SoftBank Corp. announced that its proposal entitled “Research and Development on High Energy Density Battery Pack and High-power Generation Efficiency Solar Cells for HAPS” has been adopted by Japan’s New Energy and Industrial Technology Development Organization for a new four-year project starting in fiscal 2025 (the year ending March 31, 2026).
October 7, 2025
The research is part of the initiative titled “Development and Verification of Maritime Domain Awareness Technology Using High-Altitude Unmanned Aircraft” under the “Key and Advanced Technology R&D through Cross Community Collaboration Program”*1.
In fiscal 2024, SoftBank’s feasibility study on storage battery and solar cells technologies was also adopted under the same research theme*2. Together with ENAX Inc., the National Institute of Advanced Industrial Science and Technology (AIST), and CHOSHU Industry Co., Ltd., SoftBank conducted a study identifying key technical challenges and requirements. The newly adopted project builds on the findings of that study. While contractually independent, the 2025 project is positioned as a technical continuation. Over the next four years, the four parties will jointly develop high energy density battery packs and high-efficiency solar cells to enable long-duration HAPS (High Altitude Platform Station) flights. In addition, they will pursue modularization technologies that achieve both weight reduction and environmental durability in the stratosphere, with the goal of conducting a stratospheric demonstration in fiscal 2028.
Background and future developments
HAPS aircraft fly in the stratosphere by using solar power generated during the day and stored battery power at night. In high-latitude regions such as Europe and Japan, shorter daylight hours during winter make year-round flight operations challenging. To overcome this, it is essential to reduce aircraft weight—especially the battery packs and solar cells, which make up a large share of the total weight—while also improving solar power conversion efficiency.
To reduce the weight of the battery pack, it is necessary to assess the performance of next-generation battery cells with high gravimetric energy density (Wh/kg), such as lithium metal batteries, which offer advantages in both weight and volume reduction compared to lithium-ion secondary batteries, and to develop technologies that enable their integration into battery packs. For solar cells, achieving both high efficiency and reduced weight requires tuning them to operate effectively in the stratospheric environment—where the solar spectrum differs from that on the ground and temperatures are extremely low—as well as using thinner and lighter structural materials.
In the feasibility study conducted in fiscal 2024, the research team identified key technical challenges related to storage batteries and solar cells for HAPS and formulated the necessary performance requirements. Moving forward, the project will focus on developing modularization technologies that combine environmental resistance and weight reduction, enabling the integration of high energy density battery packs and high-efficiency solar cells into HAPS systems while ensuring safety. A stratospheric demonstration is planned for fiscal 2028, with the ultimate goal of enabling year-round HAPS operations in high-latitude regions such as Japan.
