Alright, so I got some testing done...
Could not even get my inverter running on my old weak POS 12V car battery that I'm using from time to time in the electronics workshop. Scoped the +/- terminals while loading the inverter with my laptop charger, and it went down from 12 to like 7 volts and took bout 200ms to recover, so the inverter would just go in and out like crazy...
So, I guess the problem with the inverter noise while being used in the car, had to do with lack of capacitance on the 12V input terminals inside the inverter (a 1000uF 25V electrolyte was installed originally).
The inverter is connected to the cigarette lighter underneath the dash in my VW Beetle, and the battery is located underneath the backseat, so pretty long positive wire and multiple connections points in between. Right, so because of large inductanse on those long wires, why not just take one of my huge ass 6800uF 100V Nichicons and solder directly across the input terminals inside the inverter case?
While I was inside, I bit the bullet and connected a wire between the negative input (chassis GND) and the GND on the 230 "mains" AC output.
You know, just for the heck of it
Hooked the shit up in the Beetle and went for a ride. Wow! All the noise problems with regards to the inverter are completely gone!
I had like two falling out on the USB link while driving for 10 minutes because of noise from the ignition system, so that's still an issue of course, but then again, a completely different subject.
That being said; the ECU is mounted inside the engine compartment like 20-30cm away from the old fashioned distributor having the USB cable hangin' in the breeze from the ECU, alongside the car and in through the door
My experience is that when the ECU gets mounted inside the car behind the torpedo wall, interference from the ignition system is no longer an issue on the USB link. That would be the next step in other words.
Still gonna find out why the USB link drops out when the engine is being run in the test stand like seen in the video attached. Coms is via Bluetooth because USB keeps disconnecting like every ten seconds or so when running the engine like this:https://www.youtube.com/watch?v=BBI3_qtuoyU
I wonder if the reason that I get so much USb noise disconnect problems is because of the the lack of EMI shielding in terms of having a metal chassis built around the engine that absorbs a lot of the shit, and the fact that the laptop is isolated from the battery GND in the test stand. Because when I have done tests in the electronics workshop with coils/injectors firing like crazy, I don't get NEARLY as much noise problems on the USB when powering my "engine simulator" from my bench PSU. And when powering the "engine simulator" through my PSU, the laptop charger is eventually connected to the same mains GND as the PSU afaik.
Anyway, I'm glad i got to improve my crappy inverter