Should I cool the plug at TX 105*?

[Anson Fung]

TThere's an assumption in your analysis that I'm not sure is necessarily true. Is the sensor supposed to measure the conductor temperature or the handle exterior temperature?

There are design limits on both: the handle exterior can't become unsafe for a person to touch, and the conductors / cable interior has some much higher temperature limit that would be unsafe to exceed (due to the materials melting).

I think you're assuming the sensor is trying to measure the latter, but if it's measuring the former, then the wet towel isn't tricking anything. It's actually cooling the limiting component, namely the handle exterior.

I don't know if I'm right or not, but the temperature limits for the parts a person can touch are likely a lot lower than the melting points for plastic and metal.

Another thing to consider is that the wet towel trick has been known for long enough that if it actually posed a risk of tricking the sensors and letting the cable interior get too hot Tesla would have advised people to stop (or make a design change to make it not risky).

Thanks for actually giving a reasonable analysis, btw, and not just a knee jerk "you're tricking the sensor it's dangerous and irresponsible!" reaction.

Appreciate the thoughtful response! You're right that my initial post assumed that the temperature sensor was measuring the conductor temperature instead of the handle's exterior surface temperature. Also very reasonable to hypothesize that the sensor is designed to measure the handle exterior. Indeed, the IEC 62196-1 spec referenced in section 8.1.3 of the Tesla NACS specification limits exterior nonmetallic grasping surfaces at 60 C. That's a lower temperature than the maximum interface contact temperature of 105 C limit per 8.1.2 of the Tesla NACS specification.

To settle the question, we'll need to figure out where the thermal sensor(s) are located inside the supercharger handle. The Tesla DC connector datasheet suggests there are two temperature feedback components. The first is a temperature sensor (Vishay NTCALUGE2C90784) and the second is a temperature switch (HXPEE TP2-09505). The Tesla NACS specification and datasheet don't go into any more detail about where these sensors are located though.

I did a little more digging and found some information that tends to suggest that the temperature sensor is designed to measure the contactor temperature.

First is Fig. 10 in U.S. Patent No. 11,804,390 B2 (issued Aug. 10, 2021). Fig. 10 "shows the charging connector 210 without electrical socket 406. Charging connector 210 includes a temperature sensor 1002. Temperature sensor 1002 is thermally coupled to electrical socket 406. Another temperature sensor (not shown) thermally coupled to the electrical socket 404 is provided under electrical socket 404." Note that 404 and 406 are the HV+ and HV- connectors that mate with the inlet on the car. (emphasis added). This suggests that the sensor was designed not to measure exterior handle temperature, but the temperature of the contactors themselves.

Second is U.S. Patent No. 11,225,156 B2 (issued Jan. 18, 2022). Of interest are Figs. 6A, 6B, 8A, and 8B. Figs. 6A and 6B show that the thermal switch is located at the midpoint of the bottom of the handle. Figs. 8A and 8B then show how the temperature switch is placed in-line with the pilot pins. Note that its possible that the thermal switch is measuring exterior handle temperature: "It is to be understood that the inline thermal switch may be placed elsewhere in handle 600, e.g., to accommodate space constraints within handle 600 and/or to increase temperature sensitivity at particular locations within handle 600."

However, the thermal switch's operation is irrelevant when it comes to the wet towel trick. The thermal switch is like a bimetallic switch--its either on or off. When the thermal switch trips, it completely disconnects the pilot signal circuitry, which will immediately stop all charging. The reported charging speed increase with the wet towel trick thus cannot be due to the thermal switch.

 

[qdeathstar]

If it's so dangerous, how come:

a) Tesla hasn't advised people to stop
b) made a design change to eliminate the risk (if the towel actually tricks the sensor, they're probably not well positioned in the handle to measure what they intend to measure)
c) nobody has shown something dangerous actually happen. The internet would be all over a "Tesla supercharger MELTS video!"

It doesn't make much sense.

So your argument is it’s safe because Tesla hasn’t told us to stop?

That argument is almost as compelling as “it’s dangerous because Tesla hasn’t recommend we do it.”

I mean, not quite as compelling, but almost.

 

[gsmith123]

To be clear, that was 1 of 3 points I was making.

I still think it's a valid one. If people were literally melting Supercharger handles with towel trick (which is what people are claiming happens!), you don't think Tesla might consider something like tweeting they should stop or giving some official guidance (for people like us discussing it)?

They have every interest in stopping people from doing this: people might hurt themselves and presumably it would cause the handle to stop working.

Companies love covering their asses legally - the manual for a level two wall connector has a full page of warnings. What's so unreasonable about Tesla offering a warning (many be in the vehicle owners manual) if a common practice causes Supercharger handles to dangerously fail?

 

[gsmith123]

I did a little more digging and found some information that tends to suggest that the temperature sensor is designed to measure the contactor temperature.

Good find on the patents. Is it unlikely there's a third sensor thermally coupled to the hottest point of the exterior of the handle?

 

[LostVector]

There's an assumption in your analysis that I'm not sure is necessarily true. Is the sensor supposed to measure the conductor temperature or the handle exterior temperature?

There are design limits on both: the handle exterior can't become unsafe for a person to touch, and the conductors / cable interior has some much higher temperature limit that would be unsafe to exceed (due to the materials melting).

I think you're assuming the sensor is trying to measure the latter, but if it's measuring the former, then the wet towel isn't tricking anything. It's actually cooling the limiting component, namely the handle exterior.

I don't know if I'm right or not, but the temperature limits for the parts a person can touch are likely a lot lower than the melting points for plastic and metal.

Another thing to consider is that the wet towel trick has been known for long enough that if it actually posed a risk of tricking the sensors and letting the cable interior get too hot Tesla would have advised people to stop (or make a design change to make it not risky).

Thanks for actually giving a reasonable analysis, btw, and not just a knee jerk "you're tricking the sensor it's dangerous and irresponsible!" reaction.

It's not unreasonable to call it dangerous and irresponsible. High voltage and power systems are by nature quite deadly, so that covers the dangerous part. And think about the many many hours of testing of these chargers ... tens or hundreds of thousands? Do you think any of those hours involved a wet towel wrapped around the handle?

When you change the parameters of operation so that they don't fall within the bounds of how the charger was tested, then you lose the benefit of all that testing. And I would argue, you would lose the "OK" of any professional who signed off on that system. In any system like this where safety is an issue, you should use it as it was designed and tested, because that's how they got the OK for it in the first place.

I understand the desire to know what's going on under the hood and what a wet towel's actual effect is on the system, but then ... just ask that question. As others here said, the marginal benefit is very low to be actually messing around with these systems in real life. I think the theoretical question and analysis is very interesting tho, and I welcome that discussion.

 

[frankvb]

I don't want to encourage people to use this method to cool the handle, but likely (hopefully...) the system is designed not to have a single point of failure.

As it comes to the temperature of the electrical contacts, I would assume there is a temperature sensor inside the charge port (on the vehicle) as well.

 

[LostVector]

I don't want to encourage people to use this method to cool the handle, but likely (hopefully...) the system is designed not to have a single point of failure.

As it comes to the temperature of the electrical contacts, I would assume there is a temperature sensor inside the charge port (on the vehicle) as well.

Hopefully no single point of failure, yes. However, it's a common human fallacy to bucket things into the "safe" or "not safe" category ... in fact, all safety critical systems really rely on multiple layers and it's the slippery slope of stripping away those layers that causes accidents to occur.

 

[Anson Fung]

Good find on the patents. Is it unlikely there's a third sensor thermally coupled to the hottest point of the exterior of the handle?

Appreciate it! And that's a good question--its definitely possible that the cables actually deployed at superchargers use a different design than that illustrated in the figures. But likely won't be able to find any more info unless I can get my hands on some second hand decommissioned supercharger cables

 

[gsmith123]

And think about the many many hours of testing of these chargers ... tens or hundreds of thousands? Do you think any of those hours involved a wet towel wrapped around the handle?

That seems likely, actually, that they have tested it.

People have been doing the wet towel for many years. Seems pretty likely Tesla has caught wind of it and did some testing to make sure nothing terrible would happen. They certainly test water ingress in general, so a wet towel test wouldn't be too hard to do.

Can you imagine the bad press if somebody actually melted a supercharger cable or managed catch it (or the car) on fire?

I've been thinking about this a bit more. So the theory about how this is dangerous is it tricks the sensor into an artificially low reading and the conductors are then allowed to reach dangerously high temperatures.

How is a towel touching the exterior of the handle meaningfully different than just really cold, snowy weather with heavy snow or rain hitting the handle? Both have a similar effect: the handle surface is colder and there's liquid present.

Under this theory the risk would be higher the colder the handle exterior gets.

Am I missing something in this comparison?

 

[ucmndd]

It's not unreasonable to call it dangerous and irresponsible. High voltage and power systems are by nature quite deadly, so that covers the dangerous part. And think about the many many hours of testing of these chargers ... tens or hundreds of thousands? Do you think any of those hours involved a wet towel wrapped around the handle?

I know this might blow your mind, but get this:

Sometimes people charge their cars outside when it’s RAINING. Water everywhere. Literally falling from the sky.

I know, right?

The damp towel trick is useless and gimmicky in 2023. It’s ridiculous that we’re still taking about it. But dangerous? Come on.

 

[gsmith123]

Sometimes people charge their cars outside when it’s RAINING. Water everywhere. Literally falling from the sky.

To be fair to the argument in opposition of the towel trick, I don't think they're arguing that the liquid is the main problem.

It's specifically the selective cooling of part of the handle in a way that tricks the system into thinking the hottest parts are cooler than reality. The liquid ingress isn't the issue, like you point out inclement weather routinely wets handles as much as a towel.

 

[ucmndd]

It's specifically the selective cooling of part of the handle in a way that tricks the system into thinking the hottest parts are cooler than reality.

I've read through the above and it doesn't really seem like there's any evidence that "trickery" of any sort is what caused this to work (if it ever did) back in the day.

 

[gsmith123]

I've read through the above and it doesn't really seem like there's any evidence that "trickery" of any sort is what caused this to work (if it ever did) back in the day.

That's exactly the argument being put forward in this previous post about why it's risky (which to be clear, I don't agree with, but I am attempting to understand fully before disagreeing with it!).

If that's not the problem with the towel trick, then someone please correct me. Surely it's not the water because superchargers are used in rainy conditions all the time...

 

[LostVector]

I know this might blow your mind, but get this:

Sometimes people charge their cars outside when it’s RAINING. Water everywhere. Literally falling from the sky.

I know, right?

The damp towel trick is useless and gimmicky in 2023. It’s ridiculous that we’re still taking about it. But dangerous? Come on.

I know the two might appear similar, but, if you think about it harder, are a wet wrapped towel and weather/rain resistance the same problem?

Consider the NEMA 3R rating. Understanding Your Enclosure: What Is NEMA 3R?

• Falling dirt
• Accidental contact by people or objects
• Light dripping and splashing of water
• Weather conditions like rain and snow
• External ice formation on the enclosure

Those are very specific forms of weather and moisture resistance being called out. Not just "rain" and you figure out the rest.

 

[gsmith123]

I know the two might appear similar, but, if you think about it harder, are a wet wrapped towel and weather/rain resistance the same problem?

Do share. What is meaningfully different? The volume of water? The direction it's coming from? The towel itself?