The alternator shown can be the oem alternator or an additional alternator. I have left both dc-dc converters as well, so if this is the only alternator, you can simply keep the start batteries in float using the dc-dc converter.
You need an alternator that outputs the voltage of the LFP batteries. You also need an external regulator that has a charge profile for LFP chemistry as well as a temperature sensor on the alternator to prevent overheating the alternator. Balmar makes this, so I chose their images (and it’s what I have).
There is an ON/OFF battery switch to disconnect the alternator. This is used when working on the engine, to prevent a short (Rod Collins recommendation).
I show a fuse on the LFP busbar but I am not sure if there needs to be a second fuse at the alternator. Let me know if this is required and I’ll add it.
In real life, for the regulator B+ wire, I would use a fuse position in the fusebox that is connected to the busbar, but in the diagram it is all the way down, too far away from the relays for clarity.
Now comes the tricky part: you need to shut down alternator charging before a BMS disconnects the battery. If you don’t, the rectifier diodes in the alternators will burn out. The documentation of the alternator regulator will show how to have it shut down the field current, which stops the charging. In the case of Balmar, they recommend the brown ignition wire, but also allow using the red B+ wire for rare cases where it is crucial that the alternator stops charging, so I use the red B+ wire in the diagram.
For each BMS you now need to add a relay with a NC (normally connected) contact (the blue thingies near the BMS’s). The coil of the relay is energized by the warning signal from the BMS. The regulator B+ wire starts with a fuse at the busbar and from there has to go through the BMS controlled relays. Now you can choose between two options: I show the relays to be in series, which means that charging stops when any one BMS raises the warning. You can also put them in parallel, which means that charging continues, which will probably trigger a HVC of the battery that triggered the warning before. This doesn’t damage the diodes yet because there is a second battery still online, which didn’t warn yet. I find my shown method the preferred option because you need to try to prevent a HVC, which it does.
That’s it, not difficult but of course you need to run the ICE (internal combustion engine) to charge the battery.