Hi all, this is a bit awkward, because my goal ultimately is to buy an OpenEVSE charger cord w/j1772 connector and wire it to a Tesla Gen 3 HWPC. However, I think the question is a more general engineering question around j1772.
For some background - the Tesla cables have an extra two wires, and they also have NTC thermistors built into the charging handle, used to sense high temps and shut off. I'm not certain of the actual circuit for this, but I've seen some teardown and have done some research on the ready-made offerings from third parties.
I've seen some patents and schematics from various companies that implement something similar in standard j1772 connectors, without the use of extra conductors - they have thermistors that simply short some current to ground on overheat, tripping the GFCI in the EVSE. However, I'm not sure anyone is actually doing this, and this is one of my questions.
Is it common practice to implement a temperature shutoff within the j1772 handle? If I buy an OpenEVSE handle with cord prewired, or order an EVSE from some other company, do you think their charger handles would incorporate this?
I'm asking because I'm debating whether I should:
1. cut the handle off the Tesla cord from the HWPC and wire a j1772 on it and attempt to implement the thermistor circuit (this seems to be what the folks at third parties who modify HWPCs are doing)
2. Defeat the handle temperature monitor within the HWPC itself, if possible. This would enable me to buy an OpenEVSE cord/handle kit and just wire it up. As a bonus, the OpenEVSE one is longer than the Tesla stock cable.
My thinking is that if j1772 EVSEs don't generally incorporate handle temperature protection anyway, there's really no harm in defeating it in the HWPC itself and not locking myself down to Tesla cords with the extra conductors. On the other hand, I don't know why Tesla is monitoring the temperature of the handle - is it something specific to their connector design that makes it important, or could they be doing something sketchy with the j1772 charging standard that makes it required?
It is not common to implement temperature cuttoff in the handle, Tesla is the only one doing it that we are aware of. Its not a bad idea there have been a few cases of a handle/connector melting.
You can likely go either route. Implementing sensors in the handle may be challenging, there is not a lot of room near the pins.
Yeah, I'm thinking that even if I wired up a thermistor circuit, I don't really have the knowledge required to reliably monitor the right place, with the right temperature range and accounting for hysteresis in the temperature swings to trigger at the right point. It would likely take a lot of testing to develop and implement it properly. I could probably epoxy a thermistor to the back of the black slug that serves as the harness for all of the pins, whatever that is called, and maybe get to a rudimentary shut off.
I think I'll opt for trying to work around it in the HPWC, which will give a more standard experience and compatibility with standard cabling.
I carefully tore apart the Tesla connector handle yesterday. Wow, that was a lot of work. The entire thing is potted internally with a dense rubber-like material. I can see how that would be good for heat dissipation, as well as provide a fairly consistent mass for monitoring temperature.
I succeeded in getting the charger to work with my OpenEVSE charging handle.
The blue wire is 3.3v, just cut it and terminated it. T
The orange wire originally went all the way to the proximity pin, I'm not sure if the EVSE really needs to know the status of the proximity but I went ahead and connected this up while wiring the switch in the new handle.
The white wire, I determined went straight to a 10k@25c thermistor and then straight to ground. I haven't fully characterized the thermistor (I think it has a beta of around 4500k) but for now I just wired white to a 10k resistor and then to ground, in the handle.
I've ordered a full OpenEVSE handle + cable assembly. I'm not sure if it will come with the proximity wire actually wired to the cable, and not just to the button - I hear most EVSEs don't care about this and I don't see any explicit connection to the EVSE on normal j1772 proximity schematics. I can open it up and take a look, but I'm going to play with this and see if I can just use the cable as is and defeat the thermistor circuit directly in the EVSE. This would simplify things greatly in not having to use the Tesla cable and mess with assembling and crimping.
I hope that it's not inappropriate for me to continue updating about this, my hope is that other people who want to hack, understand, and mix-match various EVSE components might be interested and/or find it useful.