Advice on dual batt installation

Ok I have all the parts to tackle this project this weekend, I will be adding a second battery with a switch, BAT1 for starting, BAT2 for acc. I have read a lot on this and I think I can pull it off, although I just traced the pos cable to where it connects to the engine and 2 cables are coming out the main pos 2 or 4 gauge red one that goes to the pos terminal, and a smaller cable wrapped with black conduit, well I then see on the battery end that main cable connects to the terminal and the smaller one connects to the connector with the wire nut, but they seem like they didnt take the same route getting there (it might now be the same one).....Anyway now that your done laughing (I am a newb) where should I connect the smaller cable? to BAT2 or common on the switch? Thanks in Advance

Mercruiser 4.3 MPI

Are you sure that the smaller wire doesn't go to the fuse panel?

Are you using a Perko switch? If so, I just completed this same little task last weekend... it's VERY simple. Take everything that was on your battery's positive terminal and move it to the common post on the switch. Then take your existing battery's ground, leave it just as it is and ADD another negative cable to go to your second battery's negative post. I overthought it WAAY too much, it really was simple. Works like a charm!

Im using a blue sea battery switch, was looking for a perko but they didnt have any (west marine) salesman said it was the same thing, will give it a shot, still going to find out what that wire goes to.

Thanks

Blue seas is much higher quality then perko anyways, just make sure that its amp rating is higher then what your starter pulls and you will be fine.
david

Blue seas is much higher quality then perko anyways, just make sure that its amp rating is higher then what your starter pulls and you will be fine. Blue Seas switches come with wirering diagrams in the package and they have lots of helpful info on their website.
david

Should you just add one negative ground from batt 1 to batt 2 or split the ground before, and THEN go to both batteries seperate??

I left it on Batt1, I found out it was the wire for the ignition (maybe the fuse panel also), so if my Batt1 goes dead I wont be able to start my boat on Batt2 right? Im thinking about changing it to common on the switch. What do you think? I still dont know how the auto bilge is wired.

Ryan B: I just ran the neg from 1 to 2.