My 2 cents. My old boat has dual Lithionics 12v x 560AH batteries with external BMS. I like the lithionics brand, latest batteries are UL listed and IP rated. 1120AH or 14kwh was enough to run a 9k BTU AC all night and I had 14kwh of Solar whch was enough to put 600aH back in the following day.
I liked having 2 batteries. One battery gets knocked off by its BMS then there is one more. My engine battery was an AGM charged from the lithiums with a mastervolt smart DC-DC charger. There was a house/engine parallel switch which would start the engine from my lithiums BUT would cook the AGM if left on for more than a couple of minutes so parallel then, start the engine, turn it off. I had dual 3kw Victron multiplus running everything on the boat from the inverters.
What did I learn from my setup
- Seperate chargers and inverters are a good idea. Lets you run the boat in different AC regimes easily. The Victron charger/inverters will flow through shore power at its voltage and frequency so, seperate charger/inverter means the boat AC system is independent of shore power.
- Seperate BMS. What good is independence for each battery. Whats bad is if the boat floods then the battery DC cables are outside the battery going to the BMS and the BMS cannot deal with that situation, the salt water will short the batteries. An internal BMS avoids this problem and protects the battery in this scenario. Pump out the water and the batteries should be fine once the "external" short is removed.
- The location of the batteries on my 55 is right at the bottom of the stairs which is a crazy place to put them. A wave comes in, floods the cockpit, overflows in to the boat and now you've a problem. The Amel 53s having the batteries under the captain berth seems much smarter to me.
- Charging the batteries from the alternator is where most of the complexity comes in. When I do my Amel, I'll charge the batteries from the shore power/gen and Solar and not use the engine alternator for the lithiums. I would charge the engine AGM from the alternator but nothing else. Now, it's much easier. The Sterling alternator protector was rated Maine Yacht Center as voodoo and I trust them. The alternator is complex. I had a 360aH alternator from API with a Balmar 614 regulator. It generates 5kw of power ish. That comes with a lot of heat and you'll need a well engineering cooling system. I had to derate to 230a to keeps temps to 80c at the alternator, plus there is the wear and tear on the engine belt to worry about. Just don't use the alternator, MUCH easier...
So, when I do the Amel, my plan is 600aH of 24v Lithiums with internal BMS. Maybe 6 strings of Battleborn 24v x 100aH. Reuse my mastervolt chargers (100A and 60A) from the generator and my solar. Keep an engine AGM battery connected to the alternator and a DC-DC smart charger from the lithiums. At least thats the plan but I would not integrate the alternator in to my setup like before and I still don't like the location of the batteries at the bottom of the stair well but at least with the battleborns, the BMS should help in a flood situation. It would be nice if the battleborns were IPX rated, I'm not sure if they are waterproof and I'd like for ones which were.