１．Battery Packs Capable of Withstanding Harsh Stratospheric Conditions

SoftBank’s Research Institute of Advanced Technology is dedicating efforts to battery development for the commercialization of "HAPS" (High Altitude Platform Station). To achieve HAPS's implementation, the establishment of high-performance battery technology that supports proper operation in the stratospheric environment is essential.
（Reference：SoftBank’s Next-Generation Battery Development）

In January 2023, a successful operational demonstration was conducted in the stratospheric environment using a battery pack containing high-energy-density lithium metal battery cells. The excellent battery performance demonstrated in the stratosphere represents a significant milestone toward the commercialization of HAPS.
（Reference：SoftBank Corp. Develops Battery Pack with Next-generation Lithium-metal Battery Cells and Successfully Demonstrates Operation in the Stratosphere）

The stratosphere is a layer of highly tranquil and stable air situated at altitudes of 10 to 50 kilometers, positioned above the troposphere where clouds and precipitation occur. This environment is characterized as "low-temperature, low-pressure environment,” with extremely thin air and minimal convection. Standard batteries often experience significant performance degradation in low-temperature conditions due to increased internal resistance, making charging and discharging difficult. To address these challenges and ensure efficient operation in the stratospheric environment, battery packs designed for HAPS must incorporate temperature management features. This includes the use of thermal insulation materials on the external casing and the installation of internal heaters. These temperature management measures help maintain the battery's performance while ensuring stable operation in the stratospheric environment.

２．Charging / Discharging Tests in Simulated Stratospheric Environment

To evaluate battery performance in the stratospheric environment, charging / discharging tests are conducted using "low-temperature, low-pressure chamber." This evaluation allows the assessment of how the battery pack behaves, its durability, and the operation of the temperature control mechanism when exposed to low-temperature and near-vacuum conditions (approximately 4kPa), simulating the harsh conditions of the stratosphere and verifying the battery pack's performance under conditions close to actual operation. To further replicate the stratospheric environment and enhance the evaluation's reliability, a cover is installed to suppress air convection near the battery pack.

Two critical points in this evaluation are as follows:
1. Heater Power Consumption: Since the heater is powered by the battery pack itself rather than an external power source, it is essential to minimize power consumption. Combining the heat generated during discharge with the heater's output helps minimize power consumption. This ensures maintaining the battery's energy efficiency and enables prolonged operation in the stratospheric environment.

2. Temperature Distribution of the Battery Pack: The capacity of battery cells varies with temperature during usage. For instance, if there are areas within the battery pack with low insulation and poor heat transfer from the heater, temperature distribution may become uneven, resulting in reduced usable capacity in cells exposed to low temperatures. Therefore, optimizing the layout of heaters to achieve uniform heating throughout the battery pack is necessary.
For future developments, we aim to create an environment chamber capable of controlling temperatures below -90 degrees Celsius to simulate even closer conditions to the actual stratospheric environment. This will enable us to conduct tests that closely resemble real-world operational conditions.

３．Safety Tests at Atomospheric Pressure and Reduced Pressure Conditions

For HAPS battery packs, it is crucial to confirm their safety under reduced pressure conditions in the stratosphere. The behavior of batteries may differ under reduced pressure compared to atmospheric pressure. Therefore, conducting safety evaluations under these conditions is essential to understand the battery pack's behavior in situations close to actual operation and establish safety standards.
To conduct safety tests under reduced pressure, a custom-made "decompression chamber" was created (The chamber was designed in the size suitable for battery cells, as shown in the below picture.) This chamber is designed to withstand the internal pressure of a battery cell in case of ignition. When the internal pressure reaches near the chamber's pressure tolerance, a vent valve located at the top of the chamber is opened.

The safety evaluation, which simulates internal short-circuits in battery cells, is usually performed using a nail penetration test. However, conducting this test under reduced pressure is challenging due to facility constraints. SoftBank has developed a novel approach to address this issue. We have attached heaters to the surface of the battery cell, enabling localized heating. The specific method involves creating a structure that sandwiches the battery cell in the tank using restraint plates and thermal insulation material (as shown in the figure). By closely attaching heaters to the surface of the battery cell, heat generated during the short circuit does not escape to the external environment via the restraining plates. During the test, voltage is applied to the heaters, causing the temperature to rise to the point where the separator in the battery cell melts and short circuits. This evaluation allows for safety testing under reduced pressure conditions to be realized successfully.

As shown in the below photo, voltage wires are attached to the lead part of the battery cell, thermocouples are placed at the center of the battery cell, and current wires are connected to the heater.

During the safety test under atmospheric pressure, it was observed that when the battery cell caught fire, white smoke was generated, followed by the gas explosion involving surrounding oxygen. The white smoke is primarily caused by the vaporization of the electrolyte and its decomposition products due to internal short-circuit-induced heating. It is believed that the combination of the electrolyte components, external oxygen, and heat emitted from the battery cell created the three elements necessary for combustion (flammable material, oxygen, and ignition energy), leading to the ignition.

Safety test under atmospheric pressure

On the other hand, during the safety test under reduced pressure, the battery cell caught fire, but the gas explosion observed under atmospheric pressure did not occur. This difference is attributed to the fact that while the battery cell's internal elements necessary for combustion (electrolyte, oxygen generated from positive electrode active materials, and heat from short-circuit) were present, there was almost no external oxygen under reduced pressure. As a result, the flame's sustenance time was short, preventing the gas explosion.

Safety test under reduced pressure

Based on these results, it is suggested that the external casing of the battery pack for HAPS could be designed with reduced strength for weight reduction since the gas explosions do not occur under reduced pressure. Weight reduction is one of the critical factors in HAPS operation, contributing to expanding the service area and payload capacity. However, it is essential to implement fire-resistant and insulating structures to prevent the heat from transferring to the HAPS aircraft, as the temperature can rise to several hundred degrees during operation.
SoftBank’s Research Institute of Advanced Technology will continue to develop testing methods under conditions closer to the stratospheric environment and contribute to establishing safety standards for HAPS battery packs, while continuing their efforts in the development of HAPS-specific battery packs.