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The box of electronics you need is simpler, but it's harder for a layperson in use, as it requires being able to figure out how to wire it up. My solution can be made by an expert and used by an idiot. Your solution can really only be used by an expert.
If you don't like "fishing" to find the alternate legs, there is another way:
Really no need for an expensive/heavy/big transformer or AC->DC->AC even if you stick to "Parallel" 120V instead of finding the two hot legs to get 240V. If you know you got plugged into two different breakers, then you could just ask the vehicle to pull double amps at 120V. For instance, 30A@120V instead of the 15@240V like the QIC220 provides. With this "solution" you would want both outlets to have the same "leg orientation" because you could cause a short if you tried to join outlets that used the other "hot". It would need to detect that both plugs are using the same hot, then parallel them for 120V at double amps.
Anyways, at this point, nothing I am saying is an endorsement of any of this. There are safety issues to consider with shorting and grounding, and even trip hazards of running long extension cords around the house. Probably best to stick to commercially available products. I linked the QIC220... Are there any other existing "off the shelf" devices available to get 3kW+ from standard household outlets?
I think the QIC220 type device just makes a lot more sense from a cost/size/weight/complexity standpoint. I don't think the 'ease of use' argument negates that.
I am surprised that this topic turned into a debate of sorts!
By the way, over on the LEAF forums, a bunch of people have built their own QIC220 style devices.
People have various ways to tell when they found both "legs" to get 240V.
Mine has a voltmeter on it.
Some people put a neon light.
Some people even have a buzzer that makes a noise when you get 240V (then you flip a switch to turn it off.)
You don't need to be that technical to figure out how to use one.
Just plug in one cord, then start walking around the house trying other outlets until the box tells you that you are ready to go.
Perhaps a bigger issue with any of these solutions is that the 120V outlets tend to be on shared circuits.
So you could get your 3kW+ charging going, then someone turns on a microwave, air-condition, vacuum, or some other high current device and pops one of the breakers.