You can be testing two different technologies on two different subs. The newer one might be a manned one.
If I were to continue what I was talking about testing Lithium Ion for proof of concept submarines built for that purpose. I would like to test using LiFePo, which is a kind of Lithium Ion and one that is gaining rapid popularity among lower end EVs in China. LiFePo is the backbone of BYD's Blade battery for example, and Tesla is moving their lower end vehicles to it.
Conventional Lithium Ion uses Nickel Metal Hydride with cobalt, and that's the kind that got Chevy Bolts in trouble with fires, along with some Teslas too. NMH is the tech that the new Soryu derived class should be using as LiFePo is not used by any other manufacturer or country but in China. LiFePo has far greater heat tolerances, and doesn't produce as much heat, which makes it significantly safer. Another thing is that these batteries can survive a high degree of stress and puncture without exploding. The safety appeal of LiFePo makes it attractive for military applications. These batteries also have much longer recharge lifetime cycles, so they can remain in use a lot longer without being removed and recycled. Another advantage of LiFePo is that you can charge them at 100% and discharge them to a 100%, while NMH is at 80%/20%. They are also cheaper and doesn't use conflict minerals like Cobalt. They don't have the power density of NMH type Li Ion however, but their safety features allows them to be packed in much more denser formations which partly makes up for their lower power density, as NMH requires some space for ventilation. LiFePo also uses Phosphate, an element that is common in China, and LiFePo batteries are commonly built by battery makers in China.