You definitely need a ground wire (1/0) from the front battery to back battery back. Don't listen to any who says you don't and that they don't and it works fine for them... I'll be telling you to run a neg cable when you're trying to trouble shy your back batteries aren't doing as good as they should.do people use them with just 2 batteries, i plan on running 2 in back also what would be best to run a full negative wire from the front negative battery to the back battery neg or can i just ground the batteries to a metal in the back, also same with amps, can i just run the neg from the amps to the neg on the back batteries or should i just ground them seperate to metal