Galvanic Isolators

DC to DC converters accept one DC voltage and output another. Often used to convert from 12V to, say 24V, or the reverse.

These converters can also be used to "convert" to the same voltage, say 12V to 12V. This is very useful for providing a stable output voltage (perhaps to marine electronics) while the input voltage may temporarily fluxuate.

The Victron "Smart" line of DC to DC converters are even more powerful. Instead of just outputing one constant voltage, they output a full multi-stage charging profile for batteries of the specified output voltage.