The alternator is a charger whether you named it that
While the engine runs, the alternator is what refills the batteries. Lead-acid packs accept that current and then sit on a float. Battery University’s LiFePO4 charging article says lithium does not want that float: charge current should stop at full, around 14.6 volts on a four-cell 12-volt pack. A small outboard aimed at a large empty lithium trolling bank can stay in a high-current region longer than the alternator was designed for. That is a system problem, not a module defect.
Why “the BMS will handle it” is incomplete
A BMS can disconnect on overvoltage or overcurrent. Disconnecting under charge dumps the load on the regulator in a way the engine maker never drew. If you already have a DC-DC charger or a battery-to-battery unit, we work with that hardware. If you do not, the path from alternator to lithium still has to be designed before the bow is full of light modules.
House lithium next to a lead starter
Many cruisers keep a flooded or AGM starting battery and add LiFePO4 for the house. The isolator or combiner then decides whether the alternator sees both chemistries. Wikipedia’s battery-isolator article describes one-way devices that charge two batteries without letting a dead house pack flatten the starter. Diode isolators add voltage drop. That drop matters more when the lithium voltage window is tight.
When the engine is not the charger
Lift-kept boats in Cape Coral often refill from a shore-power charger, not from idling in a canal. In that case the alternator conversation shrinks and the pedestal program grows. Trailer bass boats that run the big motor to the first hole still charge the bow through the engine. Those are different visits.
What we ask
Engine size, whether a DC-DC or isolator already exists, and the lithium maker’s series and charge limits. We will not tell you an alternator is “fine” because the tray accepted the case. We will tell you if the charging path is still lead-acid float.
Lead starter, lithium house
The common cruiser layout keeps a flooded or AGM starter and adds LFP house. The isolator or combiner then has to charge both without tying them at rest. We look at that hardware before we bless the lithium. If the only path is a 1-2-BOTH switch left on BOTH, the chemistries are paralleled and the alternator conversation got harder, not easier.
Sources
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.