question

Why would you not use an Orion DC-DC converter for this? e.g. Orion-Tr 48/12-9

So I have done some more reading up on the potential issues using the mppt regulator for dc-dc charging. I understand that there is a possibility of the regular shorting the input(?) to protect the battery from overcharging. This may be ok when using PV but not a battery as input.

I cannot find any 48-12 chargers but there are 48-12 Orion converters which seem to be recommended for my scenario.

So my follow up question is how this might be configured. If I got the 9A converter this would meet my needs. Does the device limit the current to 9A from the 48V input? I'm thinking I would set the charging voltage to be 13.8(?) for the AGM and manually switch off when/if the SOC reaches a predetermined level.

Does this sound right?

Hello, I have this setup and I use an Orion Orion-Tr 48/12-9.

Be aware that no power can be present when you switch it on (via remote)

See here: Orion-TR 12/12-9A as charger output doesnt goes up until batterie ist disconnected (several hundred pieces) - Victron Community (victronenergy.com)

To overcome this, I've used two big diodes in parallel so there is no voltage.

This setup is working perfectly since one year, I'm outputting something like 13,5V becaus I ave an Euro 6 and want to give spare to the alternator to charge when braking

We just did an install with a 48V lithium bank and a 12V AGM back. We used 2 of the Orion TR 48|12 30 DC-DC converters. They are "dumb" devices where 12V voltage is set with a potentiometer. The behavior we see is when they are set to 13.2V, as the 12V bank gets to about 12.9 volts they will start supplying amperage. They hold the 12V bank at about 12.6V. Here is the catch, they do not charge the 12V bank, they basically cover all the loads on the DC bus, and you see their output go up and down with the loads. If the loads ever exceed the output of the DC-DC converter, it draws that power from the 12V bank. Once the loads drop below the capability of the DC-DC output, the DC-DC converter will hold the 12V bank at that SOC. So they work very well for keeping a 12V bank from falling down, but our experience is they do not charge (unlike the smaller DC-DC Chargers that can actually charge from one bank to another). That has been our experience, and we are quite happy with it as that is what we wanted, the 48V bank is the fail safe to the 12V bank falling flat and we key generator starts off the SOC of the 48V bank.