I attach basic diagram to clarify my idea. The extra diode should protect solar panels from revers current.
This connectivity allows to eliminate the extra expensive DC-DC charger. Any criticism is very welcome!
MPPT controllers are specifically designed to be hooked to a PV panel as the charging source. They vary the input voltage from the PV panel until the find the voltage that provides the best charge rate.
I don't think it would be a good idea to hook up the alternator to the MPPT -- its specifically designed for PV panels.
If you want the charge voltage from the alternator to be changed to be optimal for charging your Li battery, then a Battery-To-Battery charger would be the best way to go.
There are some B-to-B chargers that also have a solar controller built into the same hardware.
I've read about people doing this but using a DC-DC boost converter to supply the charge controller with the proper voltage. One person said it worked fine until their cheap DC-DC converter burned out after a week.
So you could do it but you would have to buy a quality DC-DC converter and at that point why not buy something engineered to do exactly what you want to do.
Renogy makes an MPPT/DC to DC charger that people seem to like.
Highly capable, cost effective & reliable solar charger controller. May be used as a DC-DC vehicle charger.
It doesn't boost the voltage like a true battery to battery charger does, but it does allow you to "plan" your wire size to 10 awg so that it doesn't burn up vs completely non controlled implementations.
I went fairly simple. Cyrix-CT 230A. I put a switch on the ground wire so I can control charging. About 20' of 2AWG wiring goes from the start battery to two 100ah LifeBlue LFP batteries.
I put a Hall Effect sensor on the alternator wire which allows me to see output. The long run of the wire reduces the 14.3 of the alternator to around 13.6 at the house batteries. I got lucky there....perfect V to charge the LFP. It's been interesting watching alternator output as different loads are turned on....lights, brake lights, radiator fans, etc.
With the switch I pick when I charge the batteries and I don't take them to 100 % soc. It isn't the hands off approach to vehicle charging but it works for me and it means no b2b box.
A MPPT charger varies its input resistance to match the output resistance of the power source for maximum power transfer. I'd be worried about the alternator voltage regulator and the MPPT getting into a cat fight - the MPPT varies it's resistance, and thus the load on the alternator, the alternator's regulator adjusts for the altered load, the MPPT senses the changed source parameters and adjusts, lather, rinse, repeat.
The other thing is that I would NOT suggest using plain old diodes for the separation. At 0.7V drop across the diode, 100A is going to burn off 70W of power in the diode, so it will need active cooling. I would suggest having a relay that switched the panels out when the engine is running and switches the alternator in.