3rd Battery in Dual Battery Setup

Glen
Connecting the two deep cycle batteries in parallel is the best way, the efficiency of the battery is improved by slower charge and discharge, by connecting in parellel you have achieved that improvment. The difference in capacity is of no importance because the charge rate is determined by the voltage and that is regulated so the current into each battery is as reqiured by the battery. Be carefull to disconnect both ends of the positive wire when removing the second battery. Eric.

Glenn, to run your fridge for 8 days from a 80W solar panel (via the battery) you better make sure the panel is in full sun every day or you might have to invest in a rather large deep cycle battery. It still would not save your coldies on several cloudy days in a row because once you deep drained the larger battery it takes even longer to top it back up from the panel.
I would take a backup, such as a 12V portable generator as well or be prepared to go for a drive.

Your unequal size aux batteries will not discharge equally, if connected in parallel.
If you use them individually, try to use only 15Ah from the smaller and maybe 40ah from the bigger, if you repeatedly drain them deeper you lessen their life somewhat and it'll take forever to get them charged back up from a solar panel.
Klaus

Glen,
because you wish to keep the 40amp hour deep cycle battery in the engine bay, I assume for your short trips, I would set up a larger 120a/h DC battery, and connect your existing 80wt solar panel direct to it, not the existing DC battery, keep the smaller DC battery charged by the Pirahna Isolator system.

You could have two inderpendant power and also recharging systems that can be switched over in an instant via a two way or quick disconnect battery switch.

Connect the new large DC battery in parallel to the small DC battery with a on/off or quick disconect battery switch between them, and have the larger DC battery charged via the solar panel - you can switch the smaller DC battery in/out of the system via the on/off switch or quick disconnect switch.

Note; wire the fridge to the solar panel lead, at the pos +terminal of the large DC battery.

You effectivly have two power systems giving you 160a/h TOTAL.

When you are driving, to recharge all 3 batteries via your Pirahna isolator have all batteries connected.

Do the maths and you will see you will be able to get this system to work for you for the 8 days you require with out going for a drive till at least the 6th day.

80wt solar puts out ~24a (day) @ only 5 hours sunshine per day, will get 6 unless you go to tassie or melbourne
Engel uses 25a to 35a (day) unless you are in the far north

As you state the 40a/h presently runs the Engel for 2 days, an EXTRA 120a/h will get you out of the vehicle and into holiday mode.

oh dear oh dear oh dear
Glenn suggest go and find someone to talk to that knows what they are talking about
those that have replied do not have a clue

Glenn has a Pirhana Dual Battery setup with a 40a/h DC battery in the engine bay, charged by 80W solar cell on the roof via solar reg.

Glenn's requirements:
It would be nice to always have the 40a/hr in the engine bay
He will be staying in the one place for 8 days
He won't be driving long enough daily to charge the DC battery
He will modify the wiring to connect/disconnect the 2nd DC battery

Glenn's QUESTION is...

IF Glenn ADD'S a larger deep cycle battery to his existing system

(1) CAN I hook it up along with the original 40A/h DC battery?
(2) Do they both need to be the same capacity?
(3) Can I just drop in the 100a/h when I need 140a/h capacity?

(1) yes
(2) no
(3) yes

there are VARIOUS methods of adding a second DC battery
some methods BETTER than others for DIFFERENT reasons
my method is kiss, inexpencive and works