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How does the car know how many amps to pull?

Dolemite

is my name
Sep 19, 2019
1,372
1,738
Seattle, WA
Disclaimer: I'm a gearhead who's new to understanding electricity-stuff, so I've been doin' me a lot of learnin'. I've spent hours trying to answer this question conclusively, but my clicking-around-on-the-internet isn't cutting it.

So, the UMC can be plugged into all sorts of outlets:
Mobile Connector

Without us manually adjusting the amps on the screen, how does the car know what to pull to avoid tripping the breaker/burning down the house? Buddy of mine who's a nuclear engineer said the car monitors the voltage drop. OK. I guess that has something to do with the formula V = IR?

I'm not sure exactly how the math works out, but it's like the car slowly ramps up I until the pace of the increase in R starts to result in a voltage drop?

I'm not even sure this is right, but if it is, how does it then dial that back down to 80% to account for the safety "overhead?"

do me a learnin'
 

Xebec

Member
Mar 20, 2019
78
79
PA
The mobile connector adapters have some electronics in them that say "this is a 15A 120V" or "this is a 240V 30A" source, and that gets communicated by the mobile connector to the car. Each connector is custom for whatever outlet you have.

The 80% is straight math based off of that (up to the limit of what the UMC can provide and car can receive).

Beyond that, I believe if there are voltage sags or drops, the car and/or UMC may adjust further..
 

Saghost

Well-Known Member
Oct 9, 2013
8,224
7,087
Delaware
The plug end you insert into the UMC has a resistor in it that tells the UMC what amperage the plug is rated for, and the UMC passes that limit to the car through the J1772 standard duty cycles, and the car then respects that limit.

This does mean you have to be very careful if you use a third party adapter to the 14-50 pigtail - the car will think it's fine to draw 32 amps from that connection, which may or may not be the case with third party adapters.

Oh, and yes, the car monitors the no load voltage and the voltage drop as it loads the line, and will lower the amperage or stop charging entirely if it feels the voltage drop is excessive or if it decides the waveform of the voltage is unreasonable. That's a secondary safety feature to prevent fires and not the primary system (any voltage drop to the car is energy being dissipated as heat somewhere outside the car.)
 
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Lasttoy

Active Member
Mar 24, 2017
1,669
975
St Augustine, Fl
Giggle. My antique has 2 inverters. So I can pull close to 80 amps, not, but it trys.
I was in Daytona Hilton, plugged in (they have 4 free chargers), the car saw 100 amp breaker, so it went for it. In a few minutes the pole was on fire. Man asked me why my car smoked his charge pole, I said it was wired wrong, my antique saw 100 amp breaker. I moved and got a 50 amp sourse and it worked fine. They had it re wired.
The new Tryp hotel here has 100 amp breaker, my car loves it.
The system senses what you plug into and adjust accordingly.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,305
8,997
Boise, ID
I was in Daytona Hilton, plugged in (they have 4 free chargers), the car saw 100 amp breaker, so it went for it. In a few minutes the pole was on fire. Man asked me why my car smoked his charge pole, I said it was wired wrong, my antique saw 100 amp breaker. I moved and got a 50 amp sourse and it worked fine. They had it re wired.
Since that is confusing and sounds a little concerning, I'll explain further what was going with that for @Dolemite . The Tesla wall connectors are adjustable current devices. You can install them on very different sized circuits, from 15A to 100A. And inside them, there is an adjustable switch that you set when you install it to match the breaker and wire size you are using. In that case at that hotel, they did have it configured wrong. They had installed it on an actual 50A breaker and smaller wiring for a 50A circuit, but the installer had set the switch wrong, so it was "announcing" to any car that plugged into it that it could supply 80A. His car tried to pull that amount, and it did overheat the connections inside from the wires that were way too small for that wrong setting.

But yes, for the mobile connector cable, the different plug adapters you can buy that fit into the cable have a little chip inside that announces the current limit by what kind of plug it is.
 

Dolemite

is my name
Sep 19, 2019
1,372
1,738
Seattle, WA
Awesome. Thanks for the info guys - I'm on board now. Glad I asked, because I hadn't exactly been fully-informed. I had no idea there was any sort of "intelligence" (no matter how crude) going on between the breaker panel & the car's plug. Understanding this had been bugging me for quite some time.

I'll be visiting my father come May, and he lives in a supercharger desert. His laundry room is right next to his garage & his dryer has an L6 outlet. Let's say I bought one of these aftermarket adapters - even if his outlet was on a 30A braker, I'd have to dial the car back down to 16A, since the adapter is rated for 20A & I'd have to account for the 20% overhead, correct?

But at $55 I'll probably just buy the materials & install a 14-50 in his garage (yes, I did A LOT of homework before doing this in my own home).

Since that is confusing and sounds a little concerning, I'll explain further what was going with that for @Dolemite . The Tesla wall connectors are adjustable current devices. You can install them on very different sized circuits, from 15A to 100A. And inside them, there is an adjustable switch that you set when you install it to match the breaker and wire size you are using. In that case at that hotel, they did have it configured wrong. They had installed it on an actual 50A breaker and smaller wiring for a 50A circuit, but the installer had set the switch wrong, so it was "announcing" to any car that plugged into it that it could supply 80A. His car tried to pull that amount, and it did overheat the connections inside from the wires that were way too small for that wrong setting.

But yes, for the mobile connector cable, the different plug adapters you can buy that fit into the cable have a little chip inside that announces the current limit by what kind of plug it is.
Makes sense now! Thanks - the HPWC was another thing I was confused about. I couldn't understand why that charged faster than a UMC, even if on the same circuit... but I've since come to learn that the UMC maxes out @ 32A. I guess whatever's in the HPWC can handle A LOT more.

Curious why it caught fire if the car's monitoring voltage drop as a secondary safety measure?

Also, did the 2013 S max out at 80A?
 
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Lasttoy

Active Member
Mar 24, 2017
1,669
975
St Augustine, Fl
I dont know, it was very strange, might have been loose connection? They have to be super tight. I installed 14.50, electrician said make sure all connections are supe tight.
The car sees it's a 30amp. But it's a 3 wire, wont draw what a 4 wire will. Normal 14.50 is 4 wire dryer is 3 wire. I have those but haven't needed it yet.
I found free ChargePoint charger at library, it gives me 22 miles an hour, free.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,305
8,997
Boise, ID
I'll be visiting my father come May, and he lives in a supercharger desert. His laundry room is right next to his garage & his dryer has an L6 outlet.
L6 is a category, so it doesn't really identify the outlet. That first part, L6 are locking plugs that are for the 208V or 240V levels. But there would be a second number that identifies how many amps the circuit is. So it would be something like an L6-20 or L6-30, etc.

Let's say I bought one of these aftermarket adapters - even if his outlet was on a 30A braker, I'd have to dial the car back down to 16A, since the adapter is rated for 20A & I'd have to account for the 20% overhead, correct?
Or let's say don't. I don't recommend adapters like that for that kind of reason you are getting at. The Tesla adapter part announces to the car how many max amps, so if you are using the 14-50 Tesla plug, it is going to think it can use the max 32A that the UMC can do. And as you point out, if the circuit is really a 20A or 30A, you would need to manually adjust that in the car to the 80% value.

But that's just not all that good. The car is supposed to save that setting according to the GPS position at that charging location, but that is not always reliable. Exact case in point: Just this week, I was driving home, and I noticed that on the map it was showing my car several blocks off from where I actually was. It wasn't a big deal because I wasn't using navigation, so I didn't think much of it. So I got home and plugged in and went inside. About an hour later, I opened up my Tesla app to check on something, and I saw that the car was currently charging at 40A. That was not supposed to be happening. For one, I have it set for scheduled charging at 1:00 AM, so it shouldn't have even been running yet. And two, I have it set for 31A at my house, so it runs a little bit cooler on my 1st generation UMC. What was going on was that the GPS still thought my car was several blocks away, so it was not using my home settings for amps or scheduled charging.

So about that adapter you linked to, we didn't have a lot of choice in 2013-2014 and had to do stuff like that. But it's 2020 now, and there are much better options. A company called EVSEAdapters.com makes substitute adapter pigtails for the Tesla UMC for plug types that Tesla doesn't make. They actually start from an official Tesla adapter and take it apart to put a different plug on the end of it, so it preserves the circuits inside it for temperature monitoring at the plug and for announcing the correct amp limit. They are fantastic!

So I would highly recommend to just buy the appropriate adapter from them for whichever outlet type you are trying to plug into for that dryer:
L6-20:
L6-20 Adapter for Tesla Model S/X/3 Gen 2 – EVSE Adapters

or L6-30:
L6-30 Adapter for Tesla™ Model S™/X™/3™ Gen 2 – EVSE Adapters

Curious why it caught fire if the car's monitoring voltage drop as a secondary safety measure?
If it's a case like that where it's undersized wire instead of a loose connection, I think it can get really hot before it sees very much voltage drop. Also, I remember that he has been telling that story for a very long time, and that might have happened before Tesla rolled out the software change that checked for the before/after voltage monitoring safety system.

Also, did the 2013 S max out at 80A?
Older Model S’s (if I remember correctly) had a 40 amp charger with the option to install a second charger.
Yes, until the approximately April 2016 facelift of the Model S, it had either the single 40A charger, or optionally two of them for 80A. My 2014 just has the 40A.

The car sees it's a 30amp. But it's a 3 wire, wont draw what a 4 wire will. Normal 14.50 is 4 wire dryer is 3 wire.
That is not correct. Being a 4 wire outlet instead of 3 wire has nothing to do with the amp draw capability. The three wire outlet types only have the connections for one voltage level. So they would be only 120V or only 240V. The four wire outlets have Hot1, Hot2, Neutral, and ground, so they have the extra connection points so they can be dual voltage available, so parts of the appliance can use 240V and parts of it can use 120V.
 

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