NASA To Test High-Performance Batteries for Advanced Air Mobility

California-based Archer Aviation and the National Aeronautics and Space Administration (NASA) are evaluating the safety of Archer’s high-performance battery cells for advanced air mobility and space applications. The partners plan to validate the technology for NASA’s mission-critical electric vertical take-off and landing (eVTOL) aircraft. 

 

Archer Aviation’s Midnight aircraft. Image used courtesy of Archer

 

Archer’s batteries have already been deployed in Midnight, its air taxi designed to hold four passengers and a pilot. The NASA partnership will build on that experience with advanced safety testing, paving the way for securing Federal Aviation Administration (FAA) certification in late 2024. 

The announcement cited maturing battery technology as a significant trend fueling the adoption of eVTOL aircraft. The lithium-ion battery cells used in Midnight feature a cylindrical cell form factor. This design choice has a long track record demonstrating safety, performance, and production scalability. Cylindrical cells have been deployed in millions of electric vehicles, representing a strong supply chain. 

However, aircraft battery manufacturing is still in the early stages. That’s part of the motivation behind Archer’s new partnership with NASA. By proving Archer’s battery cells are safe for potential mission-critical eVTOL aerospace applications, the technology can move closer to mass production, adoption, and expansion into more markets. NASA plans to share its results with the industry. 

To test the battery cells’ safety, energy, and power performance under extreme conditions, the partners will use the European Synchrotron Radiation Facility in France, one of the world’s most advanced high-speed X-rays. 

 

Six Independently Powered Battery Packs for 12 Engines

Midnight can travel up to 150 miles per hour at a range of about 100 miles, with a payload capacity exceeding 1,000 pounds. It can also conduct 20-mile back-to-back flights with about 12 minutes of charging time between each. 

Archer engineered the aircraft’s internal architecture with the idea that failures can occur at any point during vertical take-off, flight path, or landing. Six independent 800 V battery packs power a diagonal set of 12 forward and aft electric engines. This design allows flights to continue if any part of the propulsion system fails or one of the battery packs completely loses power. 

 

Each Archer Aviation battery pack supports two motors. Image used courtesy of Archer Aviation 

 

Archer started testing its Olympus lithium-ion cells in 2021 and commenced battery pack evaluations the following year. Each battery has 142 kWh of energy, 1,300 kW of maximum power, and a 10-minute charge time for the average mission. 

 

The Complexities of Engineering Aircraft Batteries

Powering aircraft with battery cells is notoriously complicated due to tight weight constraints. In early development, Archer’s engineers had to balance weight and performance needs while keeping the overall power requirements low. Optimizing the cell design enabled them to minimize the mass of the batteries. 

A parallel layout isolates each unit, enabling Archer to reduce the power requirements at the battery cell level by 20% compared to a conventional high-voltage architecture. 

FAA certification requires forcing the cell into thermal runaway to demonstrate safe performance. With this in mind, Archer considered the power and safety advantages of three different cell form factors. Pouch cells offered the highest energy and power density but weren’t as safe without a dedicated cell vent. Prismatic cells were also promising, but the electrode placement left a higher chance for short circuits. One cell failure could affect the whole pack. 

Ultimately, the team landed on cylindrical cells because of their superior energy and power density, thermal management, and proven safe architecture.

 

Archer’s battery pack design and strategy. Image used courtesy of Archer Aviation 

 

The engineers needed to ensure each pack could meet the needs of the eVTOL propulsion system. Lithium-ion batteries supply power at the highest state of charge (SoC), making them ideal for supporting the aircraft during take-off. However, power levels drop throughout the flight as the SoC is reduced. This is why it’s important to consider power levels in flight peaks. 

 

Archer’s battery performance requirements for electric aircraft. Image used courtesy of Archer Aviation 

 

The plane initially takes off vertically. A 6,500-pound aircraft requires about 1,000 horsepower to hover, roughly equivalent to a Formula 1 race car. More power is needed as the plane climbs, cruises, and descends. 

When Archer paired each set of battery packs with a corresponding engine, the aircraft could still land even if either component failed at any point. 

With more testing underway through the NASA partnership, Archer plans to secure FAA certification for Midnight later this year.