"Great set up. I'm wondering your battery selection, and I guess it all depends on what you run for gear. I am planning on a 31M for the aux and a 34m For the starter. I don't think the alternator can fully charge the 31M, so likely a dc-dc charger to toss in. I love the RedArc products I see. Just that if I'm doing dual batteries, i want one for quick starting, and another for the slow drain...Great video."
I just kept typing and here was my response. I thought this might help someone running into the same questions.
My selection of batteries was purely due to the battery trays and space available. I went with the slee off-road trays since they were very nice and I didn't want to fabricate. That said the start is a 31 and the aux is a 75/86. Ideally these would be the other way around since the 31 has twice the amp hours. I intend to eventually wire them the other way.
As for charging and dc-dc vs. a relay here is what I believe and trust me this could be debated by someone more knowledgeable than I.
Many people especially in Australia use dc-dc chargers for a couple reasons.
First, since many alternators on newer vehicles are "smart" they do not provide sufficient voltage to fully charge an AGM battery. The alternator on my land cruiser consistently puts out 14.1-14.3v which I thought would be pretty good, below 14v would not be so good.
Second big reason, even though having the higher voltage can get you fully charged or close to it, for the best battery life when frequently draining your batteries low you need to have a phased charge profile. DC-DC chargers generally have the phases required by a battery like an AGM but some only provide higher voltage.
Back when I planned the system the only DC-DC charger available in the US was the CTEK and it did not enable you to link the batteries in an emergency start situation or in high amp draw situations where having linked batteries can be helpful, like winching. Not sure how much I care since I don't have a winch yet but at the time it seemed important. I also noticed many people operate with Tmax, IBS, national luna, or blue sea setups that are all simple relays with no problem. I also considered the additional cost of the dc-dc chargers and some are a pretty significant investment. I also intend to add solar so this was part of the consideration.
I think if you are going to be on expedition with no shore power for many many days, frequently completely draining the battery, and potentially have a lower voltage alternator then a dc-dc charger is going to be preferred, or a must, but if your travel is more like a couple days here and there and your alternator is higher voltage (not smart) and you can always top off / condition your batteries with a good ac-dc charger and be fine. I am looking at adding an on-board Powermainia dual bank agm ac-dc charger so in campsites with power or when home I can plug in the cruiser and keep the batteries strong.
Here's the charger:
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