But I just realized that I can use a power fet since it will be DC? Is that correct?
Someone recommended the FQP50N06
recently. I've never used it, but it's good up to 50 Amps, so it should be good for almost anything.
I'm going to use one of those prebuilt relay boards...FYI -
Is optical isolation needed...?
Mechanical relays are isolated (mechanically rather than optically). There is no connection between the coil and the contacts. Of course, you can use a common ground and/or a common power supply, so the two sides don't have to be isolated in your circuit.
And while we're on the subject, transformers also provied isolation (electromagnetically). But transformers don't pass DC.