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Pluggable AC Input Cables?

I'm wondering if anyone has built or considered building an OpenEVSE with pluggable AC input cables. The main reason I'm thinking this is because at the three most likely places I might want to use my own EVSE, I have 3 different plugs. I have L6-30 receptacles on a 30A circuit in my garage. I have a 6-30 receptacle on a 30A circuit at the in-laws garage. At the cottage, we're upgrading the service so we'll be putting in a new circuit. I *could* put in a 30A circuit similar to one of the other two places, but since we have the cost of upgrading the service and the plug will be within a few feet of the main electrical panel, it seems to make more sense to put something like a 6-50 receptacle there rather than limit ourselves to only 30A.


I know that I can build or buy adapter cables for all of these scenarios, but doing so doesn't communicate anything about the capabilities of the supply circuit to the EVSE.


I know that I can change the maximum current via WiFi, but to my mind this seems like a tedious and error prone process. All it takes is to forget one time or make a mistake one day when tired, distracted, not thinking or whatever.


I realize that the worst case is that the breaker trips and my car doesn't charge, but if I don't notice that until the next morning when I'm planning to drive home, that's a bit of a hassle.


My thinking is that with some firmware modifications, I could use a four pin plug with a pulldown resistor built into the cable to indicate the current limitations of the supply to the EVSE. Something like a 50A RV connector seems like it could do the job, although it's probably too big for the existing OpenEVSE case, both outside and in. Some of the 50A non-NEMA locking plugs seem like they might also be options and might be able to be squeezed in - at least on the outside. I suspect space inside the case is likely still a problem.


In any case, if I can find a suitable plug with four pins that can deal with 250V and 50A, that seems possibly doable. I really only need two hots and a ground, so the neutral pin could instead be used for the pulldown resistor from the neutral to the ground pin and then we can use this as part of a voltage divider that gets fed in to an ADC to set the maximum output current. This seems more idiot proof to me and probably more convenient. I just plug in the cable that fits and it already has a pulldown resistor in it that tells the EVSE what charge current limit to communicate to the charger. It also seems that such a charger could then be used for both level 1 and 2 charging. Make a cable with a 5-15 or 5-20 plug on the other end and appropriate pulldown resistor and it would seem I have the one and only EVSE I'll ever need.


It seems simple enough, but what are the flaws in this plan?


Just thinking some more about this...


If the EVSE simply has a NEMA 14-50P plug attached to it, that could probably fulfill this desire. The neutral and ground are shorted together at the service panel, so if the firmware interpreted a 0 ohm pulldown as indicating 40 amp maximum output, then it should work fine for the 50A circuit with a 14-50R receptacle. Then I can build L6-30 and 6-30 adapter cables, but on the 14-50R end of it, put an appropriate pulldown resistor that indicates only 24A maximum output current to the firmware. It just needs to be fed into an ADC and read off. Using some kind of I2C ADC chip should be pretty simple. The one downside of this I see is that in the case plugging into a normal NEMA 14-50R receptacle, you would probably be injecting a few milliamps of DC current into the neutral and back up the ground wire, since I assume that the chassis ground is connected to the controller negative/ground rail. That said, this probably only needs to be done for a few milliseconds during startup, so maybe that's not such a big deal.

One thought was that an improperly wired receptacle or cable could potentially put 240VAC on that neutral pin, which could potentially damage your ADC and maybe the controller board itself. Some kind of protection circuit might be in order. I'm thinking something like a zener clamp and fuse might be enough?


For that matter, an incorrectly wired receptacle or cable would potentially put 120VAC or more across the pulldown resistor, which could get hot. Ensuring high enough resistor values might mitigate this somewhat. I think it's unlikely that such a cable would be left plugged in without the charger, and I'd probably plug the charger into the cable before plugging the cable into the wall, so maybe the detection sequence could be used to further mitigate this. If we use a relay to pull the pin low for maybe a second or so during startup, that should be enough to trip a breaker and de-energize the whole thing. It would also give us a way to verify if the fuse mentioned above has been blown.

So after all my ruminations, I finally realized what the NEMA 14-X plug means. Now that I realize that the plug that comes with the OpenEVSE will plug into both NEMA 14-30 and 14-50 receptacles, that changes things a bit. I clearly don't need any adapter at two out of my three most likely charging locations, so now it's just a matter of what I do at home. Do I install a NEMA 14 recptacle in my garage at home, or do I build an adapter cable? Either way, the solution then just boils down to me not being an idiot and setting the maximum charge current too high.

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