I am switching to lithium on my boat with the alternator charging the starter battery and Orion Tr 12/12 charging the lithium (300ah). But I can’t decide weather to go with the 30A or 18A option in order not to overheat the alternator. My alternator is Hitachi LR155-20 55Amps. What is your recommendation?
I won't make any recommendations due to not enough information regarding your complete electrical layout. I will however give you a few details about my 12|12 18 that I installed in my camper and ask a few questions.
My 12|12 18 DC2DC draws about 22-25 amps when fully outputting 18 amps. I would think that the 12|12 30 would do approximately the same, which would be approximately 34-38 amps.
The first question is whether your electrical wiring is adequate to support that current draw for a sustained time. You should research DC current requirements, concentrating on total cable length (both positive and negative wires) of the required current of 34a-38a as mentioned above, and then add in the environment that these cables are run though the boat. Temperature and type of enclosure for cabling does play a role in sizing.
The other consideration is the alternator itself. A the 12|12 30 draw of approximately 34a-38a, your probably going to be working that alternator more than 50% of its maximum output just to drive the DC2DC. Most alternators are sized on current draw of the existing system load with 15%-20% reserve to prevent damage due to overheating and not much more.
What about peripheral loads that already exist on the boat, have you done any kind of audit of the boat's power usage to get an idea of where your are with the alternator's output before adding the DC2DC? I suggest that you determine your existing load requirement then add what a DC2DC will consume and make your decision based on those details.
