Aux battery with relay?

Has anyone ever wired up an aux 12V battery with a relay so that some things will be turned off with the key switch? Is a relay even necessary? How much current can a stock key switch handle? I want my light bar/headlights/tail lights/horn to shut off with the key and I want the (future) stereo to stay on.

Would this work?

Also what chargers do you guys use?
This one looks like it would work well. I'm liking the dashmount adapter. It makes it easy to plug in and also has a battery life indicator.