Key flying car battery requirements identified

Technology News | June 8, 2021

By Rich Pell

Their work, say the researchers, which includes testing of a prototype battery, reveals that eVTOL batteries have more stringent requirements than electric vehicle batteries in all aspects.

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

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

In addition, say the researchers, the batteries will also need to be rapidly recharged so that there could be high revenue during rush hours, with such vehicles having frequent take-offs and landings and recharging quickly and often.

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

Weight is also a consideration for these batteries as the vehicle will have to lift and land the batteries. Once the eVTOL takes off, on short trips the average speed would be 100 miles per hour and long trips would average 200 miles per hour, say the researchers.

The researchers experimentally tested two energy-dense lithium-ion batteries that can recharge with enough energy for a 50-mile eVTOL trip in five to ten minutes. These batteries, say the researchers, could sustain more than 2,000 fast-charges over their lifetime.

The researchers used technology that they have been working on for traditional electric vehicle batteries. The key, say the researchers, is to heat the battery – in this case by incorporating a nickel foil that brings the battery rapidly to 140 degrees Fahrenheit – to allow rapid charging without the formation of lithium spikes that damage the battery and are dangerous. Heating the battery also allows rapid discharge of the energy held in the battery to allow for take offs and landings.

“Under normal circumstances, the three attributes necessary for an eVTOL battery work against each other,” says Wang. “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.”

One entirely unique aspect of flying cars, say the researchers, is that the batteries must always retain some charge. Unlike cellphone batteries, for example, that work best if fully discharged and recharged, a flying car battery can never be allowed to completely discharge in the air because power is needed to stay in the air and to land. There always needs to be a margin of safety in a flying car battery.

When a battery is empty, internal resistance to charging is low, but the higher the remaining charge, the more difficult it is to push more energy into the battery. Typically, recharging slows as the battery fills. However, say the researchers, by heating the battery, recharging can remain in the five- to ten-minute range.

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

For more, see “Challenges and key requirements of batteries for electric vertical takeoff and landing aircraft.”