Marine Electronics Forum - 3rd battery ?

Background- Twin 225 merc motors, twin battery. Starboard batt runs my electronics..

I am installing a third optima deep cycle battery in my console to do three things, run a stereo and amp, run a windlass, and be a backup in case I run the two others out somehow on the water. My original intention was to never connect it to the other batteries unless emergency warranted un-installing it and dragging it to be a jumper. To charge it, I was going to hook it to an on board 3 bank charger.

My question is this: Can I get away with running a 4 or 6 gauge wire to the back of the boat on a switch to the location of my other batteries for an emergency start and for charging when underway? I do not anticipate running a load off that, just the low amp alternator charging it and for a quick emergency start. If not, what gauge can I get away with?

Thanks again for all the knowledge out there

Thanks again for all the knowledge out there!

What's the length of the run of wire?

20 feet or so by the time it crosses the boat

id run the windlass off a starting battery for starters. do you have a charging relay installed to charge the house battery? If not youll get 2 uses out of it before its dead. Which if thats the case you will be running a cable to the back of the boat anyways. If you are worried about having to jump your boat see if you can get a 3 battery switch. So your options would be 1, 2, 3, all, off. then you can just put the house battery back to that. i would run at least 4 guage. I personally ran 1/0

My question is this: Can I get away with running a 4 or 6 gauge wire to the back of the boat on a switch to the location of my other batteries for an emergency start and for charging when underway? I do not anticipate running a load off that, just the low amp alternator charging it and for a quick emergency start. If not, what gauge can I get away with?

No. You probably cannot start your engine with a long run of #6 or #4 wire. Too much voltage drop.

Of greater concern; If you don't put a fuse or circuit breaker at each end of the positive cable, you run the risk of fire if there is a short circuit.

There are other issues with this setup, for instance, when you talk about charging, are you using a 120 volt charger or are you using a combiner to charge the battery when the engines are running? Or were you just going to connect it to one of the other batteries?

20 feet or so by the time it crosses the boat

Remember there are two wires so that would br 40 feet.