Ah okay, power isolation then. One way, or bidirectional (i.e. charge the battery too)?
A DC-DC converter module is just a transformer internally (with some odd exceptions), so that really reduces your question to light or transformer.
Even more generally, we need to use some kind of electromagnetism, because if we stick with just one (electric or magnetic), we have no power flow, and if we stick with none, we have a direct connection (DC and galvanic path). Or, well, nothing at all (no connection).

Or if we add transducers to other media, we can do whatever. For example, generators coupled via insulating shaft, or acoustic waves ("piezo transformer").
But within E&M, we can do low frequencies in transformers; high frequencies in coupled inductors; optical frequencies in photovoltaics; um, er, heat engines I guess in THz-IR bands, since we don't really have rectennas suitable for those frequencies yet; or.. nuclear something or other for x-rays+. That covers all of E&M. But yeah, clearly the most efficient and affordable among those options will be the first three...
So, a better question might be, what drives the choice of frequency and transformer type? Cost, simplicity, size, efficiency, isolation capacitance and voltage, noise, etc.?
Tim