On 5 March 2020, at Mitsubishi Heavy Industries’ shipyard in Kobe, Japan’s navy took delivery of a submarine that was missing something. JS Ōryū, the 11th Sōryū-class boat, had been built without the four Stirling air-independent engines that made the class famous. In their place were more batteries, and the old lead-acid cells were replaced with lithium-ion. It became the world’s first military submarine to go into service on diesel engines and lithium-ion batteries.
This documentary explains why a navy would remove the technology other navies were working hard to add. We start with the oldest problem in conventional submarines: the battery runs down, and the boat has to come up and snorkel. Then we cover how Japan answered that problem with Stirling engines and liquid oxygen, testing them first on the submarine Asashio, and why retired Vice Admiral Masao Kobayashi, a former commander of Japan’s submarine fleet, called lithium-ion “revolutionary”.
We also look calmly at the question everyone asks: what happens when a lithium cell goes wrong? We explain thermal runaway, the 2013 Boeing 787 battery fires, and what Japan has actually published about stopping one cell’s failure from spreading to the next. Endurance figures and the boats’ safety systems remain unpublished, and we say so.
Then the Taigei class: 84-meter submarines designed around lithium-ion from the start, built in turn by Mitsubishi and Kawasaki in Kobe, about one a year. In March 2024, two years after commissioning, the lead boat Taigei was reclassified as a test submarine. It wasn’t the first time Japan had turned a young submarine into a laboratory.
We finish with South Korea’s first lithium-ion submarine, Jang Yeong-sil, which kept its air-independent propulsion; the US Navy, which has had no conventional submarines since 1990; and where Japan’s program stands today. Chōgei was commissioned in March 2026, Sōgei is fitting out, and a seventh boat is under construction. Battery interiors and test scenes are visualizations.
Watch our Maya-class episode next to see how Japan built a warship that hunts missiles in space. Subscribe to Nihon Forge for precise breakdowns of Japan’s warships and defense engineering.
#JMSDF #Submarine #TaigeiClass
Disclaimer: Synthetic Media & Content Generation
The visual and audio content in this video, including voiceovers and photorealistic representations of aerospace and industrial environments, was generated using advanced artificial intelligence tools. Modus Assembly utilizes AI strictly as a visualization and production mechanism. This content complies with YouTube’s Altered and Synthetic Content policies.
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