how many terminal points does that switch have?
I would think that there would be one for each battery positive and one for the load(output).If that is the case then the negatives would need to be tied together
Yes, you connect the negatives between the two batteries and the positives go to the battery switch. If you aren't using an isolator and running on both your alternator will only charge until 1 of the batteries is charged, then it will stop.
You want to connect the negative lines, but you don't necessarily want to connect negative terminal to negative terminal.
I run both of my negatives to a 'common ground', which I use a heavy duty bus bar that has 4 bolt connectors. Then all of my other ground wires (from engines, breaker panel, electronics, etc) go to this bus bar too. Then I run 1 battery positive cable to each switch position (starting battery #1, deep cycle #2) and the other positive wires (engine, breaker panel, electronics, etc) to the middle switch terminal.
The bus bar allows you to isolate the batteries w/ the switch and allows easier connection of all your other ground wires.
The bus bar does work well for connections but it has absoluely no effect on the switch's ability to isolate the battery. It also works well to run a bus bar of the + feed coming off the switch, once again to make for easier connections. If your battery is in the bilge area you'll want to varnish the connections to keep saltwater corrsion from becoming an issue.
Thanks for the input. As far as I can remember, there are 2 red + leads coming out of the switch. I think i have 2 black- leads in conduit going back to outboard. Anyway, I tied both grounds together. It does start and charge with switch on 1 or 2.