I wonder if it would be easier to switch to say a part swappable battery, rather than try and make the whole thing swappable. Something smaller, but with a reasonable range, say 50 miles, might allow for a smaller ejection port on the car and possibly even light enough to handle by a person.
Batteries are still surprisingly heavy. I'm sure someone here has a more accurate lb/kWh figure, but my first gen Chevy volt can do about 35 miles on a charge and the batteries weigh several hundred pounds.
A trailer mounted auxiliary battery doesn't need to supply hundreds of amps of current.
It can supply energy slowly, perhaps at dozens-of-amps.
The only time when an EV gets into hundreds of amps territory is when it is doing real work: Work like accelerating, or going uphill (or both).
Maintaining a speed on a straight stretch of highway is not a huge burden in terms of work, and that is where I think that a trailer-mounted battery might shine brightest.
The car's internal battery can take the brunt of rapid acceleration tasks and get filled back up over time, just like it already does in normal charge-daily EV use.
So to put some of this into real terms:
20A at 355V is 7,100 Watts, or about 9.5 horsepower. That's a ton of real work.
100A at 355V is 35,500 Watts, or about 47 horsepower. Way more than plenty to keep a portly EV heading down the highway.
The connection for the trailer APU doesn't need to handle the peak current draw of hundreds of amps. It just needs to be able to (slowly) pick up some slack and thus provide greater range.
Like the BMW i3 with the gas-fired range extender: The EV drivetrain could consume a ton more peak power than the built-in genset could produce, and that's OK.
It's not like people are using these in endurance racing or something. Most real-world driving is pretty mundane, with only occasional instances of rapid acceleration.
