Also, the lithium bank will outperform any LA battery for providing high current for long time. Your old Bukh would love it.
We really do need more information such as the brand, model and type of LFP batteries you are considering. Marine specific LFP batteries can easily start engines in which case you could keep the lead acid as a reserve or for starting and charge it with a DC to DC battery to battery charger such as a Sterling Power BB1230 from the LFP bank where all charging would be routed to. Many in drop-in
LFP batteries can't handle high cranking or other high load current, due to the mosfet based BMS switches current limitations so you would not want to crank the engine with this type of LFP battery.
The cranking limitation is not a limitation of the cells inside but rather the sealed internal BMS design that uses FET based switches. Most of these BMS's/switches can trip on over-current and a high BMS temp (over-heating caused by drawing or charging with too much current.)
In early September a transient customer who had a yard in RI install a bank of drop-in batteries lost all electrical power while docking and nearly took out a 7 figure boat in 20 knots of wind. No one had bothered to look at the BMS current handling capability of the drop-in LFP bank. When he hit his bow thruster it worked for a few seconds, then poof
, no 12V power. When he finally got tied up, the batteries eventually re-set, but his alternator was toast...
Marine specific or DIY LFP banks, that use actual contactors, such as Mastervolt or the Lithionics OPE-Li3 system can handle starting, windlass, thruster larger inverter etc.. loads with ease. The new Lithionics drop-in model 12V125A-G31-5CND-LRB can also handle cranking..
One drawback to an ACR is that it is bleeding LFP energy to float the LA battery. If you have excess energy not a big deal but if you don't then every little bit helps.
We also do not yet know where the forthcoming ABYC standard will land on paralleling Lead and LFP, so a DC to DC charger is not necessarily a bad idea, if your vessel is insured and you may need a compliant LFP system.