I think the portable charger draws the same amperage whether on 110v or 220v (13A). Obviously, at 220V you are drawing twice the KW at the same current and your car can charge twice as fast. It seems logical that the portable charger can work at 220v or 110v so that it can be used around the world, but I would be concerned about the cord between the wall plug and the charger, since the American version only needs to handle 13A at 110V and if you draw the same 13A at 220V, there is a risk that the American cord could overheat or even start a fire. The European charger is the same box but the cord connecting the box to the wall socket seems beefier. ...

Although I have not opened up the "convenience charger" to look inside, it's pretty obvious what's in there: a small board that monitors the power connection and sends the negotiation signals to the J1772 plug (the big round one that goes into the car), and a relay that connects / disconnects the wall-socket-side power line to the car-side power line. (The J1772 connector, which has more than just the power and ground pins, is basically your "power and ground" pins plus those negotiation signals.)

Here's what's slightly wrong in the quoted bit above: the amount of current a wire can carry without overheating is the same

*regardless of the voltage*. Power dissipation in an ideal resistor [1] is simply "I-squared-R": the square of the current (13 amps) times the resistance (ideally, much less than one ohm). That means the power dissipation in the wires is less than 13-squared (169 amps-squared) times, say, 0.05 ohms, giving something under 9 watts. That dissipated power turns straight into heat. [2]

The power delivered to the device (car, in this case) is simply voltage-times-current. Let's say current is always exactly 13 amps. [3] The more you raise the voltage, the more power is delivered to the car. The current is always 13 amps, and the resistance "R" value is the always the same (let's keep using that 0.05 number), so at 1 volt, you deliver about 13 watts [4] and waste (as resistive heating) something under 9 watts. At 10 volts, you deliver 130 and waste 9. At 100, you deliver 1300 and waste 9. At 120, you deliver 1560 and waste 9. At 240, you deliver 3120 and waste 9. See where this is going?

It would make all kinds of sense to use, say, 100,000 volts, which would deliver 1.3 megawatts and waste that very same 9 watts. Except for one big problem: the higher you make the voltage, the more eager the electricity is to "squirt out" across things that look like insulators: rubber wire covers, air, and so on. (And a smaller but still significant problem, the Karma will only accept power at 3.3 kW, same as the original Leaf and where a lot of the early Level 2 chargers maxed out. Some EVs now take 6.6 kW, and the Tesla S will take up to 90 kW.)

Still, the key take-away here is that

*power ***loss** rises (squared!) with **curren****t**, and

*power ***delivery** rises (linearly) with **voltage**. This is why the big transmission wires you see along the freeway use 360 or 720 kilovolts: lots of power delivered using as little current as possible. But it's stepped down (to 13 kV and 480 volts and so on) for more-local distribution because that's less "squirty" and therefore does not have to be handled as carefully. If you look closely at high tension power lines, you'll see massive insulators separating the wires from the towers that hold them up. Look at local distribution lines and the insulator stacks are much shorter.

Now, back to your 120 or 240 volt stuff: the first important point here is that neither the "convenience charger" nor a "level 2" charger does any real converting, it just feeds the raw volts-and-amps through to the car. All the "smart" electronics are on the car! So from the car end, it's perfectly fine to plug the convenience charger into 240 volts. The wires won't overheat because you'll send the same 13 amps in all cases. The key question is not heating, but rather, whether the 120-volt convenience charger's wires are insulated well enough to keep a 240-volt-pressure wall connection from "squirting out" and making a short-circuit (and hence causing a fire, etc).

Footnotes:

[1] Wire isn't technically an ideal resistor (it has a little bit of capacitance and inductance) but it's close enough for our purposes. (Also, the resistance of a wire is normally pretty negligible, unless/until you get corrosion or metal fatigue or similar. That last part is very important, especially since that increases the resistance which causes localized heating which itself also increases resistance, in a very bad feedback loop.)

[2] Touch a "wall wart" power adapter for some device, like a cell phone charger or answering machine or whatever, that has been plugged in for a while: it will feel slightly warm; that's dissipated (wasted) power in the wall-wart. Incandescent bulbs waste most of their power: 60 or 75 or 100 watts, less than 5% of it becomes light and 95+% is dissipated, which is why they get so hot. You can actually tell if one power adapter is more efficient than another by which one stays coolest.

[3] This glosses over a big complication: it's the car's charging circuit that makes sure that "current" stays constant, in cooperation with the signals over the "signal" part of the J1772 connector. Basically the charger tells the car "you may have 13 amps" or "you may have 10 amps" or whatever number, and the car makes sure to stay right around that point or less. Otherwise we'd get to footnote 4, which is...

[4] Power lost via I**2R losses also lowers the voltage at the device which in turn limits current further, so at 1 volt you wouldn't actually deliver 13 and waste 9. In fact, I suspect the circuits wouldn't even power up at that point. Still, the general idea applies: more volts = better power delivery, but also more danger. In the case of these car chargers, though, the battery charger circuit dynamically fiddles with the car's "R" resistance value to keep the current at 13-or-so amps, unless that's more than negotiated or needed. Meanwhile, the box at the plug end of the convenience charger monitors voltage there, and asks the car to turn down its charge rate if the voltage it sees drops below some critical number, probably around 105 volts. That's why the convenience charger is supposed to be used by itself: if you put a lot of things on an extension cord, and some of them use a lot of power, they drag down the voltage available to other devices. If you have a very long extension cord, it adds its own resistance, dropping the voltage that the convenience charger sees, and so on.