Charging shows +1a more than amp limit set

[EVer]

Hi all

I use a 6-20 setup at home and wondering why the amps show 17/16a instead of 16/16a

It shows up in car screen and through app and I’m wondering if it’s really pulling 17amps

My voltage drop seems about right given I run 40ft of 12awg copper wire for the outlet. My 120v line when I was charging at 12a also see a 2v drop down to 118v so I’m not surprised to see 236v.

Also the charging rate I’m getting is 14mph/23kmh but I see some posters say they get as high as 18mph on 6-20 setup but they don’t state their voltage so I wonder if it’s cause they have higher than 240v and potentially are on 3 phase 277v

Thanks

 

[gfunkdave]

Not sure about your particular case, though I remember someone posting recently that their car was charging at 33A instead of the standard 32. But as I recall voltages above 240 nominal are not really supported. Some Teslas have a hard limit of about 280V and "277" can be above that.

277 is actually single phase! The 3-phase voltage there would be 480. In Canada, 347/600 is more common I think.

 

[Irata]

It could well be just a tolerance and rounding issue.

For example... if it has a tolerance when measuring the current of +/- 0.5a, then 16.5a might be measured and that rounds up to 17a on the display.

It's very likely still limited to an actual 16a.

 

[EVer]

Hey thanks for calling out precious threads

I read up on those at 33/32a or 34/32a threads

Nothing really concrete about why the amps drawn are higher than the set limit (especially because the UMC was supposedly limited to 32a so going over by 1-2 amps means the UMC can handle a bit more)

I have a feeling the voltage plays a bigger part in the charging speed, especially since I read some 6-20 setups were hitting 18mph vs Tesla stated 15mph (of which I am hitting only just over 14mph)

 

[Irata]

The voltage definitely plays a part, volts x current = power.

More power going in = faster charging

 

[EVer]

The voltage definitely plays a part, volts x current = power.

More power going in = faster charging

So wouldn’t I get better results than 14mph if my watts are higher?

My situation 236v x 17a = 4012 watts = 14mph

Ideal situation 240v x 16a = 3840 watts = 15mph per Teslas own chart

I’m not too concerned about the charge as it’s faster than 5-15 and didn’t cost me a lot to install but more curious to understand it

 

[Jeremy3292]

I’ve seen this many times on the app as well with many different chargers. don’t think it’s any cause for concern.

 

[AlanSubie4Life]

Also the charging rate I’m getting is 14mph/23kmh but I see some posters say they get as high as 18mph on 6-20 setup but they don’t state their voltage so I wonder if it’s cause they have higher than 240v

No probably not, see below.

so I’m not surprised to see 236v.

I have a feeling the voltage plays a bigger part in the charging speed, especially since I read some 6-20 setups were hitting 18mph vs Tesla stated 15mph (of which I am hitting only just over 14mph)

For sure voltage & current make a difference in the charging speed, and that 17/16A makes a small difference, whatever the reason for it (maybe 3-5%).

So, getting back to the charging rate, your avatar in your picture suggests a Performance Model 3 (I see red brakes and a spoiler) and that's the reason for the slower charging. Likely a 2021 given your join date.

Your battery, the "82.1kWh" version is about 80.7kWh, 95.5% of which is distributed over 315 rated miles. So each displayed rated mile contains 0.955*80.7kWh/315rated miles = 245Wh/rmi. This is the largest energy content per rated mile of any Model 3.

At your charge rate, the vehicle charging is about 85% efficient probably (best you can do at 240V/48A is about 88-89%, directly from Tesla provided information).

You're charging at 17A*236V. (maybe it is 16.5A, who knows)

So 17A*236V*0.85 /245Wh/rmi = 14rmi/hr. (3.4kWh/hr into the pack)

The reason others charge "faster" is that they have different vehicles, and their displayed rated miles contain less energy. So they'll charge faster (in miles) even though they're charging at the same rate (in energy).

You can do the same calculation for any vehicle and it always comes out about right. You just have to know how much energy is in each rated mile for that particular vehicle type.

For example, for the SR+ NCA (194Wh per displayed rated mile) for your exact setup, it would charge at 18rmi/hr (17.6rmi/hr). (3.4kWh/hr into the pack)

 

[EVer]

Wow thanks @AlanSubie4Life and yes it’s a 2021 M3P

That’s a great explanation for the different charge rates
Wish there was a better table online showing the different charge rates by model using the formula you showed

I assume then once I swap over to 18” for winter, the car will recalc my energy use with smaller wheels and the charge rate will diff again (I assume the 245wh/mi will be slightly lower which then shows up as higher mph charge but still pumping in 3.4kwh into the pack, though maybe less given it will be colder too)

I did see this person on 16a charging get 78km/h but that’s cause they are on 3 phase

https://www.reddit.com/r/TeslaModel3/comments/q1oo7c

 

[AlanSubie4Life]

I assume then once I swap over to 18” for winter, the car will recalc my energy use with smaller wheels

It will not. This is a constant for your vehicle. The only exception is for 2020 model year Model 3 P vehicles, which do update to three distinct different values depending on which wheels you tell it you have (it has nothing to do with the actual wheels you have - what you tell it is all that matters). This value would change only if your displayed vehicle range depends on your wheel selection (which, with that only exception, it does not).

 

[EVer]

It will not. This is a constant for your vehicle. The only exception is for 2020 model year Model 3 P vehicles, which do update to three distinct different values depending on which wheels you tell it you have (it has nothing to do with the actual wheels you have - what you tell it is all that matters). This value would change only if your displayed vehicle range depends on your wheel selection (which, with that only exception, it does not).

Thanks again

Can you share where this original source is from?

Wonder why for 2021 M3P it would not recalc the efficiency if I moved to a smaller wheel and selected it on the screen config (moving to 18” R241s for winter use)…seems odd that all the other years and models would reboot upon new wheel selection and recalc efficiency

At least I know I would get better range with the smaller and lighter wheels even if the car doesn’t acknowledge the efficiency

 

[AlanSubie4Life]

Can you share where this original source is from?

Just go to fueleconomy.gov. You’ll see there that there are three different wheel versions of the Performance for 2020.

So when you change the wheel config to match, they will match the rated range indicated there (299, 304, 322).

seems odd that all the other years and models would reboot upon new wheel selection and recalc efficiency

They don’t all do that. Just the 2020 Performance Model 3 behaves this way.

They’re just using a different constant value for the calculations. Nothing specific to the vehicle equipment, driving style, or anything else. It’s just a different hard-coded value being used.

And yes you’ll get better range with more efficient wheels and tires. Because you’ll use less energy. But for this it doesn’t really matter what the car says - all that matters is the specific vehicle equipment, driving style, and the energy left in the battery (which is indicated directly by the range remaining number).