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Direct-wire NEMA 5-20 to J1772?

I'm sure I must be missing something, but...:

Is there any reason I can't build & use a hard-wired cable with a NEMA 5-20p plug on one end & a J1772 on the other, plugging the 5-20 into a L1 20A source & the J1772 into the Tesla adapter thence the car?

AIUI, there's some circuitry involved for the cord to tell the car "hey, you can draw up to 16A [in this case] and no more!" Can that be duplicated without the Mobile Connector magic? How does your average L2 J1772 charging spot do it?

Why? I'm probably going to be parking somewhere with limited power--i.e., the 20A outlet I'm considering. I'd like something less unwieldy than the Mobile Connector, and less tempting to a light-fingered thief. A basic "extension cord," fitted to connect to a 20A wall outlet on one end & an EV J1772 receptacle (Tesla adapter, in this case) on the other would seem to fit the bill....
 

ucmndd

Well-Known Member
Mar 10, 2016
8,390
16,361
California
J1772 handshakes require 2-way communications between the car and a capable EVSE to establish a charging session.

Also, the mobile connector locks itself into your car, the only thing a "light-fingered thief" might be able to get away with is the $35 pigtail.
 
the car needs to talk with the EVSE

So, are you saying then that the magic that's in the Tesla-specific Mobile Connector is also present in the generic L2 public charger to be found around town? That implies to me that it's a standard protocol, no? I assume all such "talking" is really just voltages. How hard would it be to duplicate for a single-size connector (i.e., the the Mobile Connector needs to be able to work with all different size adapters, L1 or L2, etc.--what I envision would be a single setting for a 120V/20A connection, nothing else...).

Thanks for the response. Still trying to figure this out....
 
J1772 handshakes require 2-way communications between the car and a capable EVSE to establish a charging session.

Also, the mobile connector locks itself into your car, the only thing a "light-fingered thief" might be able to get away with is the $35 pigtail.

OK, you're saying (I think) the same thing as @BPeter. Is that handshake something difficult to reproduce, i.e., couldn't be built-in to an otherwise simple cable?

Yeah, understood on the pigtail being the most vulnerable bit. I'd also just rather have a dedicated cord with no big box hanging in the middle....
 

ucmndd

Well-Known Member
Mar 10, 2016
8,390
16,361
California
If you're handy you could build something, but it's not going to save you much/any money.

OpenEVSE has all the parts you need, but the J1772 controller itself is a hundred bucks...

 

Rocky_H

Well-Known Member
Feb 19, 2015
7,851
9,882
Boise, ID
I have seen your same question asked by someone else before on this forum. It's asking if they can use just a dead-simple cable that is a plug that goes in the wall on one end, and then just straight wires, with no electronics box in the middle, to the plug handle that goes in the car on the other end. And the answer is no. That box in the middle is necessary to make it work.

So, are you saying then that the magic that's in the Tesla-specific Mobile Connector is also present in the generic L2 public charger to be found around town?
Yes. The device is called Electric Vehicle Supply Equipment (EVSE). The Tesla mobile cable is one. Those public charging stations are one too. You must use one.
That implies to me that it's a standard protocol, no?
Yes, pretty much--it's the J1772 signaling standard.
I assume all such "talking" is really just voltages.
No, not just voltages. It's communication signaling, with pulses of certain amounts of time high and low, etc. That's to communicate information, like allowed maximum amps available, and then states, like ready, charging, interrupt, etc. Basically a small computer on a circuit board.
How hard would it be to duplicate for a single-size connector (i.e., the the Mobile Connector needs to be able to work with all different size adapters, L1 or L2, etc.--what I envision would be a single setting for a 120V/20A connection, nothing else...).
Well, you're talking about inventing and building your own EVSE, of which there are already hundreds available for sale. I don't see why you would want to reinvent the wheel like that.
Perhaps I'm looking in the wrong place--the "very cheap" EVSEs I'm finding are all north of $200. "Cheap" may be subjective, but at that price, I'd just buy the Tesla connector ($215, last I looked?), or use the one I have.
Yeah, you're really not going to find any for less than about $150-$200 at the bottom of the market. The Tesla one is $275, but is much more capable, since it has swappable adapters and can do many types of 120V or 240V outlet types.
 
That implies to me that it's a standard protocol, no? I assume all such "talking" is really just voltages.
J-1772 is indeed a standard protocol. It's an extremely simple switched resistor kind of thing. The wikipedia page has pretty good coverage: SAE J1772 - Wikipedia

This is all stuff a hobbyist electronics person could totally manage themselves. But then... it's controlling 240V AC! Just spend $200 on an EVSE.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,851
9,882
Boise, ID
J-1772 is indeed a standard protocol. It's an extremely simple switched resistor kind of thing. The wikipedia page has pretty good coverage: SAE J1772 - Wikipedia

This is all stuff a hobbyist electronics person could totally manage themselves. But then... it's controlling 240V AC! Just spend $200 on an EVSE.
But it's also required to include a GFCI monitoring circuit too, which can detect and disconnect if there is a problem.
 
But it's also required to include a GFCI monitoring circuit too, which can detect and disconnect if there is a problem.
That's an off the shelf part. Again, there's zero complexity here that someone with hobbyist-level electronics ability would be unable to surmount. It's just not worth it for a device to charge a $50k+ car, spend the $200 to get the manufactured version.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,851
9,882
Boise, ID
That's an off the shelf part.
...which someone would need to get and include and figure out how to integrate and attach into their little home-built circuit board, etc.
Again, there's zero complexity here that someone with hobbyist-level electronics ability would be unable to surmount.
I never implied it was insurmountable or impossible, jeez. I was just pointing out that it's yet another part that needs to be included and figured out, which continues to make it more time intensive and expensive. I just mentioned it for completeness, so it wasn't forgotten. This was just part of the process of pointing out to the OP that if you tabulate all of the things that would need to be included in a homemade product, the cost in development time and parts are making it not worth it.
 

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