Since the OEM charging system apparently supports AGM, I wouldn’t put the OEM starting battery back in. But, it’s apparently unable to keep both of your Odyssey AGM’s properly charged.
I still think a DC-DC charger could help you, but I confess I’m having a hard time imagining how to install it so that it plays well with the charge controller that’s part of your Genesis system. The Cole Hersee charge controller with your system connects the two batteries together any time either of them are over 13.2 volts. It would be nice if it had an upper voltage limit that it would disconnect at as well, but it doesn’t. It does limit the amperage between the batteries to 15 amps though. How the DC-DC charger will behave when the two batteries are connected through the Genesis solenoid is what I can’t imagine.
I think it would be better if the two Odyssey’s were separated all the time, except when you need them for emergency starting. That way, a DC-DC charger could be installed between them, and it would be simply another load on the starting battery and it’s charging system. But then you would need to make sure all of your “engine stopped” accessory loads are connected to the second battery.
Renogy, Redarc, and CTEK all make DC-DC chargers you could check out. I have a Redarc 25 amp in my camper, and I know it’s sealed and rated for under hood installation. Renogy and Redarc both make a 40 amp model, not sure about CTEK. I would think a 25 amp charger would be sufficient for a group 25 AGM.
:):)
