Without getting bogged down in the “Why?”, is it feasible to use a smaller battery, say 50Ah to top up a larger 125Ah battery with an Orion, without the smaller being connected to a charging source? I understand there will be inefficiency, but would there be any other problems?
And which Orions would be suitable?
I have an Orion-Tr 12/24-10 with its input terminals connected to the flooded starter battery in my van. When I turn the device on it produces 24 volts as its output. Those output terminals are connected to the input terminals (labeled PV) on my BlueSolar MPPT 75/15. The output terminals (labeled battery) are connected to my gel house battery. The starter battery has fewer Ah than the house battery so in that regard my setup is similar to what you propose, but I believe you will need a battery charger added between the 125 Ah battery and Orion device. Also, keep in mind that if you never charge the 50 Ah battery you will have spent on a lot of money on single-use equipment.
The smaller battery is portable enough to be carried to and fro, and charged at home. Just like taking a Jerry can of fuel for a larger fuel tank. Alternative is to lug a much more cumbersome generator (although the battery will need to go more frequently - not a problem since I’m there a lot of the time anyway). Plus I don’t really like running the gennie in the middle of a nature reserve just to top up a battery.
I looked at the updated Orion-Tr product line and I see the new (to me) Orion-Tr Smart DC-DC Charger (Isolated and Non-Isolated). I believe those devices are designed to meet your need. If you had one of those devices you could skip the MPPT I mentioned in my previous reply.
