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Mobile connector maximum amperage fluctuating

Darblish

Member
May 19, 2021
169
366
Columbus, OH
I’ve had my Model Y for about 2 days now, and am charging at home with a NEMA 14-50 outlet and the “corded mobile connector” (the 40-amp version). I picked the corded model because if I come home at my latest usual time and plug in with 20%, 9.6kW will get me to 80% before my earliest usual departure time; 7.7kW on the 32A connector wouldn’t quite complete the charge in time.

I saw tonight that my car was only drawing 36A, and I was initially worried there was some wiring issue, voltage drop, etc. However, the voltage is fine even with my A/C running, and my understanding is that a wiring/reliability issue would change the amperage limit but not the maximum, and I’d see something like “36/40A” in the app and on the screen. Instead I see “37/37A” (fluctuating between 36-40A depending on when I check it). I assume this means the mobile connector is presenting to the car a lower maximum amperage?

Unplugging and reconnecting after a moment works as expected, but only raises the maximum current by an amp or two and it continues to fluctuate. Waiting a minute or so before reconnecting got us back to 40A, but only for about 30 seconds.

Before I freak out and call the electrician who wired my outlet, or call Tesla for a replacement connector, I wonder: is this normal? The mobile connector is wall-mounted and is noticeably warm to the touch; the cable to the car and outlet, the outlet housing, and the metal conduit on the wall are also warmer than ambient temperature in the garage, but only slightly so. The garage itself is not climate-controlled, and it’s probably around 80° in there. I saw the issue after about 2 hours of charging, and there are no abnormal lights on the connector.

I’m not a fan of the car charging more slowly than expected, but if this is a usual response to warm-ish temperatures in the environment, then at least it will be a mild inconvenience and not a major problem.
 

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Sophias_dad

Supporting Member
Supporting Member
Jul 29, 2018
1,532
1,612
Massachusetts
I don't have a CMC, and don't use my UMC very much.

Your symptoms are those of an overheating condition being detected. Any warning lights on the UMC or messages on the car's screen?

Next time it happens, pull the plug on the CMC(be kind and stop charging immediately before you pull it) and measure the prong temperature of the two side conductors either by finger or infrared(preferred). If they are notably hot, its probably time to contact the electrician.

Do you happen to know the outlet brand? There are pictures on a thread around here with how to tell them apart, and if you didn't go out of your way to specify it during installation you probably got the $10 Leviton from Home Depot instead of the $60 Bryant/Hubbell.

 
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jmaddr

Active Member
Mar 29, 2019
1,016
1,033
Florida
Curious why you didn’t just install the wall charger to get 48 A. I would imagine the price above a new CMC would be neglible.

Either way I concur with @Sophias_dad. If the electrician used a cheap 14-50 outlet, which is very common, the connection between the wires and the outlets can be less than ideal and heat up through resistance. Even if it is a high quality outlet, if the connections are not very solid at the outlet and the panel, you can have issues. A FLIR camera or laser thermometer should confirm and help pinpoint if this is the issue.
 
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jcanoe

Active Member
Oct 2, 2020
3,423
3,474
Maryland
What is the charging amperage when the battery SOC is between 20% and 60%? At 70% you are only 20 minutes from completing charging. The Model Y may be tapering the charging level as it completes the charging session; also might be performing cell balancing. I would observe the charging cycle for a few days to see if the lower amperage is constant or follows the charging cycle.
 
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Darblish

Member
May 19, 2021
169
366
Columbus, OH
As was said, no warning lights of any kind, including the “over temperature” error listed in the manual. The lights were all green and “streaming.”

I went with the outlet instead of the wall connector because changing that out later for a non-Tesla-specific EVSE, or connecting something else if the day-to-day charger has an issue, does not require another visit from the electrician.

I think it’s a Bryant receptacle; I’ll double check. The electrician is one listed on Tesla’s website and literally only does EV-related work.

I had wondered if it was a “taper at the end of the charge” thing as was suggested, but there’s no way for the mobile connector (or any non-smart L1/L2 connector) to know the battery state of charge in order to do the tapering. That isn’t part of the signaling that happens when AC charging.
 
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Sophias_dad

Supporting Member
Supporting Member
Jul 29, 2018
1,532
1,612
Massachusetts
As was said, no warning lights of any kind, including the “over temperature” error listed in the manual. The lights were all green and “streaming.”

I went with the outlet instead of the wall connector because changing that out later for a non-Tesla-specific EVSE, or connecting something else if the day-to-day charger has an issue, does not require another visit from the electrician.

I think it’s a Bryant receptacle; I’ll double check. The electrician is one listed on Tesla’s website and literally only does EV-related work.

I had wondered if it was a “taper at the end of the charge” thing as was suggested, but there’s no way for the mobile connector (or any non-smart L1/L2 connector) to know the battery state of charge in order to do the tapering. That isn’t part of the signaling that happens when AC charging.
I doubt its the 'taper at end of charge' thing because I'm pretty sure that would be reported as 38/40A, where the car is deciding to charge slower than the UMC says it can put out.
 
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Darblish

Member
May 19, 2021
169
366
Columbus, OH
I doubt its the 'taper at end of charge' thing because I'm pretty sure that would be reported as 38/40A, where the car is deciding to charge slower than the UMC says it can put out.
I agree; I was just responding to the person who suggested that. It’s acting like it’s the CMC that is lowering the maximum amperage, and the charger in the car is continuing to draw what it can. I might try cooling off the mobile connector the next time it happens and see if that helps. I would hope it could maintain the full amperage for a charge session even in ~80° temperatures, but maybe there’s an issue with the receptacle or device.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
6,765
8,354
Boise, ID
What is the charging amperage when the battery SOC is between 20% and 60%? At 70% you are only 20 minutes from completing charging. The Model Y may be tapering the charging level as it completes the charging session;
No, it definitely isn't that. Think about the power levels involved. You're thinking of "It tapers at this %". But that's from a source that can send over 100 kW power. The tapering curve is what it is, regardless of whether it's a Supercharger or a mobile connector on a 14-50 outlet. When you say tapering at 70%, what would the power level be? It's about 40kW. That's still FAR above what a corded mobile connector provides, so it won't be tapering that 9.6 kW yet. You have to look at where does the curve finally come down to where it would be limiting to about 9.6 kW or less. And that point won't be until about 96-97%.

I had wondered if it was a “taper at the end of the charge” thing as was suggested, but there’s no way for the mobile connector (or any non-smart L1/L2 connector) to know the battery state of charge in order to do the tapering. That isn’t part of the signaling that happens when AC charging.
But it's not for that reason. The car controls the charging either way, whether it's DC or AC charging.

From the specific symptom of the display showing the second number reduced, (37/37), that is not the car just requesting fewer amps. It is the CMC announcing a smaller number of amps available. I am pretty sure this is a heat problem reducing it. But you are not getting the car messages about it because this isn't the Gen2 mobile connector that has the temperature sensor in the changeable plug head that creates that specific error message signal. Remember that the CMC is based on the original 1st Gen mobile connector and was out for a couple of years before they developed the Gen2 with the debut of the Model 3 car. So this doesn't have that same temp sensor.

So I would think there may be a bad connection in the outlet that's making a hot spot.
 
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Darblish

Member
May 19, 2021
169
366
Columbus, OH
I am pretty sure this is a heat problem reducing it. But you are not getting the car messages about it because this isn't the Gen2 mobile connector that has the temperature sensor in the changeable plug head that creates that specific error message signal. Remember that the CMC is based on the original 1st Gen mobile connector and was out for a couple of years before they developed the Gen2 with the debut of the Model 3 car. So this doesn't have that same temp sensor.

So I would think there may be a bad connection in the outlet that's making a hot spot.
What’s confusing me is that the CMC does have a separate plug and internal over-temp error (5/6 flashes of the red light), which I’m not getting either of. And if it was a temperature issue in the plug causing it, wouldn’t there need to be a temperature sensor there for the CMC to notice and lower the amperage? That’s why I was sort of leaning toward “something inside the CMC” as the cause (since it was the warmest object in the path that I was able to reach and put my hand on). The manual for the CMC says it’s usable up to 122ºF, but what I don’t know is if that means I should be getting 40A up to that ambient temperature or if that’s where it’s too hot to be useful.

I think I’ll still call the electrician and have them check things, but it’s confusing to say the least. Also weird that it took 2-ish hours to happen, since I would think that things would heat up faster than that.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
6,765
8,354
Boise, ID
What’s confusing me is that the CMC does have a separate plug and internal over-temp error (5/6 flashes of the red light), which I’m not getting either of. And if it was a temperature issue in the plug causing it, wouldn’t there need to be a temperature sensor there for the CMC to notice and lower the amperage?
Hmm, yeah, if there is a flash code for it, I thought it would have been showing something. Could be a problem in the outlet or maybe just inside the electronics body/box of the unit itself. Maybe just going bad?
 
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Darblish

Member
May 19, 2021
169
366
Columbus, OH
Hmm, yeah, if there is a flash code for it, I thought it would have been showing something. Could be a problem in the outlet or maybe just inside the electronics body/box of the unit itself. Maybe just going bad?
It could be. I didn’t drive much today so there wasn’t time for things to heat up much while charging, but tomorrow or Saturday I’m going to see what a couple of hours on the CMC feels like heat-wise compared to the same time on the UMC. I know it’s a lower amperage, but it’s at least something while I wait for the electrician to come back out.
 
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Darblish

Member
May 19, 2021
169
366
Columbus, OH
For anyone who might be following this (or care), the electrician just checked things out and believes the wiring/connections to be fine. I didn't get a chance to test with the UMC since they recommended turning off the breaker until they could get out here and check things, but I'm currently charging to try and reproduce the issue with the CMC, and I can try with the UMC in a day or two once I run the battery down a bit.

The electrician that came out is one who only does EVSE installation (AC or DCFC), so his thoughts are:
  1. The CMC might just work differently than the UMC, in that it gets warm and steps down the amperage without considering itself "over temperature."
  2. The CMC could just be bad. I may ask Tesla to look at it either way when I take the car in for post-delivery fixes next week.
  3. The receptacle could be causing resistance in some way that isn't apparent with an ohmmeter, and need to be replaced.
 
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Sarugby9000

Member
Jul 30, 2021
18
13
Virginia, USA
For anyone who might be following this (or care), the electrician just checked things out and believes the wiring/connections to be fine. I didn't get a chance to test with the UMC since they recommended turning off the breaker until they could get out here and check things, but I'm currently charging to try and reproduce the issue with the CMC, and I can try with the UMC in a day or two once I run the battery down a bit.

The electrician that came out is one who only does EVSE installation (AC or DCFC), so his thoughts are:
  1. The CMC might just work differently than the UMC, in that it gets warm and steps down the amperage without considering itself "over temperature."
  2. The CMC could just be bad. I may ask Tesla to look at it either way when I take the car in for post-delivery fixes next week.
  3. The receptacle could be causing resistance in some way that isn't apparent with an ohmmeter, and need to be replaced.
I am not sure if you answered this earlier in the thread but what is the brand of your 14-50 outlet. I originally installed a Leviton and had similar problems until it actually ended up melting the outlet and the plug of the CMC. I replaced it outlet with a Bryant and haven't had any problems since.
 
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Darblish

Member
May 19, 2021
169
366
Columbus, OH
I forgot for the 7,000th time today to unplug the CMC and check the brand on the receptacle. I also forgot to check before I reconnected things this afternoon when I picked up the car from service. One of these days I’ll check, but recent evidence seems to indicate it’s a function of the CMC itself, and is “as intended.”

After another discovery of ~36A charging a few days ago, I started poking around all of the pieces parts, and found that the hottest thing by far was the back of the body of the CMC. I took it out of the (first-party) wall mount bracket and let the connector hang loosely, and the amperage gradually ticked back up to 40A. It seems that this is a heat issue in the connector itself.

Tesla had my car for the day to fix a few delivery issues, and I brought the CMC for them to check. They charged it from ~80% to nearly 100% with their various diagnostics during/after, and found no issues with the car or connector. When I picked up, the technician said that the 40A “first gen” corded mobile connector has a wider range of acceptable temperatures/current reduction before it sounds the alarm that things are over temperature. I was assured that as long as the mounting bracket and mobile connector aren’t showing any signs of damage, and I’m not getting any errors, that this is normal.

I suppose the lesson here for anyone else who is considering the CMC instead of the UMC for an extra 8A of current, expect that you may not get the full 40A in a warm environment. I might have made a different choice had I known that before purchasing, but hopefully this helps someone.
 
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