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supercharge data from cross-country road trip

Discussion in 'Model S: Battery & Charging' started by ljwobker, Dec 8, 2014.

  1. ljwobker

    ljwobker Geek.

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    I finally got a minute to do some analysis on the supercharging data I collected during our cross-country trip (blog here -- ElectricStartupSuperTrip pseudo-blog thread!) and wanted to share it.

    Attached is an excel file that contains VisibleTesla data from the ~30 supercharging stops we made along the way. I'll try to give a brief overview so people know what they're looking at...
    • Column A is the date.
    • B is the rated miles of range reported remaining in the battery
    • C is the reported current into the battery during charging
    • D is the reported charge voltage
    • E is a COMPUTED value (more on this later) that just multiplies current and voltage - unit is kW.
    • F is reported state (percentage) of charge.
    • G is the REPORTED "RoC" from Visible tesla. (More on this later!)
    • H is the amount of time (minutes) between two subsequent rows in the table.
    • I is the COMPUTED rate of charge - I take the delta in reported range, divide by the delta in time. Unit: miles per minute of added charge.

    The original point of this particular exercise was to answer the question I asked here: Model S charge curve as a .CSV or easily readable graph?

    I also added a couple of scatter plots: the "miles per minute per charge" has all the data points, so it's a little noisy. The noisy plots are things like the very first data point where the charger hasn't spooled up to full power, or where there are gaps in the data, or similar crap like that. There's also just enough variance in how the time deltas are calculated to introduce some noise.

    The "charge rate v. rated miles" plot is one where I cleaned up the data by doing things like eliminating the first data point from each supercharging session - to get rid of the initial ramp up error, and then eliminated all values past 240 miles of rated charge since things really really slow down then, etc. But when you just plot the computed charge power (i.e. what's actually going into the battery) vs the state of charge, you get a really really consistent curve that declines as the battery fills up and fits extremely close to the expected values. To sanity check these, I used the data to build a little calculator to answer the question: "If I roll up to a supercharge with XXX miles on the dash, how long will it take to get to YYY miles"? That's the "Timer" spreadsheet. I assumed that when you plug in it takes about 2 minutes for the car to get "spun up" and after that the charge rate tracks the data in the spreadsheet. I spot checked these times with the actual data from my supercharging stops, and it looks really good. To use the timer, just subtract the starting value in minutes from the ending value in minutes. So to go from 40 miles of range to 140 miles of range -- 28-9 = 19 minutes.


    IMPORTANT OBSERVATION: the "reported" rate of charge (col G), or at least whatever VisibleTesla calls that column, has a ton of error in it. I looked at that data for a long time and could not - for the life of me - figure out what it correlated to. If anyone has thoughts on this, I would LOVE to hear them. I plotted this against the battery "fullness" in the "charge rate v. SoC" tab. The data looks like crap, and when you do the math it doesn't correlate to anything. Not realizing this data was bogus cost me a TON of time. Grrrrrr tesla! (I'm assuming that this is an error in the Tesla reporting HTML, and not a VisibleTesla computation error?)

    I hope this is helpful to others!
     

    Attached Files:

  2. apacheguy

    apacheguy Sig 255, VIN 320

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    Awesome! Thanks for posting. I look forward to comparing notes with my SpC sessions.

    First thing I notice is that there are several instances where you are holding 90 kW at or even above 40% SOC. Mine has never held 90 kW above 39%.
     
  3. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Great job collecting the data! Thanks!!!

    Would you mind sharing which version of battery you have and what the firmware version for the car was when you collected the data. This will be important long term.

    Have you considered that the "reported" rate of charge is the average rate of charge for the charging session. This has confused a lot of people. When I looked a few of my Supercharging sessions, I found that the "reported" rate of charge was the average for the session. For example if I plugged in at 0 rated miles, and in one hour added 228 rated miles, then the "reported" rate of charge would be 228 mph, even though my instantaneous charge rate at minute 60 would be 116 mph by your data.
     
  4. Kalud

    Kalud Member

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    From order date and delivery he must have a Revision D
     
  5. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    #5 stevezzzz, Dec 9, 2014
    Last edited: Dec 9, 2014
    Great work! Thanks...

    Looking at the 'charge rate in kW vs. Rated miles' tab I find it interesting that there is a knee in the curve at about 150 Rated miles. To the left of that point, at lower SOC, the ramp is steeper than it is at higher SOC (until you get to the last gasp, above 240 Rated miles). Why do you suppose that is? If anything, I would have expected the ramp to steepen as SOC increases.

    - - - Updated - - -

    Cottonwood's post gives the reason for the lousy data: it's a session average, not an instantaneous value.
     
  6. cjc9er

    cjc9er Member

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    This is very useful to have for planning purposes. Any sense of how ambient temperature extremes might affect these values? I've noticed that when the car is really cold, it can only accept small amounts of current from the supercharger until it warms up. That's why I always now supercharge at my stopovers on winter road trips when I first arrive, rather than waiting to do it the next morning (for fear of tying up a supercharger stall all night). With summer road trips, I can imagine the opposite circumstance might occur, and that the battery would charge faster if given a little time to cool down first.

    Any sense of how these curves might be different in shape or absolute values with a 60 pack instead? I typically see maximum charge power of about 105 kW if I arrive at a supercharger nearly empty in mild temperature conditions.
     
  7. ljwobker

    ljwobker Geek.

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    AAAAHHHHHH! The part about the reported charge rate being a declining average should have been obvious. But thank you for clearing that up - it makes perfect sense (sort of) that Tesla would want to report it that way.

    I also noticed that knee at ~150mi SoC, and don't have a theory for it. I have to assume there's just an inflection point in the charge chemistry of the cells.

    For the 60 vs 85 question, my best guess would be that if you think of this as a "per cell" graph, you'd just have to scale the horizontal axis by 60/85. The charge rate/capacity curve should be exactly the same per-cell for the 60 vs the 85, because they're built using the same cells/modules. So where the knee is at 150 in the 85, we'd expect the same knee around 150*60/85 (105) in a 60kwh pack. (Note, this math probably isnt' quite right because the capacities aren't actually 60/85 of usable, so maybe something like 55/80 or 50/75 is more accurate... but the concept should stand).

    Max charge rate: if a 60 will charge at 105kw when empty, we would *expect* that an 85 would thus charge at more like 150kw when empty... but at present you end up limited by the supercharger infrastructure itself rather than by what the pack can absorb. I think that we've agreed that current top end superchargers are limited to 135kw/cabinet and 120kw per single vehicle, right?

    And (thankfully) I have very little data on very cold charging. Our trip was done mostly in moderate weather, and we almost always rolled into an SC when we had been driving for a while so the car wasn't cold-soaked. The only memory I have of a cold charge would have been on the morning of 11/3 (the "charge data" tab) and nothing there exceeds ~100kw. This was the Toledo OH SC, so it's also possible that this was a 105kw charging unit? (no idea how to find THAT out...?) but I checked my notes and every other night we charged the night before, then stopped for the evening, and just got in the car and drove the next morning.
     
  8. apacheguy

    apacheguy Sig 255, VIN 320

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    FYI, an owner just posted a screen grab showing 110 kW at >40% SOC. I don't see the equivalent of this in your (@ljwobker) data.

    430 mi/hr at the Waco, TX Supercharger

    ---updated---

    Just noticed the OP in that thread is dated over 1 year ago. That's odd.
     
  9. ljwobker

    ljwobker Geek.

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    Isn't 92 rated miles about 75 "rated" miles? Something like that? In which case the 110kw is not too much of an outlier...
     
  10. apacheguy

    apacheguy Sig 255, VIN 320

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    Ah, well I wasn't even looking at rated miles. Generally, when we talk taper curves we talk in SOC since rated miles introduce so much variation from car to car.
     
  11. ljwobker

    ljwobker Geek.

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    My post above should have said: isn't 92 *ideal* miles the same as ~75 "rated" miles. Hopefully it's clear what I meant ;-) But separate from that...

    I'm confused. In my data, SoC and rated miles are exceptionally tightly correlated. The r-squared value for rated miles vs. SoC (at least as reported in the VisibleTesla data) is 0.0004 (i.e. one-twenty-fifth of a percent).

    Are you saying that different cars report different ratios of SoC to rated miles?
     
  12. apacheguy

    apacheguy Sig 255, VIN 320

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    Exactly. Loads of info on taper curves in this thread if you're willing to sort through everything else:

    Older Teslas limited to 90kW Supercharging
     
  13. ljwobker

    ljwobker Geek.

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    OK, I don't think that's what I was asking. On my car, one "percent" SoC is equal to 2.64 rated miles - with virtually zero variance. I understand that different cars may have different max ranges, so that number may shrink a little bit as the battery degrades.

    But I don't see how that's related to the taper curve of the battery, which was the question I was trying to answer (post #8 from @ApacheGuy) ... if we look at my data:

    cap1.png

    My x-axis is in rated miles, so if we adjust the 92 ideal miles to 75 rated miles, and look at the scatter plot, there are lots of values that are in the neighborhood of 110kw. So all I was saying is that I think it's perfectly normal for a car sitting at 92 ideal miles to be charging at ~110kw.
     
  14. ljwobker

    ljwobker Geek.

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    Sort of following up my own thread here... I took the data and built myself a little "charge timer" spreadsheet... maybe it's useful for people along the way:

    Tesla SuperCharge timer - Google Sheets
     
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