The lithium-versus-AGM question is really a question about the whole electrical system, not just the boxes in the battery compartment. Swapping chemistry changes what the charger, alternator and monitoring have to do.
Here is how the two compare on the points that actually change the specification.
Headline amp-hours are not comparable between chemistries. AGM banks are conventionally sized so that only a portion of nameplate capacity is used regularly, to protect cycle life. LiFePO4 tolerates much deeper routine discharge.
That difference is why a lithium bank of noticeably smaller nameplate capacity can deliver more usable energy per night at anchor.
LiFePO4 will accept charge current far more readily than AGM right up to the top of the charge. That is a benefit for short engine runs, but it puts a standard alternator under sustained heavy load it was never designed for, so alternator protection or regulation control is part of a proper lithium conversion.
Chargers, MPPT controllers and inverter/chargers all need profiles set for the chemistry, and BMS behaviour has to be integrated so a disconnect event does not damage other equipment.
Weight saving is real and matters on smaller craft and on performance boats. It is not a reason on its own for a heavy displacement motor yacht where the bank sits low and central.
Installation requirements differ too: securing, ventilation expectations, fusing and cable sizing all need reviewing rather than reused from the old bank.
AGM remains a sensible choice for engine start duty, for boats used mostly on shore power, for modest budgets where the rest of the system would otherwise need upgrading, and where deep cycling simply is not part of how the boat is used.
We do not recommend it. Charge profiles, alternator loading, fusing and BMS integration all need reviewing — a straight swap is where most of the problems we are called out to originate.
Only if the existing one cannot be set to a suitable profile. Many modern chargers can; older fixed-profile units generally cannot.
LiFePO4 is the chemistry used in marine applications precisely because of its stability characteristics, and a correctly specified installation with proper fusing, securing and BMS integration is a well-understood job.
It depends on how the boat is used. AGM costs less up front; lithium's case rests on usable capacity and cycle life, so it makes most sense for boats that regularly cycle the bank.
As a Fogstar approved and Victron installer we specify the whole system, not just the battery. Get a free assessment and a written quote.