I recently installed an Orion TR 12/12 30 in my Truck camper. Previously the leisure battery was charged straight from my starting battery with a Blue Sea ACR in between, it obviously didn’t completely charge the leisure battery but now that that battery has been upgraded to a 300ah lifepo4, the DC-DC is required. I also have a SmartSolar MPPT 75|15, single hard mounted 100w panel and a portable 120w panel. When it was wired the old way, one side benefit was that the solar panel would top off the starting battery as the batteries were wired together. Nice insurance policy when I go hiking for weeks at a time, have a small starting battery drain, or leave a dome light on. Now this new system doesn’t charge the starting battery the way it is wired so I am wondering the following:
Can I feed the output from the solar controller INTO the input of the DC-DC charger? This would basically hook the solar charger to my starting battery charging it AND into the DC-DC charger, charging my leisure battery.
You definitely can wire the solar charger to the starter battery and have the Orion connected between your starter battery and your leisure battery, many people do this. If you have a read of the engine shutdown detection section of the Orion manual you will find that you can set it to start charging when the starter is at say 13.5V, so above the full starter battery voltage and have a shutdown voltage set at say 13.2V. The neat thing is say you only have 6Amp coming in from your solar, then the Orion will pass 30A into the leisure battery, but as the starter battery falls to 13.2V, the Orion will regulate down until it holds the 13.2V, maybe you will then have 1A tail current into the starter battery and 5A going forward to the leisure battery.
Is there any diagrams on how to do this please?
If you download the manuals for a Victron MPPT and an Orion DC to DC charger from VictronEnergy.com then you will find install schematics for both devices. The Orion one will show how to connect it between an alternator and start battery and a house battery. The MPPT will show how to connect it to a battery, which in this case is the starter battery.
Wow thank you very much. This is exactly the information I was looking for. I’ll dig into the specifics of where to set each component for optimum efficiency.
Hi. I’m doing a similar thing on a motorhome but can I wires it the other way, so that the MPPT charges the leisure batteries and the Orion trickles the starter battery? Would the alternator affect it this way around? Does a mains charger have any effect on any of this? Thanks. Mark.
You could have MPPT on the leisure battery and the input of the Orion from the leisure battery and output to the starter battery and solar will keep the starter topped up. If the alternator is connected to the starter battery then the Orion will not charge the leisure battery. If you have a mains charger connected to the leisure battery then that will also activate the Orion to charge the starter battery.
That’s perfect. Thank you.
In my installation, I solved the charging (maintenance) of the starter battery from the solar panel in the following way: I used the “LOAD” output on the MPPT 100/20, which I turn on in the range from 13.42 V to 13.6V (user-defined algorithm 2: OFF<13.42V<ON<13.60V<OFF) and inserted a diode between the on-board and starter batteries that can handle a current of 30A. The diode ensures a voltage drop (0.7V) and the direction of the current only from the LiFePo4 battery towards the starter battery. My starter battery is maintained at a voltage of 12.9V, with the current flowing from the LiFePo4 to the lead starter battery reaching a maximum current of around 1.5A. I can remotely turn the LOAD output on, off or configure it in various ways. In addition, I have also inserted a switch that manually interrupts the path “Lifepo4 - diode - Starter battery”. You can also include a 12V 21W car bulb as standard as protection in the circuit, which will serve as protection against high current (the higher the current flowing through the bulb, the more the filament heats up and the more its resistance increases - a very simple and historically proven self-regulation) 
Maybe this idea of mine will help someone or inspire an even better solution.