How can multiple charging sessions show >100% efficiency (TeslaFi)?

[9-Volt]

Just curious, but how can I have multiple sessions in a row that show >100% efficiency? I'm assuming there's something wrong with the data? I'm using TeslaFi BTW.

For example, forget the 2nd charging session below, which was only 5 minutes, but the others are over 2 hours, and all >100% efficient?

I don't really care, but just curious as I recently upgraded my service to "30A 125/250v", and wondering how much more efficient it is.

 

[justsomeguy]

Ever get those instances where your car will increase in charge while just sitting? Mine are seemingly related to temperature. I'm guessing that's coinciding with an actual charging event.

 

[camalaio]

TeslaFi has a few disadvantages when reporting this info.

1. Rounded data. Data from the API it uses is rounded fairly aggressively if I recall correctly. Though both the rated KM added and energy added seems more accurate than I remember being available.
2. External system. TeslaFi can't know how the various losses and Battery Management System changes actually interacted to produce a final externally visible number. It's possible you started charging right after a battery voltage sag (e.g. driving hard) or had a large temperature swing (came in from driving to a warm garage, finished charging while much warmer than it started) which caused the estimated percentage to change slightly. Either the "bottom" or "top" of the charge could have been "wrong", even both, perhaps in opposite directions, but ultimately "corrected".
3. Only one system. TeslaFi isn't seeing voltage and current out of the wall and can't measure cord losses between the wall and the car.

You're experiencing a bit more cold recently, I wonder if the second point I made is having some influence.

 

[9-Volt]

Ahh, didn't realize those were estimated numbers based on what the BMS is reporting. I think your #2 makes sense. I'm usually hammerfesting it home up a hill before immediately plugging it in to charge, so it sounds like that's messing up the BMS/TeslaFi.

By that same token, I assume TeslaFi is calculating its Battery Degradation Report based on what the BMS reports? I know it's been covered a million times here on other posts, but I couldn't quite figure out whether the numbers produced by TeslaFi are just "guesstimated" numbers from the BMS? In other words, if I drained the battery down to <20% and charged to 100% would it re-calibrate the BMS and show a higher estimated 100% range in the chart? I know I could do that myself to find out, but I really don't have any need to do that. Again, just curious as to what I'm actually looking at with TeslaFi.

 

[p-f-g]

If, for any reason, the car was "offline" when the charge started (for example, TeslaFi stopped polling to allow sleeping) this time will not be used to calculate the used energy.

 

[Tz00]

Also, the "Added kWh" comes from the car directly via the API (which includes two decimal place accuracy). The "Used kWh" comes from TeslaFi's 1-minute sample point data for the duration of the charge.