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Battery chemistry and Gen 3

Discussion in 'Model 3' started by brianman, Jun 11, 2013.

  1. brianman

    brianman Burrito Founder

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    I remember extensive discussions (last summer?) about Tesla's bending toward battery chemistry that puts emphasis on storage capacity rather than power output. My recollection is that there's a limit on that endeavor to achieve the performance goals (especially in the P85 and P85+).

    Assumptions
    1. Gen 3 is 25% volume scale-down of Model S.
    2. Gen 3 has similar (maybe identical?) Cd.
    3. Weight scale-down is 25% or more.
    4. Minimum range of 200 EPA 5-cycle.
    5. Acceleration of 0-60mph comparable to Model S 60 kWh (5.9 sec).

    Questions
    A. Using what is publicly known or "comfortably assumed" as available to Tesla today at "production levels" and with "reasonable pricing", are there battery chemistries that satisfy all 5 assumptions at once?
    B. Does the battery volume and weight scale down similarly to 1 and 3, or disproportionately with the rest of the car?
    C. From production cost alone (not design, no software development, no assembly line retrofitting, etc.), how would such a vehicle compare cost-wise to Model S 60 kWh?

    I'm curious to see what "educated guessing" and "napkin calculations" conclude on this one.
     
  2. pr0teu5

    pr0teu5 Member

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    This is what I come up with after a bit of calculating.

    First we need to see the what we can expect from the battery technology going foreword. The roadster used panasonic 2.2 amp-hour 18650 cells while the model s used 3.4 amp-hour 18650 cells or the 85 kWh models. So if we extrapolate that out linearly we would have 4.6 amp-hour cells, or exponentially we would have 5.25 amp-hour cells. To get a reality check we can look at the Envia battery system which claims 400 kWh/kg by using a cell with far less colbalt and nickel in the cathode and more manganese. I don't know if Tesla will use this chemistry directly, the probably won't as I think they'll keep panasonic as the manufacturer, but I definitely see them using as similar chemistry in the future.

    So if an 18650 cell has a mass of around 45g we can calculate the energy density of our exponentially extrapolated cell and compare, to see of they are in the same ballpark.

    (5.25 amp-hours * 3.7 volts) / .045 kg = 431 watt-hours/kg which seems close enough to our Envia estimate to me.

    Now we need to find approximately how many kWh we need to reach our 200 mile epa goal.

    Assuming the car will be 25% lighter than the model s (4600 * .75) = 3,450 pound curb weight, which seems pretty reasonable for a car in this class.

    Next it would get tricky because we would need to know exactly to CD of the car along with it's surface area, along with wheel friction to perform a force balance at a certain speed and get the estimate range, but this would not give us the epa rated range and it would actually require a bit of effort so instead I'll do some quick and easy comparisons between the model s and the roadster.

    Roadster Model S

    epa range = 244 mi epa range = 265 mi
    curb weight = 2800 pounds curb weight = 4600 pounds
    capacity = 54 kWh capacity = 85 kwh
    mi/kwh = 4.52 mi/kwh = 3.12

    so by just assuming that the mi/kwh is linear with weight, which is almost definitely not true but should give us an approximation that isn't totally unreasonable.

    Answer = 4.0 mi/kWh

    200 miles / 4.0 mi/kWh = 50 kWH

    Which seems like a perfectly reasonable capacity for going 200 mi

    Also to accomplish this capacity with the cells we had above you would only need 2,600 or so cells rather than the 7,000 of the 85 kWh model s. Though I doubt even if cells of that capacity were available they would use it in the low end gen 3 model. They would probably use less capacious cells for the 50 kWh and use the high end cells with the same total number of cells in a more high end gen 3 model.

    So my final guess for gen 3 models and capacities are; a 50 kWh 200 mile model for $35,000 or so, and a 75 kWh 280~300 mile model for $45,000 or so.

    As an added bonus since we know the model s 85 has ~7,000 cells assuming they stick with the same number we can find out the capacity of an upgraded model s.

    7,000 * (5.25*3.7) / 1000 = 135 kWh for the highest end model. So assuming that they keep the spacing at 25 kWh intervals for $10,000 each, and that they can reintroduce they cheaper model s as it could now meet their product accretion goals we could also see a model s range that looks like this.

    85 kWh; 265 miles; $52,400 + inflation
    110 kWh; 343 miles; $62,400 + inflation
    135 kWh; 421 miles; $72,400 + inflation

    This may seem a bit unrealistic, but it meshes pretty well with the comments Elon made about releasing a 500 kWh battery and the comments that he made about giving the model s a significant update in 3 years or so with a total refresh in 6 years or more.
     
  3. brianman

    brianman Burrito Founder

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    Welcome to the forum! Nice first post. Still reading it...

    - - - Updated - - -

    Ok, now that I've skimmed it you took a few branches from my original question. In case it's easy to correct, I'd like to see how your numbers adjust with the following corrections:
    1. Today's technology not future technology.
    2. Lower power output, higher capacity cells. The goal is a lighter, smaller car with about the same performance characteristics as a P60. I presume this allows significant flexibility with respect to "Model S class" (quality) of cells and packaging but farther towards "capacity" on the "capacity vs. power output" spectrum than the Model S.
    3. Today's pricing.

    Basically, what I'm trying to get at is -- What would today's Gen 3 look like (viability and price) if the only "knobs" you could adjust relative to a Model S 60 are (1) any of today's chemistry options, (2) drop in EPA range to exactly 200mi., and (3) smaller volume/size vehicle by 25%?
     
  4. pr0teu5

    pr0teu5 Member

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    Well that question is slightly more difficult to answer as I don't know exactly how much tesla pays per kWh for their batteries or any other necessary information to determine price, but what I do know is this. With the 25% weight reduction stipulation, and assuming that my linear approximate is more or less valid you would still come up with the same weight and you would still need ~50kWh or so to get a 200 mile range at that weight. Therefore, if the weight and battery are more or less the same as they would be with future technology so too would be the acceleration characteristics.

    What would change this is the weight that everything besides the battery would have to be, and how expensive the battery would have to be. If we assume that currently tesla is paying $275 kWh, which I cannot confirm the battery itself would cost something like $13,750 in total which is probably too expensive to produce the car that tesla would like to produce for the gen 3 segment. Also the weight of the cells in the battery alone would be around 400 lbs. Note, this does not include cooling, control and other battery components these are for the cells alone so the actual battery is probably considerably heavier.

    So, I think what I'm trying to say is that it is impossible to know exactly what tesla could make on the gen 3 front with today's technology, but what I do know is that their is know way that they could meet all of their goals simultaneously. So even though a chemistry like nano-structured lithium could get them better power density, the energy density is way to low to get to 200 epa in a reasonable weight. Or they could choose to use current 3.4 amp-hour panasonic 18650 cells, but the car would have to be make out of light materials to meet the weight requirements and it would be expensive to meet the 200 epa mile mark. It is hard to say exactly how much such a vehicle would cost not only because we would have to know how much they pay per cell, but we would also need to know how much it would cost to make everything else, which I'm guessing would be highly variable depending on how tesla wants to furnish the car. So while your question is a good one I don't know that anyone could answer it without having more information than we currently do.
     
  5. jdevo2004

    jdevo2004 Member

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    #5 jdevo2004, Jun 14, 2013
    Last edited: Jun 14, 2013
    Here are some quick and dirty guestimates.
    1. Currently the Model S uses 3.1 AH batteries. 3 years from now we can assume that 4 AH batteries to be mass market available and cheap for Tesla to buy.
    2. Currently the 60 kWh version of the model S has an EPA rating of 95MPGe and an epa rating of 208 mile range.
    3. Since the physical size of 60 kWh of batteries is less than 75% of 85 kWh, it is likely that if Tesla build a Gen 3 today, they could fit at least 60 kWh of batteries into it using existing battery technology guaranteeing over 200 mile EPA rating.
    4. Since we know that the difference of weight between the current 85 kWh Model S has an EPA rating of 265 mils and the 60 kWh Model S has an EPA rating of 208 miles we know that the reduction of weight between the two vehicles accounts for a 11% increase in efficiency. The weight difference between a 85 kWh pack and a 60 kWh pack is about 250 lbs. So for every 250 less weight we gain approximately 10% efficiency.
    5. Since the Gen 3 is purported to be 25% smaller than the Model S, but probably uses less expensive and heavier components, we can assume that the Gen 3 body and drive train components will be about 15% lighter than the Model S.
    6. Since we know that individual 4 AH will be virtually identical in size and weight to current 3.1 AH batteries we have about a 25% savings in weight for batteries including packaging components.

    Now with all these assumptions we can get a pretty good idea of how much the Gen 3 will weigh, and what type of battery will be needed to achieve an EPA rating of 200 miles.
    First, a Model S without a battery pack weighs about 3500 lbs. The Gen 3 without a battery would probably weight about 15% less than this so approximately 2950 lbs. Since we have assumed that each 250 lbs loss in weight gains us 10% efficiency right off the bat the Gen 3 will be 20% more efficient from the body weight alone not including the battery.
    Now we need to think about the battery.
    60 kWh is enough to power the heavy Model S for an EPA of 208 miles. That means 58 kWh will power a Model S for 200 EPA miles. Since our Gen 3 is 20% more efficient than the Model S by weight, we can assume that 47 kWh battery in a Gen 3 will achieve a 200 EPA mile rating with current batteries technology.
    But that is not all. Since our future 4 AH batteries are 25% lighter and need fewer batteries than the 58 kWh batteries that the Model S would need there would be an additional battery weight savings of about 350 lbs. This translates to about 14 % more efficiency. So a 47 kWh Gen 3 with current batteries would only need about 41 kWh of future batteries.

    Conclusion: Gen 3 will feature at minimum a 38-43 kWh battery pack to reach the EPA estimated range of 200 miles.

    Have fun figuring out my dirty math!
     
  6. ElSupreme

    ElSupreme Model S 03182

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    I don't think efficiency drops that much with weight. You could have a 2000 lbs car that goes a really long way on almost no batteries with your methodology.

    Think of the LEAF 24kWh battery pack and is good for ~60-80 miles. EPA range of 75 miles. I expect the Gen III to be a little bigger, and a bit heavier than a LEAF. But probably much more aerodynamic. So I think a Gen III could do as well or a little better than a LEAF.

    So I think ~50-55 kWh is what will be needed for a Gen III to get to 200 miles.
     
  7. Darko

    Darko Member

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    Using the same comparative logic, 55 seems too high, as it is only 5 kWh less than the Model S 60 that already gets 208 EPA, especially if as jdevo suggests the cell efficiency increases, thereby creating compounding gains on battery weight through fewer cells per equal kWh rating. 50kWh sounds just about right. If 4mAh cells are in fact used instead of 3.1mAh, this suggest only 65% of the number of cells currently needed for the battery pack - the other components of the pack are more fixed, but it suggests that the battery can start to get reasonable in cost even without efficiency gains in production.

    Of course this might be just the entry level and Tesla offers an 85kWh Model S equivalent - say 70kWh for Gen III.
     
  8. EVNow

    EVNow Active Member

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    If we shift to useable capacity - we are looking at about 20 kWh and EPA rating of 84 miles on 100% charge. So, that gets us to 47 kWh usable for 200 miles EPA rating. Assuming 80% usage, total of 59 kWh or using 90% usage, 53 kWh. 55kWh seems to be a good bet.
     
  9. brianman

    brianman Burrito Founder

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    You're already off-topic at this point. :(
     
  10. vfx

    vfx Well-Known Member

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    I believe Elon said G3 was a 20% scaled down MS at the recent stockholders meeting.

    Regen should be strong again. Apparently the weight of the MS is why it feels less than th Roadster.
     
  11. Alpha

    Alpha Member

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    Do you have a reference for that comment?
    Is the regen "standard" setting the strongest we could get out of a Model S?
     
  12. JRP3

    JRP3 Hyperactive Member

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    I look at it this way. The S is using 3.1ah cells, there are 3.4ah cells available, a 10% improvement. Since aerodynamics are more important for long range driving than weight, and since the smaller G3 should have at least as good a cd as the S with a smaller frontal area I'm assuming better aero. Looking at the Roadster with a poor cd but small frontal area it got 240 miles from 53kWh's, or 220 wh/mi. The same test procedure for G3, (not the current EPA test method), using a 45kWh pack and assuming 225 wh/mi would give it 200 miles. I'm assuming Tesla will aim for 200 miles of "Tesla" range, the same way the S got "300" miles of range. One thing to consider when talking about the 4 ah Panasonic cells is they do not have better gravimetric energy density, just volumetric density. Obviously further battery improvements would lead to better numbers.
     
  13. PopSmith

    PopSmith Saving for a Model 3

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    Since Gen 3 isn't being released for a few more years I hope Tesla will rate it with the current 5-cycle EPA and not the old 2-cycle method, which is what Roadster and the "300-mile" Model S numbers were based off of. Using methodology that was replaced almost a decade ago (by the time Gen 3 is released) would be a poor decision on Tesla's part, in my opinion. It's better to use the more realistic 5-cycle numbers, since it's closer to what the cars achieve in the "real world".
     
  14. deonb

    deonb Active Member

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    Elon did start to refer to the Gen III as needing "200 miles of real range, regardless of weather and driving style".

    Which is pretty much on par with a 85kWh Model S today.

    I think they'll however ship with 200 miles of ideal range if that is what it takes to launch on time.
     
  15. JRP3

    JRP3 Hyperactive Member

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    I would not be surprised if they exceed my expectations, and I'm purposely being conservative with my projections, looking at what could be done with minimal battery advances.
     
  16. ElSupreme

    ElSupreme Model S 03182

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    I fully expect the Gen III to have 200 miles of real range. But not the base battery. The base battery will have 200 miles of ideal range.

    I don't think that is inconsistent with what has been said.
     
  17. brianman

    brianman Burrito Founder

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    My read...
    1. Technically they can get away with 200 miles ideal range and still be within the spirit of what he said.
    2. Elon wants 200 miles (EPA 5-cycle) rated range.
    3. Customers want 200 miles of real range.
     
  18. favo

    favo Model 3 Reservation Holder

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    Call me greedy, but I want 300 miles of real range for Gen III. (Or full charge in <15 minutes.)
     
  19. Vger

    Vger Active Member

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    Reading though all of these reasonable back-of-the-envelope musings, as both an owner and an investor, I am really smiling. To me this indicates that the feasibility of the major design goals of Gen III is really not in doubt, and not far off.

    So that means to me that the main thing that the Tesla team has to figure out is how to build these things at the target price at 10x the rate as the Model S, with 10x scaled sales and service capabilities, and still make a healthy profit. It seems like the major task for them in the next 3-4 years is not so much vehicle engineering but process engineering.
     
  20. Hache

    Hache Member

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    I fully agree with you !
     

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