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Li-ion versus NiMH

Discussion in 'Battery Discussion' started by TEG, Dec 11, 2010.

  1. TEG

    TEG TMC Moderator

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    Was the Model S demo/concept really representative of the final product?

    By then lithium cells might cost less / kWh than NiMH.
    I am not sure that we will see a return of NiMH. Li-ion might be the future unless ultracaps actually work out.
     
  2. scott451

    scott451 KWH-PWR#1349Sprt,S Sig#96

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    #2 scott451, Dec 11, 2010
    Last edited: Dec 11, 2010
    I rode in the S at the March 26th, 2009 premiere. The fit and finish was quite good. I was very impressed. I had not planned on getting an S, but after the drive I put down a deposit for a signature model (S105)!

    NiMH batteries can handle larger output current. Li-ion is limited to about 5C discharge rates. So it's harder to make a smaller (i.e. less capacity) Li-ion battery. E.g. Say you wanted to make a faster, lower cost roadster by cutting the battery weight in half. So a 26KWhr battery would give you about 45% of the original range and shave off ~350lbs. The problem is that for the same peak torque, the discharge rate per cell is now 10C (35/brick vs 69/brick), which would significantly reduce the life of the battery.

    NiMH batteries can handle high discharge rates with minimal damage. So you can build a smaller, lighter, lower capacity/range battery and still have excellent torque. NiMH batteries are only 60% efficient, so your Whr/Mile cost would be about double. So the entry cost of a NiMH based car could be significantly less than Li-ion, but the day to day operating cost would be higher. It's a good trade off for a very low end car.

    -Scott
     
  3. TEG

    TEG TMC Moderator

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    Just, FYI, I rode in it too. I expect the production model will have substantial differences.

    Aren't there some some Li-Ion chemistries that can offer higher discharge? For instance as found in A123?

    Also, FYI, I drive a NiMH EV almost every day.
    60% efficient at what? Are you saying that you only get back 60% of the energy you put in when charging?
    How much more 'efficient' are Li-Ions in comparison?
    For a Tesla pack you would probably have to average in all the energy used on active cooling and heating too.
     
  4. stopcrazypp

    stopcrazypp Well-Known Member

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    #4 stopcrazypp, Dec 12, 2010
    Last edited: Dec 12, 2010
    According to wiki, NIMH has 66% charging efficiency, but the source for that number doesn't go into detail. Wiki says Li-ion is 80-90%.
    The source for the lithium battery article is more insightful. It seems the efficiency is about the same in low duty cycles, but for NIMH it drops more rapidly when duty cycle increases.
    http://www.pluginhighway.ca/PHEV2007/proceedings/PluginHwy_PHEV2007_PaperReviewed_Valoen.pdf

    Specific power is 200W/kg (3C, DOD=80%) for the RAV4-EV, specific energy is 63Wh/kg
    http://www.evnut.com/rav_battery_data_sheet.html

    Specific power is 1310 W/kg (30C) for the new Prius, specific energy is 44 Wh/kg
    http://www.toyotapriusbattery.com/

    So it seems NIMH in EVs can't really be compared to NIMH in Hybrids and the DOD range is also much different (I think prius is between 80-40%).

    Specific power is 245W/kg (1C) to 1226W/kg (5C burst like in the Roadster) for the Panasonic NCR18650A (batteries planned for 300 mile Model S), specific energy is 245Wh/kg
    http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+BA+3+ACA4001+NCR18650A+7+SA

    So it seems even with the much higher C-rate, NIMH batteries can't match lithium. Probably other lithium cells like a123 (which also have high C-rate, but also much higher specific energy than NIMH) are more suitable.

    The advantage of NIMH is the 1000 cycle life vs the 500 cycle for lithium cobalt 18650, but there are lithium cells that can match or exceed that now. Advantage is you can warrant a 100 mile EV for 100k miles (100mi *1000 cycles = 100k mi), while Tesla needs to stick to around 200 miles to warrant for 100k miles (200mi * 500 cycles = 100k mi).
     
  5. scott451

    scott451 KWH-PWR#1349Sprt,S Sig#96

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    Yes, but NiMH is still at least 2X-5X better.

    cool! which one? Is it based on the Panasonic E85 NiMH? (wonderful battery by the way, too bad Std oil/chevron halted it's production)

    Using Tomsax's "wall [kwhr meter] to wheel" the roadster is about 350Whr/mile. Your NiMH is probably closer to 700Whr/mile. The Chevy S10 EV quoted ~700Whr/mile using NiMH. The lead acid version was closer to 400 Whr/mile
     
  6. scott451

    scott451 KWH-PWR#1349Sprt,S Sig#96

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    #6 scott451, Dec 12, 2010
    Last edited: Dec 12, 2010
    I think you're missing my point :) I believe you're saying that for the same power, Li-ion will be lighter. True. I'm trying to say that for the same peak output current, NiMH will be lighter. So for a low cost EV, NiMH's much higher C-rate, allows you to build a smaller, lower cost, battery pack (same peak output current) than you could using Li-ion cells. (assuming the $/Whr cost of NiMH and Li-ion are the same, which the are not Li-ion$ > NiMH$)

    So here's a trade the trade off (rough estimates). Which car would you buy? :smile:

    Roadster 2.5: 0-60 3.7s, 245 miles / charge, fast charge in 3.5 hrs, eMPG 100

    Roadster NiMH: 0-60 ~3.3s, 125 miles/charge $5k-$10k less, fast charge in 1 hr, eMPG 50

    My overall point is that if Tesla wants to succeed in the low cost EV market, they will have to invest in both Li-ion and NiMH.

    Yes. cycle life too.
     
  7. vfx

    vfx Well-Known Member

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    Tesla should hire Scott.
     
  8. stopcrazypp

    stopcrazypp Well-Known Member

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    #8 stopcrazypp, Dec 12, 2010
    Last edited: Dec 12, 2010
    I suppose you do have a point on C-rate assuming 121Wh/kg for the current Tesla pack vs 63Wh/kg for NIMH. It'll have half the range for roughly the same weight and volume (assuming volumetric differences is in proportion to the specific energy differences). It'll need to discharge at (245mi/125mi)*5C = ~10C for the same performance, which is probably possible although much higher than the 3C the RAV4-EV uses (I also noticed specific energy is lower 44Wh/kg for the hybrid batteries, so there are probably some differences there in terms of how they engineered the pack).

    The current charging bottleneck isn't necessarily the battery, but rather the power supply (it is already drawing 240V @ 70 amps). It is also inherently unfair to compare total charging time rather than charging rate (which is what really matters since you care about how much charging you have to do to go a certain distance, not necessarily the time to charge the battery full). In both cases, the charging rate would be about the same. Elon did say he plans a 45 minute charge for the Model S, so faster charging isn't impossible given enough power.

    Given Tesla has a relation with Panasonic and Toyota, Tesla probably will have access to those batteries when the patents expired, so it'll be interesting to see if they still apply to EVs then. The only major negative for NIMH is the rare-earth metal requirement, which is squeezing Japanese battery makers right now in term of hybrids and probably will be the reason why they are starting to move to lithium even for hybrids.
     
  9. TEG

    TEG TMC Moderator

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    Yes, Ford RangerEV.
    [​IMG]
    Calendar life seems to be a non-issue as 10 year old packs are still going strong.

    Well, at least they are "green" electrons coming from home solar...
     
  10. TEG

    TEG TMC Moderator

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    Both the RangerEV and original RAV4EV have (by Tesla standards) a wimpy motor. They both have ~25kWh packs, but only 50-70kW motors, so they apparently don't come close to taking advantage of the high C output capability of the NiMHs. I wonder why not? Inverter costs? Or maybe they found that the NiMHs last a lot longer if you don't actually push them as hard as they can handle? I think just like the SoC range (DoD), the current draw has an affect on battery longevitity, so a NiMH powered Tesla might not end up with as high a cycle life as a NiMH pack in say an old RAV4EV that had a less powerful motor.
     

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