Lithium system with B2B charger

[marioxp]

I have a similar problem, but a slightly different approach to the solution.
Two house 135Ah batteries are at the end of their life.

Now on Aliexpress you can get LiFePO4 280Ah for about 500 euros.

LiFePO4 280Ah has a much higher capacity and lifespan then acid batteries, because LiFePO4 can use 80% of capacity on acid batteries not more then 50%. The number of discharge cycles is several times higher then acid batteries.

The problem is that I can't properly charge LiFePO4 batteries with a Quick Charger and Alternator which are original on my B36 2003.

My idea is that the existing charging system charge one small acid battery (45-55Ah) which is connect with DC-DC charger https://www.renogy.com/12v-dc-to-dc-on-board-battery-charger/ which can properly charge LiFePO4 batteries. So DC-DC is constantly "overflows" current from a small acid battery into LiFePO4 batteries. Loads are connected to LiFePO4.

I estimate that the total cost would be around 800 euros, which is not small but much less than if I adjust the whole system to LiFePO4 or the similar two changes of acid batteries.

I would love to see your comments on this concept.
 

[Yngmar]

I split your reply into a separate thread as it's a quite different approach.

Lithium system with B2B charger can be okay, but the larger model is only 40A, so it depends on what your charging sources are. If you don't have anything bigger than that like an upgraded alternator, it can be fine, but it will take a while to charge your house bank that way, and B2B systems often do not charge at the full rate as they also have to consider the source battery and protect it from deep discharge and overcharging.

Your full 280Ah bank would take at least 7 hours to charge at a consistent 40A, which you're unlikely to achieve. If you only ever use a part of that bank, it can be totally fine though. So you need to look at your boats power use patterns and size of charging sources to make a decision.