how do you go about doing it?

and what sort of battery would be best suited to being the second battery?

i know all abou thte caravan type stuff with it's relays and whatnots, but i'm after specificlly ice oriented get ups so that i can run the system from that battery until it dies and still start the car with the original car battery and then still charge both

or do i just plug it into the cigar lighter socket and hope for the best

want to do it right, but also only want to do it once