I'm been powering an off-grid house with LFP (one of the several different li-ion chemistries) for just about a year now. I love them, and would never use lead again for any sort of power system, including on a boat. The Nordhavn 68 that I'm building will have LFP as well, along with about a half dozen other Nordhavn's in recent production.
The most important thing to understand is that there are different types of Li batteries and they are VERY different from a safety perspective. They are different in other ways too, but the safety difference is the most important on for a boat. Everyone has heard about the 787 battery issue, Samsung phones, and various other Li battery fires. In those applications, including in cars, packing as much power as possible into the smallest space, and lightest weight is what it's all about. As a result, they all use so-called NMC or LFC chemistries. These are much higher power density, but also much more volatile.
LFP is a lower power density, and much more stable chemistry, and the only thing I would consider for use on a boat. There are numerous studies, tests, and videos of LFP batteries having nails drive through them, dead short circuits, flame throwers, etc. LFP batteries just smoulder and off-gas, where NMC and LFC batteries catch fire and take off in a cascading fire until there is nothing left. Which would you want on your boat? LFP is very stable, and I think actually less dangerous than lead.
But the reason I love them is because they just work much, much better than lead. They cost more, the premium is reducing quickly. My boat system, which will be a manufactured, fully engineered, packaged system, is only about 50% more $$ than equivalent usable capacity AGMs.
Why I like them:
- They don't off-gas one of the most explosive gasses on the planet.
- They aren't filled with one of the most highly corrosive acids on the planet.
They aren't filled with lead which is one of the most toxic materials on the planet.
- They don't heat up when used or when charged, assuming use comparable to how you would use a lead battery.
- They accept full charge current right up until they are full. There is no more long drawn out absorption stage that drags out charging time, and generator run time. Then end result is that you can recharge is about 75% of the time it takes to charge and equivalent lead battery, using the same charger(s). And they can typically accept higher charge currents, so can charge even faster if you want. With lead, increasing charge current produces significantly diminishing returns after a certain point. With LFP, a full recharge from empty to full can very reasonably be done in 2-3 hrs.
- They are much more efficient than lead. 98-99% vs 80-85%. So what goes in, come back out. This is particularly helpful if you are also using solar since you can capture and use that much more of the produced power.
- You can run them indefinitely at any state of charge. No more worries about draining them all the way down, or not fully recharging frequently enough or full enough.
But of course there are some down sides.
All batteries will be ruined if you over-discharge them, or over charge them. But with LFP, death is a bit more abrupt and certain. So where with lead you might be able to get away with a little slip up or two, chances are you would completely kill your LFP battery.
As a result, nearly all LFP batteries have protections built in to disconnect them just short of disaster. This is akin to a low voltage cutout that is incorporated in nearly every inverter to keep it from unintentionally flattening your batteries. In every way, the same protections would benefit lead batteries, but we don't use them. Largely because of the higher cost of LFP, they are common place.
Similarly, you need to be mindful of over charging, so need to be sure all your chargers are set so they will not over charge the batteries. It's typically not a big deal to do this, but you can't just blow it off or it could be a costly mistake.
But all this is good practice anyway, and shouldn't lead you to believe LFP is in any way fragile. It's just the opposite. Keep it within well understood limits, and they just work, day in and day out, with no fuss. Honestly, much LESS fuss than lead because you dopn't have to worry about leaving them at a low level for too long, or getting them fully recharged frequently enough. You can literally ignore them.
So to the OP's question, I don't believe anyone knows yet how long LFP will actually last in a boat application. There are lots of tests demonstrating 2000 cycles, but anything more than that is just extrapolation at this point. But keep in mind that 2000 cycles is probably 10 years on a boat, and that would be for a pretty heavily used boat that is going through a full battery discharge/charge cycle 200 days per year. I'll bet few if any of us come even close to that.
For the particular boat you are looking at, the first and most important thing I would check is what chemistry the batteries are. If they are LFP, I would just happily use them. If they are something else, I would probably plan on replacing them, just for safety reasons.
