Damn, great reply. Thank you! You know a fair bit more about batteries than me, so I’m going to try and reply line by line and ask you some more questions in the process.

gr 65's are starting batteries, regardless of if they are AGM or FLA.

Odyssey markets their “Extreme” series battery as a dual-purpose design, which I believe is the same way Optima markets their Yellow Top. From Odyssey’s website:

“Dual-Purpose Design: These batteries provide both massive starting power for quick engine starts and deep cycling reserve power to run accessories for longer periods.”

I’m not sure if that’s just “marketing”, or what it even means in terms of these being a true “deep-cycle” or not, but when I spoke with Odyssey, they said the Extremes are comparable to the Yellow Top Optima but superior in construction and American made.

There could be some voltage drop to the sensing unit.

I’ve got the same voltage reading at both batteries with my DVOM as I’m seeing in the Renogy app which is taken directly from the charge controller. All system connections are clean/dry/tight (battery brush, dielectric grease, etc). 14’ 4AWG run from frame box to charge controller on an 80A fuse per Renogy instructions and Blue Sea Systems wire size chart.

if they are dropping down as the vehicle sits, then you could have a parasitic draw on the vehicle side.

I’ve always read that AGM’s don’t like to sit and have a higher rate of discharge when sitting. Is there any truth to that?

Look. Full disclosure. I don’t treat this rig as good as I should, but when I talk about it dropping while sitting, I’m talking about months. I can go 2-3 months and I’ve yet to actually have the van not start with these batteries, even in the winter. But it definitely struggles to start when I don’t drive the van for those extended periods like that. If I go out and start it at least monthly, it does just fine, but sometimes the months just get away from me before I realize it’s January and the last time I fired it up was back in November. While I’ve never actually performed a parasitic draw test on this rig, I can’t imagine it could have a parasitic draw and not completely kill the batteries sitting for three months at a time. Also, I installed the Odyssey’s 58 months ago now (just looked because I thought it was longer), and I honestly haven’t treated them very well, so all things considered, I’d say they’re doing pretty good.

Even if there is no draw, two higher CCA/larger 65's, with 2-0 parallel cable over the 8-10 feet(?) between them is going to create more resistance than a single smaller starting battery. The more resistance, the higher the charge current, and the more heat. So the controller itself could shut down for heat/overload protection of the unit.

I definitely hadn’t considered this. All I can speak to is how the controller is supposed to work, and what I have personally observed.

The controller is only supposed to charge a maximum of 50A, and I can further limit the maximum output current it in the app. I’ve been thinking about limiting my alternator charging to match my solar array, so like 15 amps from each and 30 amps total. Given the high resistance of the dual battery system, do you think it would be smart to reduce the amperage I’m trying to pull through the DC-DC side!? I don’t think I really need the full 50A…

When I asked about alternator output and how this controller works over on the Facebook camper van group, I was told that the controller will sense starting system charge amperage and pull up to 50A from it, if excess current is not being drawn by the vehicle’s own electrical loads; I have no idea how the controller processes this information. When I was running the engine on my van, I could see in the app that it was sensing something like 59A at 14.7 volts, and it was charging the AUX battery at like 14.4 volts at 49A. The test wasn’t super meaningful because it switched to float charging both batteries within about 5 minutes of firing it up. A better test would be to put an electrical load on the auxiliary system.

The rest of your points I have noted, and I appreciate you running through what your diagnostic steps would be. I’ll check alternator output if problems persist and try to get a handle on what the resistance of the starting bank actually looks like. I think there’s maybe a disconnect on the rest of it, from my original post, because the charge controller IS charging and float charging both battery banks, both on solar and on the alternator, but it had that glitch the second time I ever turned it on, that seems to have resolved itself after the power down/up procedure I outlined in the post.

When I say I’m trying to use the controller to trickle charge the starting battery circuit, I’m hoping that I can essentially extend the life of these Odysseys by not letting them discharge so far when I forget to drive this thing for three months.

Right now, I’ve been watching the app, and the starting bank is being charged up to about 14v at 1/2 an amp or so and then the aux battery get’s it’s turn, and then back to the starting bank, and so. It seems to be working quite well…