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Tesla Model S 60-kWh Is More Efficient Than 85-kWh Car: Why?


2013 Tesla Model S

2013 Tesla Model S

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Sharp-eyed Tesla Model S buffs may be wondering why the mid-range 60-kWh car, which costs $10,000 less, is rated more efficient than the top-of-the-line 85-kWh model.

The EPA said last week that the 60-kWh Model S has a range of 208 miles, and its efficiency rating is 95 MPGe (miles per gallon equivalent).

That's about 7 percent better than the 89 MPGe rating for the 85-kWh version--which has a rated range of 265 miles.

Measured from a different angle, the EPA says the 60-kWh car uses 35 kWh of electricity to go 100 miles.

The equivalent number for the longer range version is 38 kWh, meaning the cheaper car has about an 8-percent edge.

Why the difference?

Tesla Motors [NSDQ:TLSA], with its penchant for secrecy about technical matters, is mum on the subject (although a Tesla rep recently told me the company was working on an official statement to explain the difference).

Whatever the reason, the numbers make Tesla buyers who've opted for the mid-size battery  (like me) feel a little better. 

The $10,000 jump to the big battery gains only 57 miles of range, from 208 to 265 miles. That's about $175 per mile.

Looked at another way, the 85-kWh Model S uses a battery pack that's 42 percent larger to achieve 27 percent more range. Not a great tradeoff.

There are a number of possible reasons for the better efficiency numbers of the car with the mid-size battery.

*Lighter weight?

Although Tesla has always been cagey about battery specs, it's likely that the smaller battery weighs less. Lower weight means quicker acceleration with the same power, and less rolling resistance at higher speeds.

But it seems unlikely that a weight difference of 100 to 200 pounds--we're guessing here--could account for a 7-percent efficiency jump.  There's something else going on.

Tesla Motors - Model S lithium-ion battery pack

Tesla Motors - Model S lithium-ion battery pack

Enlarge Photo

*Improved battery chemistry?

An unofficial source at Tesla hinted to me that the chemistry of the 60-kWh battery is different--and better--than that of the 85-kWh battery.

The 85-kWh packs went into production more than six months ago, while 60-kWh packs are only now starting to roll off the production lines.

It's quite possible that, in the meantime, Tesla has improved its battery chemistry.

Tesla uses Panasonic commodity lithium-ion lap-top batteries in its battery packs, making them easy to update. Perhaps Panasonic has tweaked its cells recently?

*Test procedures?

The new 5-cycle EPA tests include a more vigorous acceleration profile.

The 85-kWh battery can supply more output power to the Model S drive motor (362 hp vs. 302 hp).  If the EPA test includes maximum acceleration, the 85-kWh car would accelerate slightly faster than the 60-kWh car--and thus use more energy.

If that's the case, the EPA numbers are not a precise apples-to-apples comparison, and the efficiency advantage of the cheaper car is illusory.

2012 Tesla Model S

2012 Tesla Model S

Enlarge Photo

It's also unclear whether the EPA tested a standard 85-kWh car, or the more powerful Performance version (416 hp).

If the test car was the latter model, this test-related discrepancy would be even greater.

*Software changes?

A recent software upgrade tweaked the acceleration profile of the Model S to provide more oomph at highway speed.

If included in the 60-kWh EPA test car, this may have had an effect on the  test numbers--although we would have expected any performance increase to reduce the MPGe number.

As you may have noticed, fevered speculation about technical and production minutia is a daily sport among Model S owners and hard-core Tesla buffs.

With luck, Tesla will soon put us out of our misery on this question.

Until it does, does anybody else have theories why the 60-kWh Model S is more efficient than the 85-kWh version?

David Noland is a Tesla Model S reservation holder and freelance writer who lives north of New York City.

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Comments (20)
  1. I think it is just the lower weight, nothing else. EPA does not use full acceleration while testing 5-cycles. Other ideas seems even less possible.
     
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  2. Until Tesla comes out with their "official explanation", I would tend to go with the lighter weight and test procedures to best explain the better efficiency rating.
     
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  3. I suspect a few things are working together to improve MPGe. In addition to a lighter battery pack and the associated benefits already mentioned, my understanding is that a portion of the battery cooling technology is active. It's just a guess, but the smaller battery may require less active cooling.
     
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  4. Weight is very important, but another important factor would be the maximum depth of discharge allowed for the pack of course. The smaller the pack the deeper discharge EV makers tend to allow but that comes at the expense of battery life. Maybe differences in chemistry make it possible for the 60KWh model to be deeper discharged and retain the same sort of life expectancy as the 85KWh pack.
     
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  5. I think you have nailed all the reasons.

    1. Weight. That is simple physics.
    2. Power/Performance. That potentially can changing the "gearing" or the "direct drive" mapping software. Higher performance models tend to have more "aggressive" mapping of the accerlator.
    3. Larger/more powerful motor. They are rating differently. So, the motor that designed for higher power can have slightly lower efficiency due to trade off, especially at lower power requirement.

    I would have guess that the 40KWh version will have even higher efficiency.

    Even according to Tesla's own estimate, there are efficiency differences. 40KWh is 4miles/KWh. 60KWh version is 3.8 miles/KWh. 85KWh version is 3.5mile/KWh.
     
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  6. Also, note the difference in the motor.

    40KWh verison Peak power is 4,000-10,300rpm. vs 5,000-8,000 rpm for max power for the 60KWh version and 6,000-9,500RPM for the 80KWh verion.

    Also, peak torque are different between those versions as well.

    Top speed are different too.
     
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  7. Nice catch on motor specs, my first guess was weight, but bigger differences in Highway vs. City EPA values seem to suggest otherwise. Typical trend is increased stop/go acceleration in city conditions expend greater energy.

    85kWh — 88 City — 90 Highway — 89 Combined EPA
    60kWh — 94 City — 97 Highway — 95 Combined EPA

    You'd think the ride adjusting air suspension & spoiler would help reduce highway drag on 85kWh Model S? It appears that larger tires (21" vs 19") and performance power inverter and motor tunng are having a countering effect on EPA vs 60kWh edition.

    So it's not just a battery size difference … clearly Model S editions are tuned for performance; either power, or better EPA.
     
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  8. Spoilers normally add drag at highway speeds, while keeping the car more firmly planted when going faster.
     
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  9. "EPA numbers are not a precise apples-to-apples comparison, and the efficiency advantage of the cheaper car is illusory."
    EPA numbers are NOT apples to apples anyway. In its "acceleration" or "hill" climbing cycle, it doesn't specify how "quickly" the car has to accerlate to reach those miles.
    If a car has a "tuning" SW that "detuning" the power during those test cycles, then it can "appear" to be more efficient than it actually is in real world driving.
    I am guessing that is exactly what happens in Ford C-Max/Fusion hybrid case.
    If you want better MPGe, tune to your car's performance to a lower level and see how much MPGe increases...
     
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  10. Here is a detailed page on how EPA test each cars. by reading and comparing each cycles, you will see why "slower" or lower performance cars have a huge advantage in the EPA test cycles...

    http://www.fueleconomy.gov/feg/fe_test_schedules.shtml

    And it will shine some lights on how a "more powerful" car can get worse MPG than a less powerful car in the EPA test cycles.
     
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  11. Also consider the 19 inch wheels vs the 21 inch wheels.
     
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  12. 19" is standard on both versions.
     
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  13. I like the battery size option feature that Tesla has; and, I hope lesser cars can also offer that option; simply put; you can order and pay for only the range you need and save carrying around the additional unused weight of the larger battery; I also think it's mostly the static weight.

    BTW, I propose we add an additional test for EVs; put the car on a spec dyno and run it at 65 mph until the battery output is shut down by the car; I call it the "real world freeway speed range test."
     
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  14. I can't believe the elephant in the room was left out.

    Wheels and tires.

    The 85 kWh car was tested with the big/heavy/sticky wheel/tire combination and the 60 kWh car was tested with the smaller/lighter/more-efficient wheel/tire combination.

    You can see the effects of wheel/tire choice on Tesla's own range estimator and it's very close to the EPA registered difference.
     
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  15. What if fewer battery cells means less energy to heat or cool the battery and more energy to drive?
     
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  16. Electric motors and batteries get hot when juice flows. Th electric motor loses about 7% of it's energy in the form of heat; the battery loses about 2%. 9% in total

    The 85 kWh battery is 500lbs heavier than the 60 kWr battery. The heavy car thus weigh 10% more. So, climbing a hill requires 10% more energy for the heavy car. 10% more juice flows from the battery to the motor. The heat-loss is 10% greater.

    Down the hill, there's 10% more kinetic and/or potential energy to recoup in the heavy car. Therefore, "heat-loss" is, again, 10% greater.

    At constant speed, the wind's drag is the same for both. Heat-loss is 0% greater in the heavier car.

    So, the 7% or 8% difference in kWh/mile is easy to explain
     
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  17. What's your source that the 60 kWh battery is 500 pounds lighter than the 85?

    Because if that's true, it would almost certainly explain the difference. That would make the weight of the car 4100 pounds--about 12 percent less than the 85-kWh car.
     
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  18. Musk is known to have done work for a Phd on supercondensors (Capacitors) and has expressed an opinion that capacitors and not batteries are the likely solution for future electricity storage. Now these have the ability to instantly accept and retain electricity unlike batteries that require varying periods to charge up. If he has sneaked in a few of these then regenerative braking and instant storage ,would prevent the momentum to be lost to heat as our brekes normally do .This saved energy would be instantly available for the accelleration that often follows braking and stretch the available battery cum momentum energy. It seems that graphene based capacitors holds very interesting possibilities in this regard.
     
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  19. Measured from a different angle, the EPA says the 60-kWh car uses 35 kWh of electricity to go 100 miles.
    That is one thirsty car
    My Fluence ZE which carries a 22 KwH pack consistently shows 15-16 KwH per 100 KM which translates as 23 KwH per 100 miles.
     
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  20. I do wonder why they haven't mated the less powerful but more efficient motor with the 60kw/h battery option: this would have given a lot more endurance.
     
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