12 Volt Lithium Battery...Possible ?

Discussion in 'Gen 3 Prius Technical Discussion' started by TruSound, Feb 24, 2012.

  1. RobH

    RobH Senior Member

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    Do electric cars like Tesla and Leaf still have 12V batteries? How about the Volt?
     
  2. GregP507

    GregP507 Senior Member

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    Apparently they do, for reasons similar to the Prius.
     
  3. David Beale

    David Beale Senior Member

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    You need a 12V battery as ballast for the 12V system. If you didn't have it the electronics would be upsetting the 12V power supply as it turned loads on and off. It would also be difficult to have 12V always on when the car is off. Besides Toyota wanting the traction battery completely disconnected when the car is off, it would not be a "good idea" (tm) to have a 12V converter running all the time off the traction battery.

    Now, I'm not saying you have to have a lead acid one. I would actually prefer a NiMH, but Cobasys has killed the NiMH cells larger than 6.8 A-Hr with their patents (last I heard they are owned by Shell - make your own conclusions). I did find some 20A-Hr ones from China (who ignore patents) but 10 would cost about $300, so the stock lead acid still wins. At 38ish A-Hr for $250 it's still hard to beat on an economic basis.
     
  4. GregP507

    GregP507 Senior Member

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    That's true. A very large capacitor would do the job, but a small battery is a much more sensible solution.
     
  5. Redpoint5

    Redpoint5 Senior Member

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    I disagree that a small battery is a more sensible solution and wager that in 10 years, you will hardly ever find a lead-acid battery in a vehicle.

    The capacitor will last the life of the vehicle, is not damaged by deep discharges, and can provide for any electrical demand instantly. Their weight is just a fraction of a standard battery. As the technology improves, production volume increases, and prices decrease, it will increasingly be the sensible solution.

    I completely eliminated the battery in the motorcycle shown in my avatar by replacing it with supercaps. I'll never run a battery again.

    If I knew that the Prius battery charger regulated the output amps to protect itself, I would experiment with replacing the lead-acid battery with 5x series connected BCAP3400 caps.
     
  6. GregP507

    GregP507 Senior Member

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    A supercapacitor is far more than what would be needed to filter the power supply, and much more expensive. The choice was between a "large capacitor" and a small battery.
     
  7. RobH

    RobH Senior Member

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    Let's see... 5 BCAP3400 supercapacitors at $70 each would be $350. In 10 years the price will probably be more like $35.

    How do you convert such a supercapacitor setup to amp-hours in a battery? How long would the current Prius last idle in a parking lot at the airport?
     
  8. GregP507

    GregP507 Senior Member

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    Maybe I'll be installing supercapacitors in my 10-year old Prius.
    But I doubt it.
     
  9. Redpoint5

    Redpoint5 Senior Member

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    The price right now is $68.33 on Mouser.com, but with shipping would cost about $350. If the cost of a Prius battery is $200 like I've seen quoted, then it will pay for itself after 2 battery replacements, and still be usable in your next vehicle purchase.

    The formula to calculate amp hour capacity is as follows:

    Series Farad Rating = 1/n * Single Cell Farad Rating (n is the number of capacitors connected in series)
    Amp Hours = (Max Volts - Min Volts) * Series Farad Rating / 3600

    Example:

    1/5 * 3400 Farads = 680 Series Farad Rating
    (14v - 7v) * 680 / 3600 = 1.32 Amp Hours

    The major drawback of supercaps is that the farad rating decreases as the number of series connected capacitors increases. Another drawback is that capacitors give up voltage to supply current. That means the voltage is linearly dropping as Ah are supplied. Chemical batteries maintain a relatively constant voltage as the Ah are used.

    I have yet to measure the mA of parasitic draw on the Prius, but I think someone quoted it at 30 mA, which is quite high compared to most modern cars. My 2006 Acura TSX draws only 21.5 mA while parked. I don't think 30 Ah is accurate for the Prius because the manual says to turn on the vehicle every 2-3 months, and the 40 Ah battery would only last 1.5 months until completely drained if the Prius consumed 30 mA. I'd guess the actual draw to be closer to 10 mA, but I'll have to measure this.

    Assuming the Prius computer functions all the way down to 7v, the supercaps would power the parked car for just 44 hours.

    The way I would solve the problem is to use a very small solar panel on the dash if the car was kept outside. If the car was kept inside, I'd just go with a 10 Ah LiFePO4.
     
  10. GregP507

    GregP507 Senior Member

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    I may be missing something, but I can't see what advantage there might be to using a supercapacitor instead of a battery in a Prius. All it has to do is run lights and accessories, and boot the computer into ready-mode. It does not crank the engine. Some claim it affects the performance of the car, but I've yet to see evidence of that.
     
  11. Redpoint5

    Redpoint5 Senior Member

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    I agree with Greg now.

    A small battery is a more sensible option in this application. Perhaps 10 years from now supercaps will be high enough capacity and cheap enough for it to be the right option, but not right now. The BCAP3400 just came on the market 6 months ago, so you won't find any good deals on Ebay, and only a single manufacturer (Maxwell) is making them.

    My Dodge/Cummins 2500 diesel truck takes 2x group 94R batteries to start on cold winter mornings, and currently mine are dead. The cost to replace the batteries will be $400.

    My truck uses 5.89 mA while parked to retain radio presets; so the parasitic draw is very low. My truck also lives outside where a small solar panel can recover the energy that drains as it sits. Because the truck takes over 400 cranking amps to start, supercaps make a lot of sense in this application, and will be cheaper than buying batteries.

    Since my PiP lives in the garage so that I can plug it in, and doesn't require high cranking amps, a supercap just doesn't make sense. I'll be measuring parasitic load soon and will report back my findings. I might even experiment with a 4.2 Ah LiFePO4 battery I have laying around. It weighs less than 1 pound and should be high enough capacity for the Prius to sit parked for a couple weeks. I got it for $40 from Hobbyking.com

    This 20 Ah LiFePO4 battery is only $125. It only has half the capacity of the lead-acid battery in the Prius, but it can be fully discharged without major permanent damage being done. I would expect the battery to last the life of the vehicle. It weighs just 6.6 pounds.
     
    #71 Redpoint5, Jan 23, 2015
    Last edited: Jan 23, 2015
  12. bisco

    bisco cookie crumbler

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    is this a battery that can take multiple deep discharges? that seems to be the achilles heal of the oem. other than that, they tend to last 7 or 8 years.
     
  13. Redpoint5

    Redpoint5 Senior Member

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    Yes. They can routinely be discharged to 20% capacity and still provide a long service life. Usually the battery can return to service after being drained flat. A lead-acid battery is ruined if it's drained flat even once.
     
  14. Easy Rider 2

    Easy Rider 2 Senior Member

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    Just simply NOT true.
    It depends mostly on how flat is "flat" and how long it stays that way.
    In most cases, one instance will not cause any signigicant damage IF it is recharged right away.......slowly.
     
  15. Redpoint5

    Redpoint5 Senior Member

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    We've got a misunderstanding of our definitions of "significant damage" and "ruined".

    A lead-acid battery will loose much capacity if run down even a single time. From my experience in using them in electric cars in high school, a brand new battery, once drained to the point that the vehicle will only creep along at 1/4 mph, will only deliver about 80% of the range after being recharged when compared new. I consider this to be significant damage.

    Your point that duration and depth of discharge are relevant factors is a good one, and I failed to qualify my statements of "drained flat" and "ruined". Regardless, the main point that LiFePO4 is much more resilient to deep discharges compared to lead-acid, remains valid.
     
  16. CR94

    CR94 Senior Member

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    Were you using deep-cycle batteries, or car batteries (or similar)?
     
  17. Redpoint5

    Redpoint5 Senior Member

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    With my very limited high school knowledge, I choose 2 of the highest capacity car batteries I could find with a total weight not exceeding 64 lbs. The weight was a regulation imposed by the PGE Electrathon rules. In hindsight, deep-cycle would have been good to have for testing purposes, and given an unlimited budget, fresh batteries for each race. All materials were donated, so we used what we were offered.
     
  18. Easy Rider 2

    Easy Rider 2 Senior Member

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    But that experience is not typical for most wet cell batteries.
    A 20% loss of capacity from just a single deep discharge is huge but probably would go un-noticed in most applications because the total capacity wouldn't be tested a second time.

    What size were these cars (batteries); remote control size or "go cart" size ?

    [edit] Never mind. Answered in previous post I see.
     
  19. David Beale

    David Beale Senior Member

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    A 20% loss of capacity wouldn't be noticed in a 20th century car, but in a Prius....
    Your battery will not start the car after about a week and a half, instead of two weeks. THAT would be noticed! ;)

    Read ANY material from battery manufacturers. It is "Not recommended" to fully discharge ANY lead-acid battery, INCLUDING "deep discharge" types. A deep discharge battery is more resilient to a full discharge, but still suffers damage. Perhaps a 5-10% loss of capacity, even after an immediate recharge. Compared to 20% or more loss of capacity for a "starter battery".

    A note here. A Prius can use a "deep discharge" battery without damaging it. Other vehicles that actually start the engine from the 12V battery will cause damage to a "deep discharge" battery. It may not show up immediately, but loss of capacity, bent plates (and the resulting shorts) will happen. The Prius only draws in the order of 50 A at start, and only for a half second or so. Compare that to 100 to 200+ Amps to crank over an ICE. Less in summer, more in winter. Of course there are always exceptions. There are "deep discharge" batteries out there that are also designed to deliver starting currents. Check the specifications carefully!

    A further note. MOST rechargeable lithium cells (and presumably batteries as well) contain a chip to ensure they are not "fully discharged". The later chips also protect them from over-charging. As far as I know this is the reason they are "not damaged" by full discharge, not the chemical makeup of the cell. If you do manage to "fully discharge" them, they do not come back. They are poor paperweights (due to their low mass ;) ). To fully discharge one, discharge in the normal way, and once fully discharged (voltage goes to zero as the chip cuts the connection), set it aside for a year or so. Further self-discharge will take it to destruction. Experience talking here. ;)
    So when you read the instructions (for example, Canon Camera batteries) that say to store them in a discharged state, DO NOT do it! Lithium cells/batteries like to be in a fully charged state. You cannot do damage by keeping them charged. They have no "memory" function.
     
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  20. Redpoint5

    Redpoint5 Senior Member

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    I'll just add to David's comments that there are many different chemistries of batteries that are called Lithium-ion. Lithium Iron Phosphate batteries (LiFePO4) are part of the Lithium-ion battery group, but have less overall energy storage potential, and are much more resilient to deep discharging. Their voltage makes them ideal for automotive applications because connecting 4 in series gives the proper voltage for vehicles, and does not require any charging cutoff since they can safely be charged above the 14.5 volts or so that alternators supply. LiFePO4 can be used in vehicles with no special charge or discharge circuitry.
     
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