Hello,
I’m installing a Voltage Sensitive Relay (VSR) between my AGM starter battery and my LifePO4 battery to top up the Lithium when on shore power. I have a 20A shore power charger for the AGM batteries which is configurable, and has a setting 14.4/13.5 which appears safe for the Lithium. So the AGM and LFP goes in parallel when on shore power to top up the LFP, and then again when off grid, at night, so the anchor lighting which is connected to the AGM, effectively runs off of the LFP if that makes sense. The LFP is charged by a 75|15 MPPT when off grid.
So I have to be careful not to have the batteries in parallel when I start the engine, as I have a 150A alternator, and the LFP might create such a draw as to burn it up. I have no idea how much the LFP would pull and don’t see any safe way to test it and find out. I’ve been advised to install a Normally closed relay, which opens when the ignition is turned to shut off the VSR.
Question is- does some kind of device exist that I can put on the VSR positive wire that will limit the amperage to something safe, say 30-50A? That way I could also charge the LFP when the engine runs and not have to worry about the alternator.
BTW, I’m waiting to buy a B2B that supports two way charging, and hope Victron’s XS eventually comes out with the functionality, but I’m not holding my breath. Renogy has one but I prefer Victron’s quality reputation. Anyway I’m going DIY for now as the boat needs a LOT of work done 
Thanks in advance!