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Realistic usable energy in 85kWh pack

Discussion in 'Model S: Battery & Charging' started by JohnQ, Apr 20, 2014.

  1. JohnQ

    JohnQ Active Member

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    I'm 12 months and 17,000 miles into my ownership with an 85kWh 'A' pack. Was originally 100% charging to 269. After completing my second 2,000 mile road trip yesterday, my most recent 100% charge is now 250 miles--on a 30A charger in Asheville, NC. I took a snap of my energy meter at the end of the Asheville to Burlington leg to determine my usable energy. Kind of like usable fuel in an aircraft. My preferred limit is 10 miles rated remaining. Ignore the Wh/mi efficiency, it was cold and rainy with a 10mph headwind.

    Assuming the energy calculation is accurate, I have 67.7kWh available of the total 85kWh (roughly 80%). My best case driving (no wind, dry pavement, 70F, 68mph over relatively flat ground) is 295 Wh/mile--this is based on my experience. That gives me 230 miles to work with under ideal conditions if I fully charge.

    Does anyone have a feel for whether the displayed energy usage is accurate?

    Energy usage Burlington.jpg
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    It's an estimate.
     
  3. Kraken

    Kraken Member

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    No. I doubt it. I am not as sure about the energy consumed, but as for the energy added to the car when charging I do not believe it is accurate, or at the very least it isn't consistent from one car to the next.

    More to come...
     
  4. SCW-Greg

    SCW-Greg Active Member

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  5. JohnQ

    JohnQ Active Member

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    Presuming the chart is correct, then I am "missing" approximately 5kWh (67.7kWh plus 10 miles remaining at approx 295Wh/mi=70.7kWh or 71kWh vs. the 75.9 on the chart). The inevitable question is "where's the remaining 5kWh? Is that degradation or algorithmic problems?

    Now, the chart could be wrong, the energy used since last charge could be an estimate subject to the same estimate issues that rated range has, both of those, or some other item, of course. But it matters, because I need to be able to accurately estimate how fast I can drive to make my destination given mileage, weather and terrain. Driving 63 mph in an area where the speed limit is 70 and people drive 80 creates some risk. If I can really drive 68 or 70 then I would do so.
     
  6. apacheguy

    apacheguy Sig 255, VIN 320

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    Really? Energy usage should be measured exactly, just as a Kill-A-Watt is able to meter the power draw of a home electronic device. However, SOC and rated miles are based on estimates of the cell voltages and total capacity among other things.


    Bluetinc concluded in another thread that 0 miles = 0% is triggered at the same pack voltage as in previous firmwares. Thus, there is a below 0 buffer and that is likely where some of the remaining kilowatts reside.
     
  7. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Energy usage != battery capacity.
     
  8. tezco

    tezco Sig P85

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    I think the energy used gauge is accurate, but how that energy converts to miles driven is the problem. Sometimes 67 kW = 230 miles; with slush on the road, strong headwind, below 0 temp you can kiss that 230 miles goodby. Unfortunately, miles left is the answer we seek. We just don't notice the same limitation in our ICE cars since we carry around so much energy. If we drove around with a 5 gallon tank, you'd hear the same complaints. My Audi shows wildly fluctuating range figures when the tank is nearly empty. When the engine is cold, 5 miles left. After 5 minutes I suddenly have 60 available.
     
  9. apacheguy

    apacheguy Sig 255, VIN 320

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    Precisely, but the OP asked if the displayed energy usage is accurate. I argue that it is.
     
  10. JohnQ

    JohnQ Active Member

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    That's exactly right. If I know that, after a range charge, I have 67.7 kWh until my pack is roughly 4% away from reading 0 rated miles (10 rated miles in this case) then I can more effectively plan my travel. It also gives me a better sense of the degradation I'm experiencing as I can track that available energy relatively easily. The big caveat to that is whether more energy gets "hidden" below the 0 rated range mark.

    In either case, my effective degradation would still be measurable as I won't be traveling below 0 except in the most dire emergencies. For example, 6 months ago I used 68.9 kWh and had 14 rated miles remaining. That's about an extra 8 rated miles I had at that time.
     
  11. LuckyLuke

    LuckyLuke Model S P85DL

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  12. apacheguy

    apacheguy Sig 255, VIN 320

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    Interesting. Still, 73.5 usable kW seems rather low for an 85. Some time ago someone got circa 77-79 kW on a full charge.
     
  13. SCW-Greg

    SCW-Greg Active Member

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    Not sure this factors in the 85kWh number, but you are never really allowed to charge the batteries to an actual full 100%. Voltage max is limited to just above 4v/cell. This ensures a battery life well beyond 10 years, if not 20+.

    There's some other threads on this subject (that I'm just too lazy to look up right now).
     
  14. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    It is, except... The exceptions are things that happen when the car in not "On" and ready to drive. These include general vampire loss, preheating/cooling from the App, preheating/cooling when you open the door or sit in the car, but don't "start" it, etc.
     
  15. JohnQ

    JohnQ Active Member

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    Alright, let's assume that departure occurs within a minute or two after the "Charging Complete" item comes on (which I did in each instance of the comparison). And, as @SCW-Greg mentions, we're likely never truly hitting full voltage capacity of the batteries to preserve longevity. It still appears that 6 months ago I had an extra ~2.4kWh accessible to me after charging to 100% on the slider and before hitting 0 rated range.
     
  16. Mario Kadastik

    Mario Kadastik Active Member

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    I raise your 73kWh and bet 77.8kWh :)

    1557244_470278563105941_301007228_o.jpg

    After driving that (in cold weather) I had been going with Charge now for ca 12km before arriving home and from the battery gauge of ~4% left I estimated that I could have gone another ~15km before being flat out of electrons. So doing the math of 96% = 77.8kWh means that 100% equals 81 kWh or exactly as per the chart given earlier in this thread that indicates 81.1kWh from 100% range charge. That's what I was going on as well and wasn't worried as I know I'd reach home at around 78kWh hence had ca 3kWh left as buffer. The car started to limit all kinds of things and when I turned towards home off the highway it lowered the AC and I was at about 100kW power limiter, but still going ;) I've since driven past the 0 point a couple more times, now with less kWh total that got me there that most likely are due to the range algo issues that supposedly were fixed in 5.9 that had hidden more and more range below 0, but I wouldn't know as I've not range charged with 5.9 yet. My 80% charge went from 292.5km to 316km with the 5.9 upgrade so definitely had some range hidden below 0.
     
  17. David99

    David99 Active Member

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    When I do a 100% charge, it shows 272 rated miles. The rated miles are based on 300 Wh/mile. Based on my experience when I drive at 300 Wh/mil I'm getting exactly the that range. Doing that math that would mean I can get 81.6 kWh out of my battery. I should mention that I have no intention to test this out, though :)
     
  18. JohnQ

    JohnQ Active Member

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    Why not? Rated range isn't until the car stops moving, it's how far down to 0 rated range. Go for a drive down to 10 miles and see how much energy you use. My battery now has 67.7 kWh available between 100% charge and 10 rated remaining (my preferred lower limit of available range before charging).
     

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