because this weekend it is looking likely that we will be doing some repairs / mods on Bodie's boat. similar to what you have described above looking to install 2x battery isolators but with 3x batteries.

1x battery for each outboard motor (twin Johnson outboards) and 1x separated battery for electrics to be run.

any tips or diagrams from others who have done similar would be very much appreciated

scano

look at a red arc controller or your charge input will never really charge all three batteries properly it can be done but without the regulator charge voltage is not 100%

I'd be looking at the BEP battery management systems... If you do some reading on them in sure you'll find what you need along with the appropriate diagrams... Can be $$ though 

 How much in the way of electrical gear will you be running?

Important part really is items that are run when engine is off such as fridge, deck lights etc.

For my boat (twin DF115) i have found that just the twin batteries works fine, using AGM 120ah deep cycle/start.

Because the AGM can take a much higher charge current i have the isolaters set up so I can run both outboards into either battery and put 35 amps in while it is low on charge.

Also a 120w solar panel but I have found the panel is rarely needed unless we dont move all day.

This runs a 50l Waeco, led deck lights, stereo, 150w inverter.

The AGMs can cope with the deeper discharge required, though my 115's have a lower start current than a single larger one.

Ive used this over numerous extended Abrolhos trips successfully.

 Check out the blue seas web site they have all the info/diagrams you need!

Have a look at the C-Tek Smartpass, seems to tick most boxes and easy to install .

 cool. thanks for the imput guys

would be looking at some extended trips. but hopefully having a solar panel it should sort out my charge worries.

Hey mate there's plenty of options and it depends how far you went to go but here's one option..

With a 3 battery/2bank setup I would have 1 dedicated start battery and parallel the other two deep cycles to make a higher capacity bank for your house services- sounder, plotter, stereo, fridge, washdowns, lighting etc. Seperate them with a 1/2/both battery switch. Your 240v c-Tek charger will mean your always starting any boating trip with fully charged batteries. 

Wire your start battery to 1 on the switch and house to 2 and when you go fishing leave it on 1 and your outboard will make sure starting battery is always getting topped up while your running. As its only ever starting your outboard its should never discharge much and you always have the option of combining the two banks via the switch if required. 

Redarc make a dc-dc 3 step charger and solar regulator in one unit (BCDC-25) that will look after your house bank and charge your deep cycle batteries to near 100%. It's essentially wired to your outboard alternator and also your solar panels. While your outboard is not running the BCDC draws current from your solar panels and charges your house bank which is isolated from your start battery. As soon as you start your outboard the BCDC switches from the solar panels and starts charging from the outboard alternator. The BCDC also takes the sometimes shitty alternator voltage and turns it into a voltage perfectly suited to charging deep cycle batteries to full capacity. 

I have the BCDC25 and a solar panel on my car to maintain my aux battery and it has run my fridge 24/7 for the last 18mths and hasn't missed a beat. 

Pretty sure there's a few auto sparkys on Fishwrecked that might be able to chime in with a better/cheaper option or correct if I've gotten anything wrong. 

I've been thinking about solar panels on the boat for a while. Would be awesome for extended Abrolhos, Rotto trips.

http://www.redarc.com.au/images/uploads/images/Redarc_BCDC1225_25A_in-vehicle_battery_charger_130408.pdf

goid luck