About 25 years ago, the first modern electric cars began to appear. The Society of Automotive Engineers (SAE) realized with alarm that ordinary consumers would soon be attempting to charge cars from household circuits, the first time in history that people with limited electrical competence would be making and breaking connections drawing more than 15 amps. They knew that it was only a matter of time before someone tried to charge a car at 40 amps using an 18 gauge lamp cord and start a fire. Previously, anyone making these sorts of more powerful connections--household ovens and the like--would either be an electrician, or someone with enough competence to not make such a mistake. E.g. a welder.
The approach they came up with was called J1772. Basically, they mandated that all electric car chargers would require a standard plug which would inform the car of what it was connected to, so the car wouldn't draw too much current. The way this worked was a control wire, called the "pilot", which implements a square wave where the duty cycle says what the maximum current can be. The pilot signal would be generated by a device called an EVSE: Electric Vehicle Supply Equipment. An EVSE contains a circuit which generates the specific square wave, and some number of "contactors", which are big relays that can tolerate make/break of the full current. Whether the contactor is open or closed depends upon whether the charge circumstances the EVSE sees are deemed safe. The EVSE does no power conversion--it's just passing whatever power it receives on to the car. The car must contain a battery charger appropriate to the battery in the car, and reciprocal electronics to behave correctly according to the signal it's receiving from the EVSE.
Part of this design is that the plug on the EVSE is specific to the power limits of whatever it's plugged into: a NEMA 5-15 can draw no more than 80% of 15 amps. A NEMA 14-50 no more than 80% of 50 amps, and so forth. Replacing the plug on an EVSE or plugging it into an extension cord foils the safeguards that are the whole point of the EVSE's existence. Therefor, most EVSEs are intended for a single voltage/current application. A few of them have a couple of modes, which are typically controlled by switches, to adapt to different power sources.
There's a second factor too. J1772 defines two levels: Level 1 and Level 2. In the North America, this is the distinction between 120 and 240 volt power, sometimes incorrectly called one phase and two phase. 120 is wired with Hot, Neutral and Ground, where 240 is wired with Hot1, Hot2 and either Neutral or Ground or both. (connectors without a ground have been disparaged since the 1960s and became illegal for new construction in the '90s). Typically, EVSEs are configured for either L1 or L2.
Tesla came up with something clever: Their portable EVSE (which they call a UMC: Universal Mobile Connector) comes with a connector that allows different adapters to be plugged in. Thus, if it's got a 14-50 adapter, on it, it tells the UMC to send the signal for 40 amps. if it's a 5-15, it says 12 amps. (this is simply a resistor that's tuned to the particular R/C circuit in the EVSE). The UMC also detects whether the source power is 120 or 240 and modifies its behavior. They have a half dozen adapters for this. Interestingly, the adapter does not care if 240V power is plugged into an adapter intended for 120V. As long as the wires are correct, it simply passes the power along. If you're careful to make sure everything upstream is safe for the volts and amps, it all goes smoothly. If it's not wired correctly, the car gives you an error message, and you can fix your adapter. Obviously this is dangerous to play with if you don't understand what I mean by all of this.
Tesla is doing a separate thing, which they call "supercharging", which requires much bigger conductors in their charging system. Their approach was to have their own, incompatible connector, so people don't naively plug non-compatible cars into their super high powered charge stations. They provide a small adapter so people can charge Teslas with standard J1772 connectors--all the electrical stuff is identical between J1772 and Tesla connectors.
I've studied quite a few EVSEs and I'm pretty convinced the Tesla UMC is the best on the market. It's well designed and built--even though there's a pretty exterior, the whole thing is basically potted in a huge block of rubber, and it would take pretty extreme treatment to break it. It's also priced pretty reasonably: $550. You can buy a version with the tesla plug replaced with a standard J1772 plug here. Quickcharge also has their own 16 amp J1772 adapter for quite a bit less money. QCP is run by a guy named Tony Williams who is both knowledgeable and reputable. There are a bunch of others. Probably the most complete line is Clipper Creek.
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