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Adding capacitors to get Fun and Efficient

Discussion in 'Battery Discussion' started by Dirrigc, May 3, 2017.

  1. Dirrigc

    Dirrigc New Member

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    So I love the kick you get when you slam the accelerator but then you get a depressing huge orange mountain in your consumption chart. I understand this mainly comes from pulling hard on batteries and Peukert's law.

    So why doesn't Tesla put in capacitor that could give max power for a few seconds; maybe 5s... after that you start breaking the law anyway. That would:
    1- avoid losses due to pulling hard on batteries and Peukert's law = get the fun and the efficiency
    2- make regen more efficient and available in freezing weather as recharging a capacitor is more efficient than recharging a battery

    A few seconds of capacitors shouldn't take too much space or weight (300kW x 5s = 1500kWs = 0.42 kWh)
    At 10Wh/kg you would need 40 kg of capacitors + some power electronics (10-20kg? I dont know). Seems manageable.

    Capacitors either charge from battery or only on regen (customizable in user settings).
    If the capacitor is empty then you just pull on the battery as normal and get the fun but without the efficiency.

    I'm not sure what catastrophic failure of this 0.4 kWh capacitor would look like. I know capacitors discharge super fast and can vaporize cables but 0.4kWh could be small enough to limit the "catastrophicness" of failure.

    And then you get the fun and the efficiency... you could even have cool beeping sounds telling you when you could get your next "free kick". so why didn't they do it?!
     
  2. Saghost

    Saghost Active Member

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    It's not that simple. Capacitors absorb and release their energy through voltage change, which means the only way to use them as energy storage is to put them behind a DC DC converter.

    Here are two options picked from quick Google searches for commercially available ultracapacitors:

    Supercapacitors / Ultracapacitors 2.7V 3000F Threaded terminal: Amazon.com: Industrial & Scientific

    BMOD0500 P016 B01 Maxwell Technologies | Mouser

    With the first, a 2.7V operating limit and 3000F rating means the maximum energy one can absorb is 21.9 kJ. That's just over 6 Wh, meaning that if you use the entire range, you need 70 of them to get your 420 Wh pack.

    At $60 each, that's $4200, and at 510g per, that's 35.7 kg - without any wiring or enclosure or safety systems, let alone the DC DC converter. You'd also need a space that's 24" by 17" by 6" just for the cells.

    The second is much worse. The 16V limit seemed to offer big advantages in capacity - which turned out to be 128 kJ / 35.5 Wh - but you still need 12 of them, and that means both more mass and more money (66kg, $7k)

    Having to pass through the dc dc converter twice probably eats a similar amount of efficiency to the battery charging losses, so I'm not sure you're gaining anything there.

    I'll admit I would really like full regen at all times, but I'm not sure I would pay $7-$10k more for it (gotta include the cost for the converter, enclosure, and safety systems.)
     
    • Informative x 2
  3. oktane

    oktane Active Member

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  4. Pando

    Pando Member

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    Peukert's law applies mainly to lead-acid batteries, not necessarily to lithium-ion.

    If you used a capacitor then the huge orange mountain will just be spread out over time, but it will still be there. If it's using regen to charge the capacitor instead of the battery, you just don't get as much energy put back into the battery, and you're not seeing the actual consumption as it's now buffered by the capacitor, but the net consumption is about the same.

    Then again, a flux capacitor may do the trick. :)
     
    • Funny x 1
  5. jaguar36

    jaguar36 Active Member

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    Why wouldn't you just put 60kg worth of more batteries?
     
    • Like x 1
  6. TexLaw

    TexLaw Member

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    I think you're buying more problems than you're selling with this idea. Cost and complexity just aren't worth it. Now, if you were concerned solely with acceleration performance, e.g., some sort of race car, then there may be worth it.
     
  7. transpondster

    transpondster Member

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    isn't Ludicrous 1/4 mile something like 1800 A * 360V * 11 s ~= 2 kWh. Best available supercaps are about 7 Wh/kg that means 300 kg if you somehow manage to discharge to 0 V and about 600 kg if looking for real numbers, hmm and we are back at Tesla's pack weight...
     
  8. Dirrigc

    Dirrigc New Member

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    I don't know for you, but for me "net consumption" is really not the same between "grandpa driving" never going above 30-50 kW power and "aggressive driving" when you frequently hit 200+ kW even if you always stick to pure 1 pedal driving. I get an increase of over 50% in Wh/mile. In theory, you should get almost the same. I understand the gap comes from:


    I explain
     
  9. Dirrigc

    Dirrigc New Member

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    I don't know for you, but for me "net consumption" is really not the same between "grandpa driving" = never going above 30-50 kW power and "aggressive driving" = when you frequently hit 200+ kW, and this even if you always stick to pure 1 pedal driving (= full regen of all kinetic energy). I get an increase of over 50-100% in Wh/mile. In theory, you should get almost the same. I understand the gap comes from:
    1- more wind drag as your average speed increases (same peak speed at local speed limit, but you hit the limit faster)
    2- battery inefficiencies (voltage drop when pull hard + Peukert's law)
    3- regen inefficiencies

    I think 2 and 3 are significant (as 1 cant account for that much) but I read everywhere Peukert's law hardly applies to Li-ion, but that doesn't match the 50-100% increase I see. So what share of inefficiency does each type account for? Are there other inefficiencies i forgot?

    Capacitors could solve 2 and 3, but economics and complexity don't work (and efficiency of DC-DC might be an issue). Thanks Saghor for input.

    Maybe in 2-3 years, if/when ultra-capacitors get much better, such technology will make electric cars even more efficient.
    Or batteries' high current discharge and regen will improve to the point of making the inefficiency disappear; keeping the car simple.
     
  10. Saghost

    Saghost Active Member

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    Higher current flow means higher losses throughout the wiring - a function of current squared, so when the car goes to 1300A wide open, it really hits hard.

    The motors themselves are less efficient at very high power levels than moderate ones typically, though I haven't seen a map for any of the modern Tesla motors to know for certain.
     
  11. arnis

    arnis Member

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    Super-capacitors are in development phase right now and are almost ready for mass market (finally) .
    They will find their place pretty soon in multiple areas.
    They do actually have multiple advantages over batteries.
    For example in hybridization business. Especially on heavy vehicles.
    They can take much more regen and also give out as much as needed.
    No need for conditioning. Cold is not an obstacle. etc

    But, in case of Model S X and 3, there appears to be no need as the main
    pack can handle pretty much all our needs. There are some things
    that could be improved (ludicrous, cold soak regen, regen efficiency).

    More reading from a company born pretty much near my home:
    Ultracapacitors and supercapacitors | Skeleton Technologies
    (check out multiple pages, incl HV modules and specs)
     
  12. GoTslaGo

    GoTslaGo Learning Member

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