How to Charge Boat Lithium Batteries from an Alternator

How to Charge Boat Lithium Batteries from an Alternator: steps

Understand why the direct connection fails
An empty LiFePO4 bank behaves electrically like a very hungry load. It accepts everything the alternator can produce and keeps asking until nearly full. A lead-acid bank's internal resistance rose as it charged, and that rising resistance is what protected alternators for decades.
Fit a DC-DC charger and set its ceiling
Take the smaller of a quarter of the bank's capacity and half the alternator's rating, then round down to a size you can buy. A 300 Ah bank behind a 90 A alternator gives min(75, 45) = 45, so a 40 A charger.
Check the alternator's real sustained output
Badge ratings are usually cold, high-rpm bench figures. An old, small-case or poorly ventilated alternator sustains less, and the charger should be sized against what it can actually hold all day.
Give the charger an ignition trigger
So it charges only with the engine running and cannot drain the cranking battery into the house bank at anchor.
Watch the alternator temperature the first few times
Run the engine with the bank well down and check how hot the alternator gets under sustained charge. This is the test that tells you whether your sizing was right on your boat rather than on paper.
Consider external regulation on large systems
Above about 120 A of alternator output, DC-DC chargers become expensive and need paralleling. An externally regulated, temperature-sensed alternator specified for lithium is the better answer at that scale.

Connecting the lithium bank straight to the existing alternator.

Always through a current-limiting DC-DC charger, sized as above.