I want to have a A+B switch to get power from bouth battery banks for engine start in emergency situastions. How would the Orion TR Smart DC/DC charger handle this situasion, when bouth terminals in fact will be connected to the same (and only) bank a little while?
Is the Orion used to charge the second battery?
I would say an emergency switch is fine, since starting the vehicle will happen before the Orion starts up in any case. Switch to combine, start the vehicle, switch back before the Orion completes its startup process - and more to the point, before your alternator ramps up and tries to start charging all the batteries at the same time and possibly overloads.
Remembering to switch back after the engine is started will be the primary concern.
