April 16, 2020: In March 2020 the Japanese navy put its first lithium-ion battery-equipped submarine into service. This sub is the 11th of the Soryu class. The 12 ^th and last of the Soryu class will also have lithium-ion batteries. These two subs are referred to as “Super Soryus” because of their new battery tech and the higher cost that goes with having this feature.

Several nations (South Korea, China, Germany and the United States) have been working on this technology and those efforts became particularly intense after 2015. The main obstacle was the safety of lithium-ion batteries in a submarine. Lithium-ion batteries are known to be dangerous under certain conditions. Consumer products (cell phones, laptops) have had problems. Not a lot but enough of the hundreds of millions of cellphones and laptops using lithium batteries have burst into flames or exploded to make the general public aware of the risk. These overheating problems had to be minimized to levels that made lithium-ion batteries safer than the current lead-acid batteries used for over a century in submarines. Several nations believe they have achieved the needed safety levels and Japan is the first to put a lithium-ion boat into service. This is encouraging for China, South Korea and Germany who are planning on offering upgrades (from lead-acid to lithium-ion) for existing subs. Orders for such conversions have not been forthcoming because there have not been any military subs in service with the new battery tech. Now there is one boat and another will join it in 2021.

The advantages of lithium-ion batteries are many. First, lithium-ion stores twice as much power as equivalent (in size and weight) lead-acid batteries. Lithium-ion batteries can release more power than lead-acid and take less time to recharge. Lithium-ion batteries do not degrade over time and have more recharging cycles. Lithium-ion batteries can enable subs to move faster under battery power. Putting out low levels (for low speed) of power, lithium-ion batteries can provide almost as much submerged time as current AIP (Air Independent Power) systems. This means that smaller coastal subs can be designed without diesel engines because lithium-ion batteries provide enough power for the short voyages coastal subs are intended for. These coastal boats don’t even have to return to port to be recharged as this can be done by a surface ship equipped with the proper cables and power regulation system to quickly recharge lithium-ion batteries. Users of cell phones and laptops have already been getting this fast-charge capability and for those who used the older battery tech appreciate how much shorter recharge times are now.

The last two Soryu class subs are dispensing with the AIP systems they were designed to use and rely on lithium-ion batteries to provide the underwater endurance similar to that provided by AIP. This approach is also be watched closely by submarine builders because adding AIP is more expensive than installing lithium-ion batteries in place of lead-acid ones. The key factor is the safe operation of submarine lithium-ion batteries under all conditions, including accidents that damage the hull and internal equipment. This is something you can’t really test, only design for. The Japanese lithium ion battery manufacturer insists they have all this covered. Only time at sea will tell.

The quantity and quality of its submarines is important for Japan. Since the 1970s, Japan has maintained a fleet of at least 18 diesel-electric submarines. A decade ago, in the face of growing Chinese naval power, it was decided to increase the submarine force to 21 or 24 boats. Currently, there are 22 subs in service; 11 Soryu class (with another entering service in 2021) and 11 Oyashio class. Two of the Oyashios now serve as training boats and are used to produce sailors qualified to serve in submarine crews.

Since 1980, Japan has replaced their subs after about 25 years, with newer designs based on experience with the previous classes. The current expansion was accomplished by building more of the new Soryu class. A decade ago there were two Soryu class boats in service and four under construction. These 2,900 ton boats have a crew of 65, six 533mm (21 inch) torpedo tubes and 30 torpedoes or Harpoon anti-ship missiles. There are also two 76mm tubes for launching acoustic countermeasures. Sonar and electronics are superior to the previous class. These boats also have AIP (Air Independent Propulsion) that enables them to remain submerged for a week or more at a time. Top surface speed is 24 kilometers an hour, top submerged speed is 37 kilometers an hour.

A successor to the Soryu class is basically an improved Soryu with lithium-ion batteries plus many lesser technical improvements, and displacing 3,000 tons. The first of these new boats is under construction and should enter service in 2022. This first-in-class will undergo intensive use to determine which of the new features requires modification or replacement. Another eleven are to be built and replace the existing Oyashio class subs.

Currently, Japan also has eleven 2,700 ton Oyashio class subs, built 1994-2008. With a crew of 70, they are armed with six 533mm (21 inch) torpedo tubes and 27 torpedoes or Harpoon anti-ship missiles. Their sonar equipment is superior to that of the Harushio class. Top surface speed is 24 kilometers an hour, top submerged speed is 37 kilometers an hour.

Japan has retired its seven Harushio class boats, including two diverted to training duties. These 2,400 ton boats were built 1987-1997 and have crews of 65-70 sailors. They are armed with six 533mm (21 inch) torpedo tubes and 26 torpedoes or Harpoon anti-ship missiles. They have hull-mounted and towed sonar. Top surface speed is 24 kilometers an hour, top submerged speed is 37 kilometers an hour.

China currently has about 76 submarines, none of them as effective as the Japanese boats, despite 19 of them being nuclear. The Japanese crews are also better trained, but the Chinese are building better ships with more intensively trained crews. Two other Chinese neighbors, South Korea and Australia are also increasing their submarine forces.