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Engineers: Lowest Wh/mile possible for Gen3 at 55mph? Battery pack size

EVNow

Well-Known Member
Sep 5, 2009
15,001
37,451
Seattle, WA
Because it will be narrower it will have a new battery pack format. I'd guess it will start with a 50kWh pack and achieve 20x EPA range.
That 50 kWh has to be useable - so, atleast 55 kWh full pack.

Comparing i3 & Leaf shows that the gain from using light weight materials is rather minimal on EPA ratings. Tesla will be better off aiming to reduce drag rather than weight.
 

stopcrazypp

Well-Known Member
Dec 8, 2007
12,646
8,538
That 50 kWh has to be useable - so, atleast 55 kWh full pack.

Comparing i3 & Leaf shows that the gain from using light weight materials is rather minimal on EPA ratings. Tesla will be better off aiming to reduce drag rather than weight.
Yeah. For all the kudos the i3 gets for light weight materials, the effect isn't too huge (~5% better than Fit EV for a ~15% weight difference).

BMW i3 at 2799 lbs: 124/138/111 MPGe (combined/city/highway)
Fit EV at 3252 lbs: 118/132/105
Leaf at 3243 lbs: 114/126/101
85D at 4647 lbs: 100/95/106

Interestingly, the 85D actually is more efficient on the highway cycle than the Fit EV and Leaf. Also, surprisingly the Leaf is lighter than the Fit EV; I guess most of the Fit EV's advantage is due to aerodynamics.
 
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ItsNotAboutTheMoney

Well-Known Member
Jul 12, 2012
12,179
10,563
Maine

Ah great, this site now has somebody who'll be able to take construction photos when the Bangor Supercharger is being built. :)
I should be able to get some Augusta photos once I find out where the chargers are. (Nothing on the Augusta Planning Board meeting agenda for March).

The non-tractive loads need work. Particularly in the cold extremes where pack and cabin heating consume considerable amounts of energy. It may be that integrating a non-combustible insulating layer into the battery pack, and improving cabin insulation (and glass coatings) will pay dividends. It would help in the heat too, keeping exterior heat from soaking back into the pack and cabin once it's been removed. There are probably efficiency gains available in the heatpump/ac units too.

Yes, this is a big deal. In the Model S 60kWh to 85kWh is about an $8k upgrade (deducting the $2k Supercharger fee for an MS60). I really hope that for the Model 3 Tesla will produce an "intermediate" pack, understanding that with the Model 3 potential buyers will be more cost conscious and might just need a little extra. I recognize that in places like Maine they'll have shorter spacing between Superchargers, but extra energy take extra charging time so an intermediate battery could help in two ways.
 
Yeah. For all the kudos the i3 gets for light weight materials, the effect isn't too huge (~5% better than Fit EV for a ~15% weight difference).

BMW i3 at 2799 lbs: 124/138/111 MPGe (combined/city/highway)
Fit EV at 3252 lbs: 118/132/105
Leaf at 3243 lbs: 114/126/101
85D at 4647 lbs: 100/95/106

Interestingly, the 85D actually is more efficient on the highway cycle than the Fit EV and Leaf. Also, surprisingly the Leaf is lighter than the Fit EV; I guess most of the Fit EV's advantage is due to aerodynamics.


I took the published dimensions of the cars and made a model that approximated the highway mpge from CdA and mass ( dominated by CdA ).
( The model was correct within 0.5% for all the cars except the Fit, where the error was 1.25% )
Then calculated a theoretical Model 3 car - a car that is exactly the same shape as the Model S ( same drag coefficient ) except that it is much narrower at 1800 mm wide and 1435 mm tall and weighs 3700 pounds.
The model predicts it will take 49.8 kWh of battery to achieve 200 miles EPA highway range for that car.


Data from random googling ( in addition to mass and mpge data from above ):
BMW i3
1775 wide
1578 high
.29 Cd

Tesla
1963 wide
1435 height
.24 Cd

Leaf
1770 wide
1550 high
.32 Cd

Fit EV
1720 wide
1560 height
.30 Cd

Notice that all those cars are much narrower than the Model S but are ridiculously tall and have terrible Cd ratings.
 

stopcrazypp

Well-Known Member
Dec 8, 2007
12,646
8,538
So Fit is bad but Leaf is really bad. That seems to explain the Mpge numbers.

What is frustrating a little is that range is primarily important for highway driving. Tesla gets this but the EPA does not. The Leaf gets an 84 range but that isn't realistic at highway speeds but getting over 100 in town is pretty easy. The algorithm that the EPA uses is flawed (IMO) for EV range. This is especially true when you get to 200 miles. What the buyer wants to know is can I make it to a supercharger or my parent's house etc. Not whether they can be a taxi.

I think everyone expects the Model 3 to be aerodynamic. But has anyone done the math on a smaller model S and what the cabin space would be like? Luggage space for the S is fine (better than fine) but rear seat headroom is tight. Are we expecting a really tiny back seat? The aerodynamics on a smaller car are harder. So it might be asking a lot for the Model 3 to have the same aerodynamics as the S (not talking frontal area just Cd). More power to them if they keep the Cd the same, but the EPA won't reward them appropriately for doing it.
 
Yeah. For all the kudos the i3 gets for light weight materials, the effect isn't too huge (~5% better than Fit EV for a ~15% weight difference).

It may take more energy to get a heavier car going, but you recapture most of that energy when you stop and it matters almost not at all for highway driving. Energy lost to wind resistance is lost forever.
 
It may take more energy to get a heavier car going, but you recapture most of that energy when you stop and it matters almost not at all for highway driving. Energy lost to wind resistance is lost forever.

Rolling resistance increases in proportion to mass (and speed), you're continually bleeding away energy to it. Yes, aero losses grow more quickly with speed, but it is a continual drain. A focus on lightness pays dividends in handling, acceleration, and energy needs. They really can't afford NOT to consider an aluminium intensive structure in order to minimize the M3 battery size (battery cost for target range is still their biggest challenge in meeting the $35k target price), and really ought to consider carbon fiber IMO!. If nothing else it makes building the car hugely simpler with the potential for a radically lower body parts count, and simpler handling requirements. Material handling and assembly costs matter too!

Perhaps save carbon fiber for the Model 3.5 "adventurous" version, it's risky and they've no in-house experience with it...
 
Wheels & Tires
The Model S is a big heavy performance car and starts out with 275/25ZR22 tires. The basic Model 3 will have perhaps the same as the base BMW 320i, which I believe is 225/50VR17 tires – so, a lot less rolling resistance, and somewhat improved aerodynamics, than Model S. Tesla will likely be lobbying Dunlop, Pirelli etc. to come up with a special tire for their needs – ultra-low rolling resistance – for the OEM parts.
I totally agree with you that Tesla will create aerodynamic wheels and that these shall be the standard equipment on the base car. Great way to increase range by a few percent, and will pay for themselves (as far as Tesla's costs are concerned) by shrinking the size of battery required to achieve EPA200 by that same few percent. Other wheels will be available for extra money.

Since when has the Model S sold with 22" rims/tires?

Looking at tire rack I see 2014 model S with either 245/45-19 or 245/35-21 (with possible 265/35-21 for staggered rears).

As to Model 3 and saving on rolling resistance only first number matters the most. You can have super narrow 15" tires or super narrow 21" tires. They'll look different and cost different but can be made to roll the same.

I agree that they'll likely drop the wheel size down to 17" stock and 19" optional but the key will be to get the narrowest tire you can on those 17" or 19" rims.

It'd be nice to see a 185/xx/17 or 215/xx/19 tire used but I'd even be happy with something more like a 205/xx/17 or 225/xx/19.
 
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Two ideas to increase efficiency, the second one a bit "out of the box":

1.) Temporary/rentable battery modules How about if one could buy a minimal size battery pack (say 30kWh), which might be all one needs for most of the year, and then be able to temporarily rent one or two additional packs/modules that could be simply added to the existing pack (a la RAM to an laptop in years gone by), if one needed to take a longer trip. At the end of the trip, the extra weight and rarely-used capacity of the two packs is returned, and a certain fee per day is charged for their use.


This would eliminate the problem of hauling around a lot of unused weight most of the time (decreasing Wh/mile), but still enable long trips to be made using the superchargers, etc. The added packs would need to be nearly identical in geometry to the "base" pack, so that their addition would not change the center of gravity, etc. But I don't think that would be a problem for Tesla's engineers if it were designed into the car from the get-go. The installation should be "plug and play" -- taking only a few minutes to do, with all of the electronics and software automatically recognizing and adjusting to the new capacity. (i.e., no special knowledge or action would be required of the user/owner/driver.)

2.) "Dynamic aerodynamic" frontal area transformation. Sorry for the awkward phrasing, but what I'm imagining (and have mentioned elsewhere) is the front of the car automatically changing to a more aerodynamic shape at higher speeds. I'm not sure how much could be gained from a simple system that made the grill area more "pointy", but if was non-trivial and could be done with some simple mechanism, it might be worth it. Of course it would all be automated -- extending above 40mph (say) and then retracting as the speed came down, 100% collapsible, etc.

Thoughts?

Is #2 that crazy? Even if not, I suspect that it might violate one regulation or another, if replacing mirrors with cameras is so controversial and difficult.

But I see 'battery module renting' far more practical and possible, and I'd love to see it offered. Besides improving overall Wh/mile efficiency, it would make the base model that much more affordable. Those who don't want to mess with renting and returning capacity would still be able to buy 60 or 90kWh versions, of course.
 

GregRF

Squirrel Power
Supporting Member
Jul 22, 2014
521
1,054
CA
2.) "Dynamic aerodynamic" frontal area transformation. Sorry for the awkward phrasing, but what I'm imagining (and have mentioned elsewhere) is the front of the car automatically changing to a more aerodynamic shape at higher speeds. I'm not sure how much could be gained from a simple system that made the grill area more "pointy", but if was non-trivial and could be done with some simple mechanism, it might be worth it. Of course it would all be automated -- extending above 40mph (say) and then retracting as the speed came down, 100% collapsible, etc.

Thoughts?

Not going to get a whole lot of improvement out of the front. Being able to extend out the kamm tail of the back could give significant gains.
 

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