On my boat, I currently have an agm start battery and 300ah agm house battery bank. My alternator is 80 amps and I have a 50 amp ProMariner shore power charger. My plan is to keep the agm start battery and replace the agm house batteries with a 400ah lithium iron phosphate battery. The agm start battery will be charged by the shore power charger when at the dock and charged by the alternator when the engine is running. I plan to charge the LFP battery using a Victron Orion XS 12/12 50a charger connected to the agm start battery. A question I have is, will the Victron dc to dc charger be able to charge the LFP battery from the agm battery while the agm is being charged and floated by the ProMariner shore power charger, and if not, what might work in this setup to best charge the LFP battery while at the dock.
The Orion will work well in this scenario, many people use your proposed setup. You will need to setup the engine shutdown so that it charges at something like 13.2V or higher on the start battery which proves that the start battery is receiving charge from either the alternator or ac charger.
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