. It requires a lot of power to go vertically up and down."

Additionally, a flying car battery must ideally be able to be quickly recharged. Unlike most flying vehicles, eVTOLs will likely be taking off and landing rather frequently.

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

In the lab, researchers tested the performance of a pair of energy-dense lithium-ion batteries capable of delivering the kind of power needed to sustain a 50-mile, 5- to 10-minute eVTOL trip.

The experiments showed the batteries were good for 2,000 fast-charges over the course of their lifetimes.

Tests involving batteries the team is developing for electric road vehicles -- which are designed to offer a longer driving range with a faster charging time -- showed heat is key to preventing lithium spikes, which can damage batteries and lead to dangerous battery failures.

To avoid this, Wang and his colleagues were able to rapidly heat the batteries by incorporating nickel foil into the design.

Researchers found suitable heating also allowed the batteries to deliver a rapid burst of power -- the type of discharge required for take-offs and landings -- more efficiently.

"Under normal circumstances, the three attributes necessary for an eVTOL battery work against each other," Wang said. "High energy density reduces fast charging and fast charging usually reduces the number of possible recharge cycles. But we are able to do all three in a single battery."

It's easy to rapidly charge a battery that's nearly drained, but frequent takeoffs and landings will require rapid charging of half-full batteries -- a more difficult task. However, the latest research suggests sufficient heating can solve this problem, too.

"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," Wang said. "I believe we have demonstrated that the eVTOL is commercially viable."

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