Amprius Technologies’ ultra-high-energy silicon anode lithium-ion batteries have helped AALTO break an endurance record, completing a continuous 67-day flight in the stratosphere with its Zephyr high-altitude pseudo-satellite (HAPS) aircraft.
In April 2025, AALTO, a U.K.-based Airbus subsidiary specializing in stratospheric UAVs, successfully concluded Zephyr’s record-setting mission, which surpassed its own previous 64-day milestone.
Operating at altitudes near 70,000 feet, the solar-powered Zephyr utilizes daytime solar energy to recharge its battery system, relying entirely on Amprius’ silicon anode batteries to sustain flight through the dark, multi-night cycles of the stratosphere. The energy density and durability of these lithium-ion cells were instrumental in maintaining uninterrupted flight, showcasing a new benchmark for long-duration unmanned aerial endurance.
Dr. Kang Sun, CEO of Amprius Technologies, said, “We are extremely proud to see our batteries enable this groundbreaking 67-day flight. This accomplishment demonstrates the transformative impact of our silicon anode battery technology in aerospace. By providing the energy needed to keep Zephyr flying through the night, our silicon anode cells made it possible to achieve a new level of endurance for stratospheric aircraft. We are honored to partner with AALTO in pushing the boundaries of what high-altitude platforms can do for both commercial and defense applications.”
Pierre-Antoine Aubourg, Chief Technical Officer at AALTO, added, “Achieving this 67-day flight is a major milestone for Zephyr and HAPS technology more widely. Amprius’ high-energy batteries provided the reliable power storage we needed to maintain Zephyr’s day and night operations. Their technology was instrumental in enabling us to break our own endurance record, demonstrating that persistent stratospheric missions and capabilities are now a reality. We look forward to continuing our collaboration with Amprius as we focus on our remaining flight program for 2025.”
Amprius Technologies’ batteries stand out as some of the few commercially available power sources capable of meeting the demanding energy and weight requirements essential for persistent HAPS missions. This successful collaboration between Amprius and AALTO builds on prior achievements, including the 64-day endurance flight in 2022, further validating the performance and reliability of the Zephyr system and its energy storage solutions.
The Zephyr flight opens new horizons for high-altitude pseudo-satellites in both commercial and defense sectors. With the ability to remain aloft for months, Zephyr offers satellite-like persistence combined with UAV-like operational flexibility, ideal for continuous border surveillance, secure communication relays, environmental monitoring, and other critical missions requiring uninterrupted coverage.
