LMAO! 80/20 rule stipulates that when a breaker reaches 80% capacity, it will trip. You try to push 12A over a 15A circuit, and you can guarantee that POS electric car won't charge. If you have a dedicated 20A circuit, you might have a fighting chance, but most people will just assume, "Oh, the plug fits in this outlet, it must work."
A 12ga, 50 ft. 15A rated extension cord will get quite warm pushing 12A. They might get away with 8ga or 10ga 5-30, but that would require a dual-pole breaker on a dedicated circuit.
Gasoline is just so much easier.
If your breakers are tripping at 80% of their rating, something is wrong with them.