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Different charging sources

I have a Victron MPPT 75/15 with a 100W solar panel and 66Ah (2x33 actually) lithium battery. All works fine. I'm about to install this in an adventure-style trailer (based on small utility trailer with tent on roof of trailer). I'm driving a Golf Bluemotion w/ start-stop (i.e., bigger alternator, but voltage all over the place). I'm looking to be a bit sustainable and would appreciate any feedback on a couple ideas:

1. Power from the car should run the fridge and give some charge while driving. Power via the trailer wiring harness is 2.5mm2 (14 ga) and I'd like to limit current to say 4A (~50W). For sure there are DC-DC chargers out there that will manage this for a couple hundred $ or € or £ (Victron indeed makes a nice one), but my needs (current speaking) are so limited. On ebay, there are dozens of CC/CV boost converter starting at $5 if only wanting 2A. For $20, easily up into the max 6A in / max 100W out range (and they're adjustable, so I can choose the amps I'm comfortable with). I don't see these being discussed much in these circles... is it just because their power is too limited for most people?

2. While the power from the car (above) would keep things running + charging while I'm on the road, I would like the option to charge from AC at a campsite / in garage. I've recycled an old 250W desktop computer power supply (PS) into a 12v (18A) DC power supply. Again on ebay (etc.), there are cheap 12-24v 10A step up converters. This seems like a perfect fit for me. Am I missing something fatal?

3. While a CC/CV boost converter can directly charge lithium batteries without a charge controller (if I had lead acid, I'd worry less about cost of screwing up), is there a reason why both the power from the car (3A@20v) and the power from the PS (10A@24v) couldn't both go through the MPPT 75/15. All within MPPT ratings, and I just need to give some thought as to whether I need a 3-way switch (to ensure only 1 charge source is ever connected) or use diodes to prevent current randomly going backwards into places it shouldn't (e.g., don't want to have my solar panel pushing current back into the car and screwing up electronics) Am I missing something fatal? If I didn't have a 3-way switch and had the craziest situation where I had my 100W solar panel out (say 5A@18v), car running, and trailer plugged in for AC charging, this is effectively like the "can I hook up 3 unmatched solar panels in parallel" discussion, no? (I'll read more into this one, but I think 16-20v from solar panel and 10A@24v from the AC charging is too far off? i.e., equivalent to very unmatched solar panels...)

I know a few questions. Thanks for any input!

MPPT - Solar Charge Controllercharging battery
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