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Attempting to estimate Battery Size to fit my requirements

Discussion in 'Reserving, Ordering, Production, Delivery' started by DavidQ, Jan 17, 2017.

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  1. DavidQ

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    #1 DavidQ, Jan 17, 2017
    Last edited: Jan 18, 2017
    OK, time to take a deep breath and write my first post. Here goes.

    I've been trying to clarify in my own mind what I consider to be the minimum range that I need for my Model 3, so that when the times comes I can choose the best battery size. I've been following threads on EPA vs. NEFZ vs. real life, the effects of cold, wind, altitude change, ageing and what safety margin one should leave for each segment of a journey. I've also been following threads on what people think will be the available battery sizes, and the scaling of range against the Model S battery sizes. Lots of unknowns and assumptions, but it's keeping me occupied while I share the impatience obviously felt by everyone else!

    The ranges on the US Tesla site are EPA, whereas those on the German site (where I live) are NEFZ apart from that given for the Model 3 (344 kms being a direct conversion from 215 miles). I've therefore based the following on the EPA estimates converted to kms rather than the numbers on the German site. The EPA/NEFZ ratios for the various models are pretty consistent (0.82-0.85) so I think this is a reasonable starting point.

    I've also assumed battery sizes of 55kWh, 70kWh and 85kWh. If I look at the scaling factors for the Model S between S60, S60D, S75, S75D, S90D and SP100D, and further assume that I want dual motors, then the D models through the S90D scale approximately linearly against the S60 as baseline. I hope this is what will happen for the Model 3, so the EPA ranges for my hypothetical 70D and 85D are 439kms and 533kms respectively.

    Then I've applied the following factors:

    EPA to typical: 0.90
    Cold weather: 0.95
    Wind and altitude: 0.95
    Ageing: 0.95
    Trip Safety Margin: 0.85

    In order to find the effective range. Not all of these will need to be applied for a particular journey, but together I hope they determine the worst case degradation that I should experience. This results in the attached spreadsheet.

    Back to my original motivation, which is to determine the minimum distance I think I need to be able to go without stopping for a recharge. Once a week we make a round trip in the evening of just under 300 kms, being about 30mins of "landstra├če" and 1:15hr of "autobahn". Since we are generally teaching and the traffic is sometimes heavy, we don't want to rely upon an intermediate recharge on the way there, and afterwards we just want to get home. We can also ensure that the car is 100% charged when we leave home. We make other longer trips during the year, but for them intermediate recharges are fine (especially since I normally stop every 2 hours). So this determines my minimum distance. Looking at the spreadsheet, this means that I really need to go for the 85kWh battery size, although there is quite a bit of additional headroom if my hypothetical battery sizes are incorrect - probably a 75D would be sufficient.

    Out of interest, we also want to be able to tow my "toy" (which is what I've chosen as my avatar), which weighs about 360kg and sits on a trailer with a combined weight of less than 750kg. I really hope that the Model 3 will new able to tow this weight. The Model X is too large (and expensive) for us, so I'll really have to rethink the Model 3 if it's towing capacity is too small (mind you, I haven't seen any other EV that can tow). Based on our experience towing this with our existing ICE car, I've assumed a 0.70 degradation factor for these long trips. That's based on driving at typically 125-130 kph on the autobahn when not towing, and at 80-90 kph when towing (the German speed limit for my trailer is 80kph). As you can see from the table, that still gives me a reasonable range when towing, at least greater than my self-imposed 2 hour limit.

    Any thoughts?

    DavidRange Spreadsheet.jpeg
     
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    • TrevP

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      Hi and welcome to the board!!

      This is the kind of thinking that everyone is wondering about including myself. Given the lack of real information on the details all we can do is speculate at the moment.

      We do know there will be a tow hitch option on Model 3 but what the intent is we don't know. For all we know it could be for bicycle racks alone! Best case it would be maximum of a class 2 towing ability.

      As for range etc the best place to put your money is on the biggest battery you can afford. Initial physical dimensions of the Model 3 compared to a Model S puts the car at about 90% the size so the battery pack is not significantly smaller. We know the new 2170 cells will have about 46% more volume and most likely higher voltages (to handle Supercharging rates above 120kWh) along with newer chemistry and higher engergy density. I think it totally possible given even with the smaller size the packs could go as high as 100kWh but of course those would be expensive.

      You're smart to keep measurements on the EPA testing cycle and convert as those are more strict and realistic numbers. Ideally I'd like the biggest pack I can get to minimize range loss in the winter months. The longest drive I would do regularly would be 300kms and if I could do that on a single charge in the winter without touching a Supercharger then I'd be very content. If not then I'll have to make consessions but so far finances are looking promising to get at least a possible mid range car. We'll be keeping a close eye on developments but calculations are difficult given the complete lack of known and confirmed battery sizes. The only indication we're had is a quote from a Tesla official last year that it would be below 60Kwh (on a base model). Given the recent Bolt range estimates I'm sure Tesla will at least meet those but most likely exceed them.
       
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      • garsh

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        I think it's a great exercise to figure out the minimum size you would require for your regular, long trip. But if you find that you're able to afford it, I will recommend going with an even larger battery than the minimum you require (if offered).

        But, this is from the perspective of somebody who has lived with a Nissan Leaf for the last 5 years. It's usually just fine for my typical commute, but sometimes you need to go out of the way (construction, accident, stop at a store/doctors office that's just a bit off the normal route), and an extra buffer will be really appreciated when that happens.
         
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        • DavidQ

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          Thanks @TrevP and @garsh (is that the correct etiquette for referring to someone?) for the feedback. We've never had an EV before, although I've been thinking of switching my wife's Citroen C1 (7 years old) for a second-hand electric Smart 2-seat soon so we can get some experience - perhaps not too relevant for the Model 3, however! I expect to be going for most of the options, whatever they are. I certainly agree that it makes sense to go for as high a range as feasible, since range anxiety will still be a concern, certainly for the first months when coming to grips with the new world. I've been looking for web sites that offer advice or apps and now have a whole lot of bookmarks to play with. Getting a feel for the best ones will really have to wait until we have something to actually try them out on. One thing I'm having a hard time with is actually finding such things by roaming through this forum, the Tesla one and the TMC one. It would be nice if there were a centralised site acting as a portal to other resources.

          We've just built a new house and garage and haven't yet installed solar panels. The garage has a flat roof of about 50m^2 and the house has well over that, mainly facing south-west. I do have 3-phase 480v "Drehstrom" installed to the garage (not sure of how many Amps though), but haven't started to think about what charge points to install, or whether to install Powerwall(s), both for the house and for the garage. Interesting (and exciting) times...

          David
           
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          • Topher

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            Elon promised a 'Tow hitch'. The bike rack for Model X is called an 'accessory hitch' by Tesla. From that I determine that actual towing will be possible. Weight is the question.

            The voltage of a battery cell is determine by chemistry. That is why AAA, AA, C, and D are all 1.5 volts despite size differences. Best bet is that the 2170 will be the same voltage as the 18650.

            Thank you kindly.
             
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            • TrevP

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              #6 TrevP, Jan 17, 2017
              Last edited: Jan 18, 2017
              There is ample evidence that Tesla has indeed been working on increasing the voltages of their cells.

              Jeff Dahn (who works for Tesla under contract) spoke about it in his video about why Lithium cells degrade but will also speak at the International Battery Seminar & Exhibit next month. He's going to be talking amongst other things that increasing the voltage in essential but proper additives are required to prevent the battery degradation. I think Tesla/Panasonic along with his research as all but eliminated it. This would explain Elon's comment about having the most advanced cell in the industry.

              https://electrek.co/2017/01/16/tesla-gigafactory-battery-cell-production/

              Other evidence of cell voltage increases are from the 60 kWh pack.

              https://cleantechnica.com/2016/08/29/tesla-battery-improvements-tesla-model-3-randy-carlson-dives/

              Lastly, the voltages need to be increased in order to handle higher output from the Supercharger 3 system that Elon mentioned.

              All of this points to the new 2170 cells are being incredibly powerful for their size. Combine that with needing less of them in the Model 3 plus massive economies of scale results in a significant cost reduction on the Model 3.

              When I did my video on the Model 3 hidden details I counted 8 battery modules in the pack. The number of cells is uncertain as the video quality was not great but to me it seems something like 8x16 cells (128 per module) but don't quote me on that. A Model S battery module is 14 x 31 cells (434 + a few extra in a couple of ends for 444 total) with a few extra cm between the rows for coolant lines. 8x16 2170 cells is just a bit smaller than the Model S module which would account for the smaller requirements for the new pack. Surely they have some cooling tech in there that's being testbed by the new 100kWh pack (mentioned by JB).
              Scuttlebutt going around is that not only are the cell higher voltage but also have higher amp/hour ratings (5000mAh each)

              If you look very closely at the image below you will see the 8 modules I speak of. A bit closer look reveals what indeed looks like 8 cells wide in each module.
              Screen Shot 2017-01-17 at 5.06.17 PM.png

              This pictures is the best I can do to clean up the HD broadcast with some filtering so it's extremely hard at counting how many cells deep the modules are. Based on the width of 8 cells I'm saying it's about 16 deep.

              Screen Shot 2017-01-17 at 5.09.21 PM.png
               
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              • Badback

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                Trev, that's 5000mAh. M is mega.
                 
              • Topher

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                Well if Tesla is increasing the voltage of the cells, they certainly seem to be doing it in the manner I described, by changing the chemistry. Voltage is not directly linked with power, but it helps.

                A design goal, does not an innovation make. But moreover I don't see why it is necessarily true. In order to charge a battery cell you need to put a voltage across it that is larger than the nominal voltage. If you wanted a 1000V Supercharger, you would need to split that voltage among at least 270 3.7V Li-ion cells. Surely, the lowest Model 3 will have way more than that (to get 55kWh, you would need 3000 cells, even at 5000mAh). Thus a supercharger is current limited not voltage limited. Now, if the battery chemistry is current limited, upping the voltage probably increases the charge rate some, but the voltage increase in a cell seems likely to be a few percent at most (surface coating the electrode would seem to get 3.8V for example), certainly not going to help much if you want to quadruple the charging power. There is a reason that battery cells are ALL in the low single digit voltages.

                Thank you kindly.
                 

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