For flying cars to soar, new batteries are needed

10 June 2021
For urban air mobility to proliferate 30x increase in energy throughput and a 3x power requirements are needed for electric batteries. Source: Penn State

Research is ongoing to find a potential electric battery that will extend the energy capacity to allow electric vehicles (EVs) to travel longer and charge faster.

Electric vertical takeoff and landing (eVTOL) aircraft, also known as flying cars or air taxis, have additional requirements for electric batteries. As such, Penn State researchers are investigating what type of batteries will be needed for autonomous aerial vehicles.

"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, 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 a remarkably high energy density to stay in the air and need high power to take off and land as well as to vertically climb up and down. These batteries also will need to be capable of rapid recharging to meet demand during rush hours.

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

Additionally, weight is a consideration for these batteries as the vehicle will have to lift and land the batteries as well as travel at an average speed of 100 miles per hour for trips taking up to 200 miles per hour.

Penn State researchers tested two energy-dense lithium-ion batteries that can recharge with enough energy for a 50-mile eVTOL trip in five to 10 minutes. These batteries can also withstand over 2,000 fast charges during a lifetime.

The key to the batteries is to heat the battery to allow rapid charging without the formation of lithium spikes that damage the battery and could lead to catastrophic failure of the battery. The heating 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," 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."

Never drain the battery

One caution with flying car batteries is that they must retain at least some charge. Unlike cellphone batteries that can be 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.

When a battery is empty, internal resistance to charging is low but the higher the remaining charge, the more difficult it is to push energy into the battery. Recharging slows as the battery fills and heating the battery makes the recharge process faster in the five to 10 minute range.

The full research can be found in the journal Joule.

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