DiploStrat
Expedition Leader
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Having done a little more research on the setup, I now think the problem is at least in part due to the mismatch in main and aux batteries. Main is a 70ah lead acid, aux I have recently upgraded to a 110ah AGM.
What I think may be happening is that the alternator is thinking the battery system is fully charged, and not providing more power. Hence the aux battery is never bring fully recharged as it is only trickling charge at best.
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You could not have a bigger mismatch than I did, 150Ah of starter and 600Ah of camper battery. As long as the batteries are grossly of the same type, e.g. lead acid, and only combined when under charge, size doesn't matter.
You can have the my-starter-battery-is-charged-so-my-camper-battery-won't-charge, but it happens when the wire between the two batteries is too small. To be sure, as long as the voltage of one battery is higher than that of the other, it WILL charge the battery with the lower voltage; it will just do it very slowly.
Above and beyond questions of the National Luna malfunctioning, the next question is the the size/length of your wiring. It doesn't matter if you use a manual switch, a relay, an intelligent relay, or a battery to battery charger, if the wiring is too small, the performance will be poor.
You seem to be thrashing a bit, this may help: https://cookfb.files.wordpress.com/2016/09/dual-battery-slides-2016.pdf
Some of your comments indicate that you might benefit from reading this: https://cookfb.files.wordpress.com/2016/09/battery-charge-slides.pdf
These, and their companion pieces are on my website under "Documents." You may find them useful.