cool idea. Though IMHO sorta illogical to not use the alt. to kick the lithium up quickly and be done with it. Granted that is assuming one has a large enough one to do so already.
I agree, and will do so but haven't 'assembled' that functionality yet even though I have the parts already.
On a factory-built RV it may already do something like what you're talking about between the starter and house batteries. My RV has a large solenoid up front that engages anytime key is on to direct alt charging to house batt. When you turn the key off it disengages at which point those two batteries are isolated from each other unless you press the 'emergency start' switch in the rear. The same functionality would work for the lithium alt-charging connection. You can also use any cheap <$20 solar charge controller's load outputs to run a solenoid, possibly an existing solenoid, off your custom voltage setpoints. At least any SCC that has a screen and buttons..
It still leaves the issue of controlling current such that you don't run the alternator at max load into the lithium bank for long periods (especially at idle, when the rpm-based forced air cooling from the internal fan is weak). I have not tested the alt-charging side of this yet, but i am already feeding my 'shore power' batt chargers through an 80a PWM scc that has a configurable charge current limit (and it does work, i did verify that). I think IF you don't have an alternator that could smoke the 80a controller in the first instant (i hope that is my case), that it would then pwm the output down to <80a average and you could set it to a level that's comfortable for your alternator longevity, with the starter battery alleviating any serious side effects of the 'ripple' of PWM'ing a large current on the alternator output. The PWM scc doesn't need a minimum 'voltage delta' between input and output to function (like an mppt would), but ultimately your actual charge current would depend on that delta to some extent. I.e. i think you can get good flow into lifepo4 with .1v delta, but possibly not anywhere near the alternator's rating. So it still depends on your alternator holding the main 12v system at a voltage at least a couple tenths higher than your lithium is at.
The other gotcha to this idea that i already mentioned, is whether or not your lithium bank is chassis grounded in the vehicle. Mine is, for no good reason, that's just how i initially wired it. The problem there is if you have an SCC where all the positive terminals are internally bridged together, the SCC would have no control/authority because the chassis ground would connect the lead and lithium together, and the common positive in the SCC would connect them together, and the negative-side switching in the SCC would accomplish nothing. The solution would be either NOT have your lithium be chassis grounded, or chassis grounded through another relay only when not being charged . I'm having brain fog over all the possible reasons to have your lithium bank chassis grounded or not, so i'm just throwing those options out there.
My basic plan in my RV is to take the onboard 12v ~40a charger (which puts out 13.9v open circuited), in parallel with a couple of other small ~20a 120v chargers, and put it through my 80a SCC with current limiting, limit it to ~60ish amps, and set 'bulk/absorption' AND float voltage to 13.4. At that point my lithium will charge to 13.4 with no 'leakage' to the lead side until it gets close to 13.4, after which there will be slight 'maintenance/float' charging through diode into the lead acid @~12.7v, of a few amps. Low single digits from what i've seen so far. That's charging from AC. If nothing is pulling from the lead acid batts when parked (because inverter is hooked to the lithiums), that should be sufficient to keep them 'full' at 12.6 by at least fully offsetting whatever small drains there are (internal batt self-drain + some 12v interior lamps maybe), if not actually 'charging' them in the electro-chemical sense. Alternator would do 'actual lead acid charging' when engine was running.
On the DC side there is already a solenoid up front that hooks starter battery (thus alternator) to house battery when key is on. With slight rewiring of the existing cabling in the RV i can parallel lead acid house batt to starter batt permanently, and use that functionality to hook Alt charging to lithium. When key turns off the lithium and lead are disconnected.
The whole diode side of things is entirely optional here. It's just a workaround to 'stealing' the shore charger away from the lead but giving some passive 'floating' on shore power using a diode i already had anyway. The other option is skip the diode entirely and use the load port on the SCC in this scenario to ALSO trigger that same Lead to Lithium relay that turns on with ignition key, to also turn on when lithium his something like 13.4, and turn it off when it hits 13.3, or 13.5 to 13.4, etc. That would make the lithium charger be the 'shore charger' for the lead, but make sure the lithium is full first, and not allow the lithium to discharge into the lead in the event of a fault such as a lead acid batt gets a dead cell, short in the 12v side etc. Downside is, assuming my onboard charger HAD any 3-stage charging and temperature compensation at all, you'd be losing that and it would be 'dumb' lead acid charging unless you stuck another SCC configured for lead acid into THAT line. Part of the extenuating justification for my silly diode (if i even keep it) is the .7v drop means i can leave it 'dumb' with no downside other than it won't actually properly recharge the lead from a real discharge, just prevent it from discharging from small draws when parked.
So really the only thing needed to do this that wasn't already in my RV is the SCC. The wiring from front to back, and the solenoids, already exist.