in the new release build or experimental? If experimental I'd say you use power cells to replace them. But I haven't tested this yet since I really can't thanks to the experimental branch being a bit broken with the inventory atm, last few experimental updates broke inventory so you can't open it properly (can't use mouse)
The power regenerates over time when you are not in the craft. I think for future updates there will be a faster way to recharge it, probably docking with the bigger subs or adding a new battery?
If you go back into the engine bay, you should see small, hand-sized cylinders. These should be power cells and give you a reading on how charged they are. Having a power cell in your inventory will let you interact with a cell and replace it. You can find 3 such cylinders in the lifepod for example.
I believe power cells are also used to recharge the Seamoth, instead of batteries, which refuel the SeaGlide
batteries recharge the SeaGlide, you're prompted to do so via the R key, If you have batteries in your inventory they get used. I don't know if they added the energy cell to the seamoth yet but they have plans for it. It was on trello after all.
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I believe power cells are also used to recharge the Seamoth, instead of batteries, which refuel the SeaGlide