You should get this done professionally IMHO.

Not saying you can't do it, only that how it is wired depends on what you want - i.e a 12 v system or a 24 v system.

You need to by a 'cole hersey' type switch (Like they have in a 4wd with dual batterys in a 12 V setup).

Having said this - it's not as easey as that!

1. It needs to be a Marine Switch

2. There are 3? different types of Marine switches.

Depending on your outboard type brand etc will determine what sort you need.

You see, some modern & 4 stroke outboards have ecm computers, fuel injection etc etc.

These need a constant source of power, without spikes etc. Also those with modern alternators need a constant load. Some 2 stroke outboards of the older variety only have a lighting coil and have a different requirement for power supply and load.

Now - the 3 way marine switches (On,One,Two,Both,Off), are NOT all the same.

Some make a new circuit before they break the old circuit - and some dont.

This has implications for continuity / integrity of power supply to the OB ECM & Injection systems, with possibly bad consequences if you switch on the run for the new type outboards. Also - a break before you make type can splatter a diode in the newer alternators if you have break before make type switches.

If you have the make before break type switch and don't wire it accordingly - because you have two ircuits open at the same time you can cause brief 24 v spikes, as you switch!

Certain types of these switches aren't sheilded and can and do start vesel fires in the bilge from fuel fumes in the event of fuel line / tank leaks, so it's vital to get the right type for your application.

Lastly you should never switch one of these switchs thru the off position to get between 1, 2, or both as the load for the alternator momentarily isn't there and you will eventually fry it!

I know we tend to think it's easey and we can do it ourselves - but if you can follow any of the above - you should realise it's not always as simple as we think.

What will interuptions of supply, spikes etc do to your onboard electronics instruments?

These issues become compounded where we might have both 12 & 24 v systems on some vessels - and then moreso when we have dual banks of dual batterys (i.e. 4 Batterys) 2 per side on dual outboard systems.

Sometimes they can be wired such that in the event of a battery bank failure on one side - the switches can be set to crank the opposite side engine from the same bank etc.

Just a bit of insight into the complexity of dual battery setups with new OB's and other onboard systems.

This is a marine electronic experts field IMHO.

Then again - electrickery & I don't get along, and I believe in leaving such things to the experts, but having a little background knowlege as to when is NOT the right time to DIY and when is the right time to DIY.

As a proponent of DIY in most things boating related, this is one area where I suggest due prudence!

Cheers!

Thanks Flywest I didnt think it was so complex. Will get the proper people to do the job, CHEERS.

Yours could be a simple setup Kev, and maybe my caution is not warranted - but - if I give out the advice to you - without all the caveats, someone else might read it and apply the same advice to their situation, and get the wrong answer if you follow me!

There are marine auto electricians who know what they are doing around the place, but you need to track a good one down IMHO!

I had a situation once where I wanted to wire 3 Batterys into a system for a bream style boat, one for electronics, one for the trolling engine and one for starting the outboard!

Now - the switches don't come in 3 Battery position types - only, "Off,One,Two or Both!".

So I didn't know what to do - but the Bro in Law whos a mechanic - had a look at the switch and by alering the polarity inside the switch, (moving a cable onto a different terminal), we were able to have the starter battery always online and the other two switchable to do any job from trolling to electronics to starting, but the complexity of it all had me totaly stumped!

To his credit it worked!
My mind just doesn't comprehend such things well, whereas his does!

This is just one example from experience where sometimes these clever types actually earn their keep in that they can do things us mere mortals might never think of! ;o)

Good luck with it - let us know how it works out!

Cheers!

It is not that complicated to wire in dual batterys. We wire them in this way in the majority or the boats we fit, fuel injection or not. Kev if installing it in an ally boat make sure you put a piece of cutting board or something behind the switch to stop it earthing out on the hull. i have drawn up a diagram that is in the boat maintenance. its not the best but i think it explains it pretty well, by the way i am a marine mechanic.

[img_assist|fid=25070|thumb=0|alt=dual battery wiring diagram|caption=simplest way to wire dual batterys]

You could well be right - and it looks OK to me!.

That said consider this for a moment!

In any big boat running a 240 V genset that can kill you if it shorts out - in a metal hulled boat including alloy - the hull is the earth!

According to what I've read, it should be the same in smaller alloy boats, with 12V systems earthed (equipotentially bonded) to the hull with an earth strap!!

If you think about it - the kill switch lanyard is earthed to the engine block! The engine block is earthed to the hull by virtue of the bolts going thru the alloy transom.

Thus the hull is already the earth!

Therefore is the cable betwixt the two batterys earths poles really necessary or should both of them just earth to the hull?

OK I know no one does it this way and everyone runs earth cables!

Is that why so many alloy hulls have galvanic corrosion / electrolysis probblems with only some parts of the electrical system correctly earthed in an equipotential bond with the hull?

All good questions, to which after litterlly years of debate on this very subject, on boards such as this, there is no definitive answer IMHO.

It is a question best left for the experts IMHO!

The big boys with their luxury 200 ft mega yachts made outta aluminium (like the 600 cray boats in the WA fleet and the likes a Greg Normands battleship etc) all seemed to have it figured by experts and they all go for the hull equipotentially bonded as the earth to protect the occupants from death in the event of a 240 ground situation..

Maybe theres a lesson in there somewhere for us 12V tin boat players! ;o)

Cheers!

kev never asked how to wire dual batterys in a 200 foot boat with a genset, he asked how to wire them into a stacer with a 90 merc.

It should be different?
Do tell - how?
I don't believe there is a right or wrong in this instance!!
I am no marine electrician and don't profess to be!!

Cheers!