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Removing ballast from a 40KWh Model S

Discussion in 'Model S' started by Herbys, Jul 3, 2012.

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  1. Herbys

    Herbys Member

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    I understand that the 40KWh and 85KWh variations of the model S weigh the same due to the model with the smaller battery having some added ballast where the missing battery packs go in order to simplify the crash testing process.
    Once a car is delivered, I wonder if removing the ballast would be possible.
    According to my calculations, 45KW of battery should weigh about 800 pounds. Saving 800 pounds of weight would do wonders for acceleration (maybe getting the standard model S into the 4 second territory, simple math would put the car at around 4.7 seconds in 0-60) and save some gas. It may even help with handling (though that, I don't know).
    Of course it all depends on:
    * if the battery pack is accessible without disassembling the whole car. I guess yes since it should be replaceable.
    * if the 40KW pack is centered on the car. If they just left the rear of front pack and removed the rest, then removing the pack might upset weight balance, though it might also help, it depends on how the balance changes.
    * if the ballast is a solid block that can be removed (and I guess replaced with something innocuous, equally solid and lighter).

    Does anyone have any ideas on this? This could be a very straightforward "tune" to do to an already fast car. I'm betting on an aftermarker "upgrade" in the form of lighter ballast.
     
  2. VolkerP

    VolkerP EU Model S P-37

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    Recently we heard form a Tesla exec (Franz? JB?) that there are weight differences between the packs. But these are small enough not to have to adjust the suspension settings when swapping packs of different capacity. In no way there will be 800lbs of ballast in the lighter packs. That figure should be near total pack weight.
     
  3. ddruz

    ddruz Member

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    I received this response from Stephen Smith of Tesla on June 26 in response to an email I sent him questioning differences in battery weight:

    "With regards to your question about battery and the relation to weight, I honestly do not have an answer for you at this time. We released that the car’s weight at 4,647.3 with the 85 kWh battery, but it is unclear to me at this time if that will be the weight for all battery configurations, or if the car will get lighter with each step down in battery. I don’t think it is a necessity to keep the vehicle at the same weight, so it is entirely possible that cars with lower batteries will weight slightly less than their higher energy holding counterparts."
     
  4. dsm363

    dsm363 Roadster + Sig Model S

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    You would definitely void the battery warranty so would have to be willing to risk the $15,000 (guess on price) or so battery to shave a second or so off of the 0-60 time. You would also need the equipment and lift to remove the battery.
     
  5. grisnjam

    grisnjam P6316

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    Also note that each of the 3 battery packs use different battery chemistry. Therefore the smaller packs may weight closer the larger pack then a linear curve would suggest due to less energy dense cells. I can't imagine that after use an all aluminum frame they Tesla would hide some lead bars in the 40 kWh pack ;)
     
  6. W8MM

    W8MM R1.5 #325 + Mdl S #01380

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    Ditto! Besides, it would add cost and air is free.
     
  7. TEG

    TEG TMC Moderator

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    I think Tesla has gived mixed messages about the 60kW pack.
    Sometimes they say same # of cells as 85kWh but different (less energy dense) chemistry, and other times they say less of the same cells as in the 85kWh.
    I think the "party line" is to leave us guessing which they may do.

    It may not be 3 different chemistries..
     
  8. brianman

    brianman Burrito Founder

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    What I had internalized as fact was...

    85 kWh cell count = 60 kWh cell count
    60 kWh chemistry = 40 kWh chemistry
    85 kWh overall weight ~= 60 kWh overall weight ~= 40 kWh overall weight

    ... where "~=" means approximately equal.

    To me that meant 2 chemistries.
     
  9. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    On the other hand, there might just be more coolant in the smaller battery packs. This is not so far fetched because the smaller the pack the more cooling it will need.
     
  10. TEG

    TEG TMC Moderator

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    You can find old stories from years back where they said that the 40kWh and 60kWh would have the same chemistry, and only the 85kWh would have the new type of cells. Further it had been suggested that the 60kWh and 85kWh would have the same # of cells.
    But more recently there were murmurings such as "you could also do a 60kWh just by using a lesser amount of the new cells."
    So we are left guessing.
    Also, I don't recall comments that the 40kWh would weight the same as the larger packs. I always assumed it would weigh less.

    Year ago we speculated that the 40kWh might perform better due to lesser weight, but it seems the reality is less performance due to lower max power output.
     
  11. stopcrazypp

    stopcrazypp Well-Known Member

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    Same with me. I've always heard the 40kWh pack would use less cells (and the same ones as the 60kWh), so it would weight significantly less.

    It's actually impossible for the 40kWh pack to weigh about the same (without a ballast). The smallest brand name 18650 cell you can buy today is 2200mah. The cells used in the 85kWh pack is 3100mah and they weigh slightly more (0.5-1.5 grams). But let's assume they weigh the same for easy math.

    40kWh/(3.6V*2200mah) = 5050 cells.
    85kWh/(3.6V*3100mah) = 7616 cells.

    That's a difference of 2566 cells or 33% difference. And if you go to a denser cell, the cell count (and weight) for the 40kWh pack will only go down.
     
  12. NigelM

    NigelM Recovering Member

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    Any serious difference in weight will affect handling, so would require some specific locating of ballast (assuming ballast is necessary) and therefore removing said ballast would potentially be a bad idea.
     
  13. VolkerP

    VolkerP EU Model S P-37

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    The very idea of a car is to use it in different ballast scenarios. Think passengers, shopping. We're talking ~180 lbs weight difference between 40kWh and 60kWh pack here.
    So, yes, I can feel a different behavior when my car is full, but I can adjust my driving (e.g. hit the brakes harder & earlier).
     
  14. NigelM

    NigelM Recovering Member

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    That's why I referred to a "serious" difference in weight, maybe I should have said "significant". I don't see 180lbs as significant in a car weighing 4,000+
     
  15. W8MM

    W8MM R1.5 #325 + Mdl S #01380

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    When one reduces mass, handling should actually improve. The positioning of the battery mass is under full control of the designers and they know where to put it to best effect. The positive effect on acceleration from lower total mass is counteracted by the higher internal resistance of fewer cells in parallel which reduces the current available to the motor.
     
  16. VolkerP

    VolkerP EU Model S P-37

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    My bad. Didn't digest your post word for word. :redface:
     
  17. Robert.Boston

    Robert.Boston Model S VIN P01536

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    Using ultra-light components + ballast is a time-honored tradition in sailing. What you want to avoid is weight in the wrong place.

    Still, I agree that adding ballast to the 40kWh would seem odd. Far better to make some minor adjustments to the suspension.
     
  18. Dr Chill

    Dr Chill Member

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    There is no way Tesla will add ballast to the car thereby reducing the battery range. There will definitely be weight differences between the cars; it just remains to be seen how much. Why there isn't transparency by Tesla regarding this issue is still unclear to me.
     
  19. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    My guess would be that they don't have the exact numbers as delivered yet. And it's still possible that they will add additional cooling to the smaller battery packs negating the weight difference. Adding coolant is different than adding ballast because the coolant serves a function. Because each of the cells of a smaller battery will heat up more at a given power level, it's entirely reasonable that they would provide more cooling.
     
  20. zdre

    zdre 40kWh Model S P6415

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    I can see 20 more pounds of coolant, but nowhere close to 180lbs. I really hope that 40kWh battery weighs less. I think that adding a ballast would kill a significant number of sales.
     

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