I have a big system in my car and I want to run an auxilary battery that would get charged by the alternator but would be isolated from the main battery (not connected to it).

I am not exactly sure how to do this but I think I am supposed to connect the ground going to the main battery to the backup battery. Am I right or is there another way of hooking this up?

This probably isn't the correct place to post this but this section gets the most exposure to the public. Can anybody help?