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Tesla's Model 3 2170 cells=same energy desity as 18650's

Discussion in 'Model 3' started by George S. Bower, Sep 25, 2017.

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  1. George S. Bower

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    On a volumetric energy density basis compared to 18650's in the 100D

    I've checked my math numerous times and I can't find an error. If the numbers are right it'd really not earth shaking news since the whole idea of the 2170's is to lower costs. If correct it possibly implies that Tesla Panasonic just repackaged the existing 18650 cell into a new form factor and there is no "secret sauce" in the new 2170's.

    The key to the calcs is the usable energy in the EPA report.
    http://www.teslarati.com/wp-content/uploads/2017/09/Tesla-Model-3-EPA-CSI-HTSLV00.0L13.pdf

    usable energy for 2170's in Model 3 is 78.3 kwh in the new model 3 long range pack
    Tesla Model 3 actually has 334 miles of range according to EPA data
    It actually is there more than once in the report on pages 6 and 7 at the bottom of the page it says:

    "Manufacturer Test Comments Internal Test results for MY2017 Model 3 Long Range. Range determined by using SAE J1634 Multi-cycle test procedure. END-SOC 78270 wh."

    also it says:

    "Integrated Amp-hours 222.81
    Average System Voltage 351"

    multiplying those 2 numbers = 78273 watt hours

    Therefore usable energy= 78.3 kwh

    number of cells in big model 3 battery=4416
    Tesla Model 3: Exclusive first look at Tesla’s new battery pack architecture

    therefor usable energy per cell is 78.3/4416=.0177 kwh per cell

    model S 100D pack has 8256 cells and 98.4 kwh usable energy
    Teardown of new 100 kWh Tesla battery pack reveals new cooling system and 102 kWh capacity

    therefore usable energy in one 18650 cell=.01192 kwh/cell

    Therefore increase in usable energy in one 2170 cell is 48% greater than the 18650 (.01192/.0177)

    Volume in 2170 cell is 47% greater than the volume in the 18650 (simple volume calc for 18X65 vs 21X70 cells)

    so energy went up 48%
    volume went up 47%

    basically the same within 1%

    Do these numbers make sense? Can you guys find any holes in this analysis or math errors??

    Thanks,
    George
     
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  2. Thomas Edison

    Thomas Edison Member

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    You can't know how they compare until you can measure the mAh on an individual 2170 cell.
     
  3. jaguar36

    jaguar36 Active Member

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    Using usable capacity is never going to be particularly accurate as it isn't well defined, has changed over time and per model.

    That being said, I wouldn't be shocked if they are pretty much the same. As you mentioned Tesla was far more concerned about lowering the cost of the cells rather than the energy density. Weight is always a problem, but the cost of the 3 was the big issue. If they had a choice between using a fancy new chemistry with a higher energy density, or the same chemistry at a lower cost, they most likely would go with the lower cost. (assuming the higher energy density wasn't so much such that the weight savings would lower vehicle cost further)
     
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  4. George S. Bower

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    Seems fairly well defined and well documented by Jason Hughes. He hacks the numbers right out of Tesla's software in the car

    Here's the link for all the usable kwh's in batteries except the 100D
    Tesla’s hacked Battery Management System exposes the real usable capacity of its battery packs

    Here's the usable kwh's for the 100D
    Teardown of new 100 kWh Tesla battery pack reveals new cooling system and 102 kWh capacity
     
  5. davedavedave

    davedavedave Member

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    I'd add the more important that the volumetric energy density is the specific energy density - i.e., mass. Volume of the pack won't directly affect range or accelerate, but mass will. I've read that the 2170s are lighter by volume than the 18650s.
     
  6. stopcrazypp

    stopcrazypp Well-Known Member

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    Note the useable energy varies based on how large the buffer is (this is unknown in the Model 3), but also on the load put on the battery. The usable energy of the cell run on a HWFET cycle would be different from one run on the UDDS cycle.
     
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  7. jaguar36

    jaguar36 Active Member

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    It was well defined for that model and that software version. That's not to say its the same for others (in fact different packs have different size buffers and I'm pretty sure its changed over time too)
     
  8. kengchang

    kengchang Member

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    Given the same energy density, 2170 should be lighter than 18650 since say every 2-3 2170 is equivalent 4-6 18650 (just for example, don't know the exact number) All the battery casing etc adds up
     
  9. George S. Bower

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    Do you have a reference to back that up? The usable energy should be the absolute floor that the battery is allowed to go to. If you look at Jason Hughes usable and total there usually is only 5% buffer. AFAIK after the usable energy is reached the car shuts down. Prior to hitting the floor (usable energy) the car will go into reduced power.

    Now I suppose you could say that Tesla has just put an arbitrarily low usable energy into the software like on the software limited Tesla's but that doesn't seem to be the case here because:

    Tesla already down quoted the range numbers from 335 to 310. When the EPA tested the car and got 335 miles range it was for the usable energy I quoted. So it seems doubtful to me that the EPA's usable energy number has been purposely lowered by Tesla.
     
  10. stopcrazypp

    stopcrazypp Well-Known Member

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    #10 stopcrazypp, Sep 25, 2017
    Last edited: Sep 25, 2017
    Maybe I'm not making it clear, but to determine improvements in the battery, we care about the total capacity, not the usable capacity. So the fact that the buffer is unknown on the Model 3 makes a significant difference.

    Also the bottom buffer is not based on a fixed percent, but rather on a fixed kWh depending on pack. The larger packs use a 4kWh bottom buffer, the smaller use a 2.4kWh. We don't know this number for the 3.
    Calculate usable battery capacity based on rated miles values

    As for my second point, because of the internal resistance of the battery cells, the measured capacity will vary based on load.
    Using test results of similar 18650 cells, the total capacity of the 85kWh pack would be 82kWh on the HWFET test (6 hour test) vs 86kWh on the UDDS (16 hour test).
    That Motor Trend 1 foot rollout
     
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  11. AnxietyRanger

    AnxietyRanger Well-Known Member

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    Well, the thing is: How much room does the new battery casing take compared to the old? How much or little wasted space is there inside the pack? And how different are the cooling and other support structure needs within the packs? Even if the energy density within the cell is the same, which sounds quite possible to me, being able to get away with less metal etc. within the pack would suggest being able to fit more cells. Also, as the new cells are a bit higher (some feel they'd still fit the Model S/X thouhg), that is bound to add up over the surface area of the pack as well...

    Even if the chemistry is just same-o-same, being able to fit more cells more efficiently can certainly provide gains beyond mere cost savings...

    There is the rumor of 130 kWh Model S/X battery being tested by Tesla...

    WSS.
     
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  12. George S. Bower

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    difference between usable and total for M3 should be 4 kwh.
    redoing calcs based on total instead of usable : .
    energy up 50%
    Vol up 47%


    The usable energy is in the EPA report for the 2 cycles. It didn't change much between the UDDS cycle and the HWY cycle.
    usable energy UDDS=78.273 kwh
    usable energy HWY=77.92 Kwh

    I think we are splitting hairs here. volumetric energy density still looks like it is within 1- 3% of the old cells no matter how we run the calcs.
     
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  13. stopcrazypp

    stopcrazypp Well-Known Member

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    Where did you get the 4kWh for the Model 3?

    My point there is that this means the results done under different conditions for the Model S vs Model 3 cells can't be directly compared (they don't necessarily have the same internal resistance either). If you have the results for the Model S directly from the EPA, then those can be compared, but you can't just take wk057's numbers and expect those to be comparable to the EPA numbers.
     
  14. david_42

    david_42 Member

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    As far as I can tell, the 2170 uses the same (or very similar) chemistry. The form-factor is a trade-off between reducing the number of cells and managing heat. A 2170 is larger, but the Kwh/kg is the same.
     
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  15. AnxietyRanger

    AnxietyRanger Well-Known Member

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    But I guess the question is: how much space does that larger form-factor possibly save (less wasted space?) - i.e. how much kWh could fit in the same/similar space...
     
  16. bonaire

    bonaire Active Member

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    Fewer 2170 vs. 18650 offer:
    - fewer weld points
    - ability to build a pack faster
    - more capacity per-cell (and fewer cells)
    - a 2170 machine can spit out more kWh per unit time with the same # of cells of a 18650 machine

    The capacity add per-cell is appx 46% given the volume of the cylinder. Overall, the square size of the box used for Model 3 should be 30% better in terms of capacity per square meter but cylinder size is only 17% larger. that is using the same NCA chemistry (high capacity, high volatility). NMC would be lower volatility and slightly lower capacity. Until a newer chemistry is used, this change is physics-oriented and this seems to help production speeds. More impact with fewer cells should one or two go bad. The 18650 is a bit more "lossy" capable. 2170 is a better physics choice - but perhaps Panasonic offered 18650 at better price point in 2012/2014 when contracts were signed versus what they can now deliver through new assembly lines and the GF1.

    Volume 18650: 16540
    Volume 2170: 24245
     
  17. AnxietyRanger

    AnxietyRanger Well-Known Member

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  18. apacheguy

    apacheguy S Sig #255

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    This is consistent with what @wk057 has stated. The Model 3 likely uses similar cell chemistry as that in the 100 kWh. That, in turn, is based off of the older 85 kWh chemistry and is less likely to resemble the 90 kWh cells.
     
  19. George S. Bower

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    Do you have a link to where he said that?

    Boy you got me on that one:

    AFAIK the evolution in battery chemistry, anode/cathode design is 85kwh--->90kwh (added more silicone to the anode)---->100 kwh

    Therefore the 100 kwh cells should be the same chemistry/anode/cathode design as the 90 kwh cells.
     
  20. apacheguy

    apacheguy S Sig #255

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