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Battery Replacement

Discussion in 'Roadster' started by richkae, Sep 15, 2010.

  1. richkae

    richkae VIN587

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    The roadster battery is built - I believe - with 2200 mAh cells. 2900 mAh cells are starting to be available.
    Panasonic has said that they will have 4000 mAh cells by 2013.
    I am actually excited about the prospect of replacing my battery in a few years with something that dramatically improves the car.

    If 2 new batteries were available for your roadster:
    1) A smaller one with the same 53 kWh but weighed 500 pounds ( 400 pounds less )
    2) One the same size with 94 kWh and weighs the same 900 pounds.

    The 53 kWh battery, because of the reduced weight would increase the highway range by about 7-10% ( I'd call it 240 miles - up from 220 ), and the 0-60 should drop to about 3.5 seconds. The 1/4 mile would drop to about 12.2 seconds from 12.95 seconds.

    The 94 kWh battery would increase the range to about 400 miles, but performance would be the same.

    Of course the smaller battery should also cost less.
    Which would you choose?
     
  2. bolosky

    bolosky Member

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    Unless the new batteries have higher power than the existing ones, this wouldn't be the case. With fewer cells, you'd hit the ESS's power limit earlier, and would wind up with increased 0-60, not decreased. Way back in '08 when some Tesla folks came to Microsoft, we asked just this question (except keeping the existing cells), and that's the answer they gave us.

    Personally, I'd take the addiitonal range regardless. I'm not that into racing, and so it's plenty fast enough for me.
     
  3. Adm

    Adm Active Member

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    Wow, it's a tough decision. I've only driven a Roadster Sport once, but I'd say it has enough punch and I don't think any "normal" Roadster owners would call their car underpowered.
    Maybe more important that power would be an improvement in handling that would come with weight loss.

    For myself range would be more important. There is however a point where you have to decide how much range you need. I've reserved a Model S, but I have no idea yet where my optimum range is. I'd say 300 mi will be enough, especially if quick charging will be available but if I would have 400 mi I’d probably always be able to charge at home over night apart from a longer vacation trip. I doubt that I would drive in that case so I would probably fly and rent a car (EV of course) on location.

    I expect to need a battery replacement in 2016 or later but I have no clue where battery development will be than. I can only say I am very excited to find out where EV’s will go from here.
     
  4. dwegmull

    dwegmull 2013 Model S 85

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    If the smaller form factor with the same capacity would be possible, I could use a bigger trunk!
     
  5. vfx

    vfx Well-Known Member

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    Amen to the trunk size but what about handling? if the battery is say, 200 or 400 lbs less, that has got to be a game changer to handling. I think a fix from the adjustable suspension just won't cut it.
     
  6. samcarney

    samcarney Sam C

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    I would have to go with the added range. I think 400 miles is a good day's worth of driving, check-in to a hotel, plug-in and continue on in the morning. California here I come!
     
  7. kgb

    kgb Member

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    This is a game changer.

    The original question is would you rather better performance or better range. But if you think about it, option 2: better range, means everything remains the same, but the battery holds more charge for more range.

    With option 1: smaller battery, everything changes. The trunk getting bigger is the change I would care about most, but according to everything listed above, we'd see:

    1) Better highway range due to lighter vehicle
    2) Better handling due to lighter vehicle
    3) Better acceleration due to lighter vehicle
    4) Larger trunk due to smaller battery
    5) Bigger subwoofer? due to smaller battery
    6) Bumpier ride due to lighter vehicle

    In any case, point is mute. This iteration of Roadster is dead in 2012. Battery in the trunk will cease with next gen Roadster. It is unlikely that a major redesign is going to occur prior to 2012.
     
  8. mpt

    mpt Electrics are back

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    Can I have both? Two battery packs upper and lower? Crazy but I can't choose, I want to drive up to Boston which is 250 highway miles, but I'll only do that once or twice in the life of the car so it'd seem a waste to carry 400LBs of batteries around for occasional use... a bit like buying a 7 seater SUV.

    The reduced weight battery would also be lower down improving handling further.

    I pick smaller battery.
     
  9. tomsax

    tomsax Member

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    If I could have same performance and same range, I'd pick that over paying the increased cost in battery amortization and reduced energy efficiency in a longer range pack.

    Ultimately, road tripping is more about charge rate than battery pack size.

    When you're going on a road trip, you only get the benefit of the large pack once per day. If you want to drive 800 miles in a single run, with a 200-hundred mile pack, you need 600 miles of charging. With a 400-mile pack you need 400 miles of charging. The 400-mile pack isn't twice as good as a 200-mile pack for long drives. In fact, driving a 100-mile range car would require 700 miles of charging, not much more than the 200-mile pack, although it would be pretty frequent stops.

    If you regularly do a 350-mile trip, the 400-mile pack has real appeal. Still, ignoring any change in efficiency, if you can get 240V/70A charging on the road, a current Roadster with a 400-ideal-mile pack could drive 350 miles at 62 mph with 10 minutes of charging, for a total of time of 5:45. Today's Roadster with a 240-ideal-mile pack could do the same trip at 62 mph with 2:45 of charging for a total time of 8:22. (If you want to drive 75 mph, the total times would be 6:33 vs. 9:20.) If you can do something useful or fun during that charging time, the smaller pack is not so bad.

    With Level 3 charging (which current Roadsters will probably never be able to use) the benefit of a bigger battery pack is even less pronounced. In fact, maybe the smaller pack is better: if I'm going to drive 350 miles in one stretch, I want to stop for 30 or 40 minutes in the middle to stretch and get a bite to eat.

    Ironically, a Nissan Leaf with access to Level 3 charging will be a faster road trip vehicle than a Roadster for sufficiently long road trips. If I assume the Leaf has the same drive efficiency as a Roadster and a battery pack equivalent to 100 ideal miles, and neglect the time it takes to pull over and plug in, the Leaf ties the Roadster at 280 miles and is faster at longer trips. My simplifying assumptions are most likely biased toward the Leaf, but making a more accurate calculation just pushes the crossover point out a little farther.
     
  10. bolosky

    bolosky Member

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    Like i said before, this is very unlikely. Unless the power density of the new cells is much better than that of the existing ones (which it probably is not), the smaller battery pack would have smaller power output, which in turn would mean less acceleration.

    Think about it this way: each cell stores a certain number of watt-hours or energy (a little less than 8 Wh for the current cells) and provoides a certain number of watts of power (about 30W for the current cells). If you were to switch to cells that stored twice as much energy (16 Wh/cell) and then had half as many of them you'd have the same total energy and so comparable range, but in order to get the same ~200 kW from the pack you'd need to have each cell deliver 60W. If they don't (which I suspect they won't), then you have less power and so less acceleration.

    For this reason, you're not going to see half-sized Roadster packs.
     
  11. richkae

    richkae VIN587

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    I am pretty sure the batteries are improving in both power density and energy density.

    I've looked at a few links selling 2200mAh cells and 2900mAh cells.
    The capacity is calculated at 1C discharge and max rated discharge is 2C which I think means the max discharge is 2x2200mAh = 4400mA for the 2200s and 2x2900 = 5800mA for the 2900s
    I think that means that the 2900s have increased power density and energy density by the same factor.
     
  12. stopcrazypp

    stopcrazypp Well-Known Member

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    Your idea is right. Batteries are rated usually at 1C. Unless there was something different about the chemistry that decreased the max C rate, then improvements in energy density also mean proportional improvement in power density.
     
  13. beakmeister

    beakmeister Member

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    If the smaller, more powerful battery still gave the same peak power, I would go with an even smaller battery that could take me just 200 miles. Lighter is better. (Sport)
     
  14. bolosky

    bolosky Member

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    The math doesn't work out here. If the max discharge rate for the whole pack is 2C (and the pack should be the same as the individual cells), then it would discharge 53kWh in 0.5h = 106kW, but the actual power is roughly twice that. That is, the current cells are actually discharging at more like 4C, not 2C.
     
  15. TEG

    TEG TMC Moderator

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    Yes, I have pondered that, as I think others have as well.

    I suspect the answer is that Tesla has figured out how to 'coddle' the cells (through careful temperature management), yet still 'abuse' them at times (for 4C 'hole shots') and have proved that they can work reliably in this application. I also gather that they do lose the occasional cell but the pack has enough so you can tolerate a few isolated failures without needing to repair the pack. (Sort of like the way a hard disk can develop bad sectors that are just remapped and ignored.)
     

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