Why batteries mattered most to early submarines

Which early-submarine technology was most important: engines or batteries?

Batteries: a submarine can adapt different engines, but it cannot operate safely or effectively without workable, non-lethal energy storage.

The answer places energy storage above the particular internal-combustion engine because a submarine can use several kinds of machinery while surfaced. Once submerged, propulsion and habitability depend on the battery installation. Early cells could release lethal fumes and kill crews even when the boat's hull and engines otherwise worked. The K-class steam submarines are invoked as an extreme reminder that an awkward surface plant can still function in technical terms. Safe, reliable batteries therefore mattered more to routine underwater operation than choosing the theoretically ideal combustion engine.