This is the second in a series of articles investigating electric vertical takeoff and landing aircraft.
eVTOL operation is characterized by a higher discharge profile, especially during take-off and landing.
“Traditionally, batteries are either designed to provide a lot of energy or high power,” said Richard Wang, founder and CEO of Cuberg. “For eVTOL batteries, it is important to have a good balance between energy and power. The power is needed to allow take-off and landing, and the energy is needed to have sufficient cruising range.”
In order to sustain the eVTOL use case, with repeated trips where the battery is mostly depleted, eVTOL batteries are designed to charge extremely quickly, often in 5 to 10 minutes. “Prototype batteries can sustain up to a 50-mile trip, with energy density of 270 watt-hours per kilogram,” said Manuel Terranova, CEO and president of Peaxy, San Jose, Calif. “They typically can support up to 2,000 fast charging cycles over their lifetime. eVTOL batteries need to support high-power cell discharge, mostly during takeoff and landing. Not only is the level of discharge high, but the length of time where the battery is performing at peak capacity is much longer than in the EV case. While battery size and weight are a consideration with EVs, it’s even a larger factor with aircraft since it has a significant impact on the payload/range capabilities, not to mention making the aircraft too heavy to be flight worthy.”
eVTOL batteries are similar or larger than EV batteries in size, with more demanding usage profiles regarding power during charge and discharge. eVTOL batteries need to sustain a higher power level during takeoff and landing, typically two to four times higher than levels required for EV acceleration. Although high battery energy density is a plus in both EV and eVTOL applications, the energy density is usually traded off for power capability in eVTOLs, which makes high energy density technologies of higher importance for eVTOL.
Temperature can have a major influence on the level of eVTOL battery charge and discharge must be considered. With the high-power at takeoff and landing also comes a high temperature increase of the battery. “High-power operation conditions lead to self-heating, therefore, eVTOL batteries are not as sensitive to lower temperature,” says Ionel Stefan, chief technology officer at Amprius Technologies Inc. “Typically, high temperature operation under active control is the norm.” Cuberg manages temperature through both passive and active cooling systems integrated into its aviation-specific battery systems.
“Prototype batteries can be heated to help them discharge their excess energy more quickly, allowing a quicker recharge when its resistance to charge is the lowest,” Terranova said. “Temperature also plays a role in fuel gauging. An eVTOL aircraft regularly executing two takeoffs and two landings on a single charge will impact degradation differently versus one that only executes one takeoff and landing.”
Santa Cruz, Calif.-based Joby Aviation’s pouch cells in its production airplanes are rated at 288 watt-hours per kilogram at the cell level, and the company demonstrated in its lab that they’re capable of more than 10,000 representative flight cycles. The result of assembling those cells into certifiable, aerospace-grade battery packs is a specific energy of 235 watt-hours per kilogram at the pack level.
Dynamic dispatching cycles need to be factored into state-of-health (SoH) and state-of-charge (SoC) computations, including loaded vs. unloaded takeoffs, distance traveled, power output profiles, ambient temperatures, wind conditions, etc. “Determining battery SoH and cell-level degradation becomes a considerable challenge if data is not properly curated and automatically analyzed after every charge/discharge cycle,” Terranova said. “The nature of lithium-ion cells and batteries negates the operating assumptions that pack-level or system-level SoH computations are ‘good enough.’”