In the DC-DC manual it suggests a 10 square mm cable gauge for a 5-10m run. As a first step I intend to charge the house batteries, 24V 240Ah, directly from the alternator of the truck (24V system) so I expect that the cable requirements will be the same as running it through the Orion DC-DC charger. Is this true?
I also plan to fit a SmartShunt and possibly a battery balancer. Which SmartShunt is the appropriate size, 300, 500, 1000? And, with a battery set-up of 4x 12V 120Ah AGMs, can I get by with using only one battery balancer?
Depending on the cable lenght it could be a problem when charging the batteries directly from the alternator. In this case the current is not limited and the voltage drop across long cables could lead in a high voltage drop.
That’s why I would recommend to use a DC/DC charger and then accordingly use the required cables.
Regarding the shunt, what is the highest current you expect to have (charge or discharge)? Running on AGM batteries I would asume that 300A is absolutely sufficient.
Regarding the battery balancer, - If you put 2 batteries in series and 2 parallel (2s2p) then 1 balancer should be fine. The parallel batteries will balance themselves.
Regarding the shunt, if I were to “future-proof” and consider fitting solar and maybe upgrading to Lithium batteries, would I need to go to the 500A version?
Well, 300A is still a lot, - just a rough calculation: 24V x 300A = 7,2kW
So are you planning to install more then 7kWp of solar power? Or more then 7kW of Load?
If yes, then you have to change the batteries anyway. The AGM will not be able to take so much power without any harm.