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"24+ hours remain" with only a few miles of charge remaining

Benito1283

Member
Sep 25, 2021
51
93
Denver, CO
I've had my MYLR for a week now and am absolutely loving it. I don't drive a ton and am currently using a 120v/20amp outlet (16 amp max) for charging which I believe will be more than sufficient. I typically plug in every evening with my range around 220-230 miles and set charge limit at 80%. I select the amps/current that will end up completing charge around 8-10am. I figure there's no reason to charge at a higher current.

But in the mornings I'm getting some weird readings. Example: the other night I set it at 7 amps which said will complete charging by ~10am. When I woke up around 7am, the range was at 256 miles but said "24+ hours remain" even charging at 3 mi/hr (pic 1). So I turned the current up to 10 amps and it changed to 4hr40min remaining at 4 mi/hr... but there was only 3 miles of range left to charge (pic 2).

It is starting to get into the mid/upper 30s here overnight. So I'm wondering if it using the shore power to heat the battery w/o adding range at all? I assume it is doing what it needs to, and I should just leave it. It think it ends up completing its charge roughly when it estimated, if not a couple hours after. I'm just confused about where my electricity is going and why I'm getting these strange charging ETAs.
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
12,427
14,804
Riverside Co. CA
It is starting to get into the mid/upper 30s here overnight. So I'm wondering if it using the shore power to heat the battery w/o adding range at all?

Yes

BTW, there is absolutely no benefit to "turning the amps down to 7" or anything. ALL home charging is considered slow charging (even 60amp 240V is slow charging for a tesla, because "fast" is supercharging).

Whats happening is you are not adding power fast enough to counter both heating the battery and charging.

TL; DR -- stop turning the charging speed down, on a 120V circuit, there is absolutely, positively, zero benefit to doing so, and as it gets colder, you will likely see much more of this, given that your stated location is someplace with actual winter.
 

Benito1283

Member
Sep 25, 2021
51
93
Denver, CO
Yes

BTW, there is absolutely no benefit to "turning the amps down to 7" or anything. ALL home charging is considered slow charging (even 60amp 240V is slow charging for a tesla, because "fast" is supercharging).

Whats happening is you are not adding power fast enough to counter both heating the battery and charging.

TL; DR -- stop turning the charging speed down, on a 120V circuit, there is absolutely, positively, zero benefit to doing so, and as it gets colder, you will likely see much more of this, given that your stated location is someplace with actual winter.
That's what I figured. Glad everything's working as it should. I will turn up the current as needed. Thanks!
 

Benito1283

Member
Sep 25, 2021
51
93
Denver, CO
To further support jjrandorin's explanation and recommendation for higher charging current (for anyone else reading), it sounds like charging at lower current at 120v reduces efficiency quite a bit. Efficiency (120v) is 75% at 6 amps, 85% at 12 amps, and 87% at 16 amps. With 240v charging the efficiency bumps into mid 90%.
 
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Level 1

Member
Aug 10, 2021
79
70
Earth
TL; DR -- stop turning the charging speed down, on a 120V circuit, there is absolutely, positively, zero benefit to doing so
* unless the outlet you are plugging into is suspect, you are using a thinner gauge or longer extension cord, there are other electrical loads on the circuit you are plugged into, etc.

Basically, in general, keep the charge current higher for maximum efficiency. But there are valid cases where you would want to use a lower charging current on a 120V circuit.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
12,427
14,804
Riverside Co. CA
* unless the outlet you are plugging into is suspect, you are using a thinner gauge or longer extension cord, there are other electrical loads on the circuit you are plugged into, etc.

Basically, in general, keep the charge current higher for maximum efficiency. But there are valid cases where you would want to use a lower charging current on a 120V circuit.

I would counter and say that in that case that outlet shouldnt be used for charging at all, because its not safe. Daisy chained outlets, thinner gauge wire than supported for a continuous load, long extension cords if they are not the proper gauge, all should not be used.

So, no, I disagree. In those cases you state the outlet should not be used, and "turning down the amps" is not a safe alternative.
 

ATPMSD

Member
Mar 12, 2021
593
555
Atlanta, GA
Crank her up to 16 amps, there is no harm in leaving the car plugged in once the charge has completed. If there is a voltage problem the car will lower the amperage value automatically. This typical occurs when there are other devices plugged into the same circuit, which is a bad idea and should be rectified if that is the case.
 

Level 1

Member
Aug 10, 2021
79
70
Earth
Daisy chained outlets, thinner gauge wire than supported for a continuous load, long extension cords if they are not the proper gauge, all should not be used.
I'll politely disagree with your disagreement. 😁

Suspect outlets, sure. Use your judgement.

But the majority of residential receptacles are daisy-chained, as it is just more economical to do so (wiring + circuit breaker + panel utilization + labor costs), and modern code guidance on receptacle quantity and spacing just favors this. Receptacles are designed for this, circuit wiring is sized for it, etc. Unless dedicated for a specific appliance, you're far more likely to find a receptacle that shares its circuit with other receptacles, than one on its own circut. To say that no daisy-chained receptacle could be used, for the load it is installed and rated for, would not be reasonable. Likewise, if a multi-receptacle circuit has a 20A breaker and a 16A continuous load limit, there would be no problem if a 2A fan and a car charger self-limited to 12A were both connected to that circuit at the same time.

I can't tell if we are agreeing or disagreeing on this part... but I'd say that thinner gauge wiring under continuous load and extension cords of "not the proper gauge" are a bit of a misnomer in this case, because what is "proper" is dictated by the load, and our load is configurable. It may be that the cord has a continuous current limit that is lower than the receptacle - and this is fine - so long as the load draws less current than the cord rating. A 16A continuous load on a 12A extension cord on a 20A receptacle is a recipe for disaster. But an 8A load on a 15A-rated extension cord on a 20A receptacle is perfectly within the standards/requirements those cords were certified under.

It's always been the end user's responsibility to manage total electrical loads on multi-receptacle circuits - it's something we do on a day-to-day basis, just not always properly or successfully. (Hair dryers, etc.) 😉
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
12,427
14,804
Riverside Co. CA
To say that no daisy-chained receptacle could be used, for the load it is installed and rated for, would not be reasonable.

If we are talking EV charging, which stresses an electrical system more than just about anything else, being a continuous load , its absolutely "reasonable" to say no daisy chained outlet should be used. A Microwave is supposed to have a dedicated circuit as well, but its not a continuous load, that is used while people are sleeping, for hours and hours at a time. If one wants to plug their microwave into a shared circuit and just monitor it, not a big deal. An EV, not the same thing at all.

If this is something you do where you are (you have your location as "earth"), thats on you, but if we are going to advise people on a public forum who are looking for "correct" answers, advising someone to plug into a daisy chained circuit for EV charging is just wrong.

1. its against code to plug in an EV charger to any shared circuit
2. Its not safe to rely on "turning down the amps" in the car because that is not a fullproof way to guarantee it wont attempt to charge faster.
3. The cost of it going wrong could be a blown breaker, or, a fire in your home while you are sleeping

No, I am not being melodramatic. People normalize electricity but its dangerous if not configured / used properly. EV charging can be specifically dangerous because its a continuous load for a long time. 120V charging by definition would be running almost constantly to try to charge a car with a 60-80kW battery.
 
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