LiFePO4 batteries will accept high charge current right up until they are nearly full, which is very different behaviour from lead-acid, which tapers off early and lets the alternator relax. Left unmanaged, that difference causes two separate failure modes on boats: alternators overheating from sustained full-load operation, and BMS disconnect events sending a voltage spike back through the charging system.
Neither is a reason to avoid engine charging with lithium, but both need designing around rather than discovered the hard way at sea.
A lead-acid bank's rising resistance as it approaches full charge naturally reduces the current the alternator has to supply, so most alternators rarely run at their rated output for more than a few minutes. A lithium bank keeps demanding near-maximum current for far longer because its internal resistance stays low almost to full charge.
Standard automotive-derived alternators are not designed for that duty cycle. Sustained high output overheats the windings and diodes, and thermal protection built into many units will cut output to protect itself, which looks like a fault but is the alternator correctly protecting itself from a battery bank that never lets it rest.
On a long passage under engine with a large lithium bank at low state of charge, alternator case and stud temperatures can climb well beyond what the unit sees in normal road-car use. A temperature sensor feeding into the regulation, either the alternator's own or an external unit, is the reliable way to protect it rather than relying on guesswork about run time.
Larger frame alternators, uprated cooling, and reduced duty cycle through external regulation are the practical responses, and which combination is right depends on bank size, typical engine hours and how deep the bank is usually run down before a passage.
An external regulator replaces the alternator's built-in regulation with a unit that can be set for a lithium-appropriate charge profile and, critically, can ramp current based on temperature and time rather than simply following the battery's demand. This protects the alternator without needing to undersize the charge current unnecessarily.
A DC-DC charger sitting between a smaller, standard alternator and the lithium bank is the other common approach, particularly where the existing alternator is not being replaced. It limits the current the alternator sees to a safe, steady figure and manages the charge profile independently, at the cost of a fixed and generally lower charge rate than a directly connected, properly regulated alternator would deliver.
Every lithium battery has a battery management system that will disconnect the bank instantly on a fault, high cell voltage, low temperature, or an internal issue. If that disconnect happens while the alternator is delivering full charge current with nowhere for the energy to go, the resulting voltage spike, a load dump, can destroy the alternator's diodes or damage other equipment on the same circuit.
A correctly specified system either uses a BMS with a pre-warning signal that ramps the alternator down before disconnecting, or fits alternator protection devices designed to absorb a load dump. Relying on the BMS to simply cut the circuit with no coordination to the charge source is a known cause of alternator failure on lithium conversions done without proper design.
Before converting an existing alternator-charged boat to lithium, the alternator's condition, frame size and duty cycle should be assessed against the new bank's expected charge acceptance, and a decision made on external regulation versus a DC-DC charger based on typical engine hours and how the boat is used. This is not a component swap, it is a charging system redesign, and it should be documented and commissioned as one.
Physically the boat will run, but without protection the alternator is at real risk of overheating or load dump damage, and we would not do this without at minimum a DC-DC charger or temperature-based limiting in place.
Any BMS-protected lithium battery can disconnect abruptly on a fault condition. The risk to the alternator comes from that disconnect happening with no coordination to the charge source, not from the battery chemistry itself.
For many cruising boats, yes, particularly where engine hours are moderate and a simpler retrofit is preferred over uprating the alternator.
It varies by unit, but sustained case temperatures well above what the manufacturer specifies for continuous duty are the point at which thermal protection or damage becomes likely, which is why sensor-based control is preferable to relying on run-time estimates.
Most, but not all, alternators can be externally regulated. Some internally regulated units need modification or replacement, so this is worth confirming before specifying a regulator.
We assess the existing alternator and design the charging path, regulation, DC-DC or protection, around your bank and engine hours.