How are deep sea technologies powered?

We normally use a conventional lead acid car battery and we sink the whole thing into oil which is inert. So, it's pressure compensated. There's no air spaces, nothing to collapse under pressure and as long as the oil is inert, the electrical contact still works. But lead acid battery capacities are slightly limited so we have other systems where we use lithium ions but they have to be inside pressure housings because the last thing you want to do the lithium is get it wet. And there's a certain danger element involved with lithium as well when things don't go right, but batteries are a big issue with us especially in autonomous systems.