L2 charging,,, Car pulls more amps than the set amp limit

[STS-134]

Agreed, software would not be doing what software is supposed to if this is the case. Especially strange since the UI is showing the correct EVSE and incorrect charger values. It may be the vehicle has an internal check that it would fault on, but EVSE the is tripping first.

It's almost like Tesla still hasn't learned the lessons from Therac-25. The limit should be enforced by hardware but also reported to software if the software attempts to exceed it.

I'll experiment with it when I get the SOC down at some point. Anyways, if this is a common issue then I'd expect others to report similar problems.

Not a good idea to ever assume that others will do it.

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[mongo]

It's almost like Tesla still hasn't learned the lessons from Therac-25. The limit should be enforced by hardware but also reported to software if the software attempts to exceed it.

In the worst case a breaker trips, same deal as a HW failure.
Look into @wk057's reporting on SUA if you're concerned Tesla doesn't use HW checks on critical functions.

 

[wk057]

The chargers handle rates < 80A reported by the EVSE as a proportional reading at about 0.1A divisions so that active EVSEs can report available power in better than 1A divisions. If the EVSE is not reporting properly, it could be like 16.4A or something. The car is generally allowed +/- an amp of wiggle room from the reported value.

Tesla's UMC v1 will report 15.5A available on the pilot signal from the EVSE, for example. Most others tend to round down as well in my experience.

So if the EVSE is reporting 16.4A available, and the car is pulling 16.5, it'll show 17A on the display, but still be perfectly acceptable.

Also, the limits set on the display are soft limits. The car enforces them as possible, but still uses the limit of the EVSE as the true limit. It's not the car's fault if your EVSE is misconfigured. For example, the car will blatantly ignore the user set limits in events like having to warm the pack for use in ultra cold climates when the set value is insufficient, but still never go above the EVSE's pilot reported current.

If the EVSE is saying x.xA are available, there should be available and with a reasonable error margin, hence the 80% rule on circuit sizing. 16.5 A isn't going to make or break a 20A circuit.

 

[DuncanM]

In the worst case a breaker trips, same deal as a HW failure.
Look into @wk057's reporting on SUA if you're concerned Tesla doesn't use HW checks on critical functions.

At least I know that the EVSE will act to prevent continuous over current.

 

[DuncanM]

So... I ordered a NEMA 6-20 adapter plug for my Telsa Mobile Charger and I tested it out last night to compare it with my Morec NEMA 6-20 EVSE. I increased the car's amp limit to from 15 to 16 for this charging location and with my Tesla EVSE the car reported 17/16A 237v for about 20 minutes until I lowered the amp limit to 15 and then it reported 15/15A 235V, This is a bit concerning.

 

[wk057]

It's perfectly fine.

If your wiring can't budget for the normal margin of error on a 6-20 continuous load (+/- an amp), then you probably shouldn't be charging a car from it at all.

 

[davewill]

The display of 17/16 is not unusual in and of itself. We see that here from time to time. It seems to just be a rounding issue in either the reading of the pilot signal, or the measurment of the actual current. It was more concerning that your EVSE was faulting because that was evidence that the car was actually drawing more than the EVSE was advertising. That could have been a bug in the car or the EVSE. Since that behavior has gone away after the reboot, I'd just watch out for it happening again. If it doesn't, then I think everything is fine.