Dhays, it depends a bit on your use habits. When we are using the boat, it is typically a night at anchor followed by a 4 - 8 hour engine run. Occasionally two nights at anchor, very rarely 3. Occasionally a night in a marina but we never plug in, treat it as another night at anchor from the DC power point of view.
In that usage scenario you want as much charge as you can get from the alternator going to the house bank. The engine start battery will never get below 99% SOC unless you have a starting problem, or frequently hold your finger on the thruster button for many minutes at a time. Starting a QSB should take about 1 AH, 10 thruster applications of 2 seconds each to dock is perhaps 3 AH. (More than that argues for some lessons in docking
). The only real load on the engine start battery is the engine electronics and alternator field current, something like 15A on my boat. Putting a 50A limit on current from the alternator to the house bank is a serious limitation for me.
Against this is the need to provide a place to dump the alternator dynamic energy in the rare event of an LFP disconnect. This is why you still want constantly connected AGMs in the mix.
I think the way I am going to approach this is to run the alternator output through a Victron Argofet isolator, feeding the house and start batteries. Very little current will go to the start side, enough to replenish the 1 AH I used to start, and keep the 15A load fed (unless for some reason the start bank is run down). Most of the current will go to the LFP house bank. In the event of a BMS cutoff, the energy can be absorbed by the AGM start bank for the few hundred millisecond transient.
There are two problems with this scheme for me. One is that the voltage regulation has to be set for LFP, this is less than ideal for AGM but really not that bad. It may result in a slightly shorter life for them but probably no real difference. Second problem is, my current setup has a BlueSea relay connecting the house and start during alternator charging. This works poorly for a few technical reasons, but having the two banks connected allows the house battery to share the load when the thruster is used, so that the engine does not see much of a voltage sag. Not a problem with a really huge start bank, but I have only two G24, perfectly adequate to start the engine but can sag a bit under the 500A load of the thruster. With the two banks isolated by the Argofet, the house cannot contribute to the thruster.
I think my solution is going to be, change the function of the BlueSea relay to simply be on when the ignition is on, shorting the outputs of the Argofet together and cross connecting the battery banks anytime the engine (and alternator) is running. This recovers the load sharing, eliminates the (very small) voltage drop across the Argofet and is very simple to implement. In fact the relay is wired that way now, due to the voltage settings of the relay, which do not work with Lifeline AGMs.
Why not just use the relay and forget the Argofet? That accomplishes the same things, in that the alternator load can be dumped to the AGMs whenever the alternator is running. But it assumes the relay always does what it is supposed to. It is probably as reliable as the Argofet, but together there is a redundant failsafe path (and also a new point of failure, the Argofet).
Conceptually, you could connect the alternator to the LFP house, and use a DC-DC to charge the engine. This would require only a small DC-DC as the load is small. My sailboat is set up exactly this way (all AGM but house is 24V and engine is 12V). But it depends on the transient response of the DC-DC compared to that of the alternator regulator, to protect against a BMS disconnect, and that may well be inadequate, no manufacturer seems to publish it.
For you do to what I plan to, you'd need an Argofet rather than the Orion, but you would also have to change the alternator regulator to external, because it would be required to regulate for LFP, the Orion is not there to do that function.
If your use is anything like mine, I think it would be far more satisfactory. The fast charge capability of the LFP is useless, without a fast charge source.
