12 Volt Lithium Battery...Possible ?

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

  1. GregP507

    GregP507 Senior Member

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  2. neez

    neez Member

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    The only advantage i see is weight, which pays huge dividends in motorcycles, a little in race cars, but not so much for everyone else, unless you want to start stacking 2-3 of these for a large battery bank, which is a consideration for start and stop technology on non-hybrids. But the prius already has a large traction battery for that.

    Without a jump in MPG, i really don't see the benefit of doing this as opposed to an optima yellowtop.

    Actually i just read the datasheet on those batteries you linked. They are not meant for underhood operation which is fine for the prius, but again it appears they are worried about explosion as well, and aren't to be used in sub-freezing temperatures, which pretty much cancels out use in all northern states.
     
  3. GregP507

    GregP507 Senior Member

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    I have a lithium-ion car-booster pack that's about the size and weight of a big hoagie, and cost me 40 bucks. It will boost a vehicle from a dead flat. I can't see why something similar couldn't be used as a car battery.

    Stanley - SSLION - Simple Start Digital Battery Booster | Sears Outlet
     
  4. neez

    neez Member

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    It's not a matter of "can't be done". Because it can be done. It's a matter of "why"? Why spend the extra money, sacrifice reliability and longevity, for a negligible gain??
     
  5. GregP507

    GregP507 Senior Member

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    Or sacrifice new battery sales at least once a year.
     
  6. neez

    neez Member

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    I don't foresee replacing my $150 optima yellowtop for many years.

    As stated above, those lithium packs can't even be used below freezing. They truly are for race vehicles, not intended for everyday drivers unless you live in florida or southern california.
     
  7. GregP507

    GregP507 Senior Member

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    I've had an Intimidator yellow-top AGM battery for over 6 years and it's still going strong.

    I was talking about the standard crappy lead-foam automotive batteries that don't seem to survive going flat very many times before you have to replace them.
     
  8. David Beale

    David Beale Senior Member

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    Resurrecting an older thread here.

    First, the object of replacing the 12V lead acid battery should be to make the car more reliable when it sits for a long time.
    So you would have to increase capacity by at least double to carry you through the parasitic drain.
    Using a Lithium unit would also drop some weight, though in a car it wouldn't be very noticeable (which is why most of the units made are for motorcycles and ATVs where 5-10 lbs IS noticeable).

    Second, whatever you choose it will be more expensive than the high priced stock Toyota lead acid unit.

    Third, a lithium battery designed to replace a lead acid unit in a vehicle will have charging and discharge protection built in, so it will take care of itself.

    With all that in mind, and what caused me to resurrect this thread, check out "Anti-Gravity Batteries". They make both replacements and "booster units". I've got a booster one (18 AHr) coming to me. Not cheap at almost $200 but very interesting. The replacement units are quite a bit more, at around $400!
     
  9. GregP507

    GregP507 Senior Member

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    I can't think of anything more reliable than 9 Duracell D-cells in a cardboard tube.
    :LOL:
     
  10. bwilson4web

    bwilson4web i3 and Prime

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    Late to the discussion:
    • 11 NiMH cells in series, 10 Ahr solder tails - would have an operational voltage range matching a 12 V lead acid
    • Requires a low-voltage protection - discharging to under 1 V/cell risks permanent failure
    • Shunt regulator can keep four, paired and one triple set cells balanced
    But it makes no sense in a Prius as the weight savings are 'in the noise.' However in a light airplane or motorcycle, it would be just fine and integrate with existing 12V alternators provided there are no parasitic loads. In a light plane, there are weight and complexity considerations.

    The Prius parasitic loads have been reported as ~30 ma. (I have not measured them, yet.) It makes more sense to install a solar battery tender like the one on the moon-roof Prius. A solar panel only needs to provide a watt or so to handle the load during the day and enough charge to handle the night load. This also becomes a tradeoff:
    • simple linear regulator and more solar cell area - waste excess charge and cutoff in dim light
    • switching regulator and less solar cell area - boosts dim light to extend charging window
    • modular regulator and solar cell system - minimizes shadow effects for smallest cell area
    Bob Wilson
     
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  11. Redpoint5

    Redpoint5 Senior Member

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    Thank you Bob for providing the parasitic load you heard through the grapevine. That gives an estimate of what capacity a battery replacement would need to be. What is the Ah rating of the stock battery?

    I'm wondering if the Pb-acid battery could be replaced with a bank of supercapacitors? I'm already operating the motorcycle in my avatar on 6-series connected 400 farad supercapacitors. It has replaced the battery and only weighs about 1 pound. The downside is that the bike can only sit for about 3 days before it can no longer start the bike.

    My concern with replacing the battery with a bank of supercaps is that the charge controller that feeds the 12v battery could be damaged by excessive power draw. Capacitors have very little internal resistance and can accept enormous currents. The charge controller would need to be self-regulated to not exceed its output capabilities.

    I don't see why LiFePO4 wouldn't work, assuming the battery isn't charged in sub-freezing temps. 4 of these 20 Ah batteries in series would provide 27 days of juice for the Prius to sit, assuming a 30 mA parasitic drain. At $140 the price is right.

    The point of replacing the Pb-acid battery would be the following:

    1. Extend battery life
    2. Reduce money spent on frequent battery replacement
    3. Fuel efficiency (Pb-acid batteries are only 50% efficient at accepting a charge compared to 80% LiFePO4 efficiency and 95% for supercaps)
    4. Reduced weight
     
  12. CR94

    CR94 Senior Member

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    That's only about half the nominal capacity of the standard lead-acid. If 20Ah is good enough, you could also save cost and weight by using a half-size lead-acid.
     
  13. Redpoint5

    Redpoint5 Senior Member

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    Not exactly. Lead-acid batteries are quickly damaged by deep discharges, and even at 50% depth of discharge, severe damage is being done. A LiFePO4 battery sustains much less damage from being discharged deeply. I'd say you can somewhat safely get 10 Ah out of the 40Ah lead-acid battery, and 15 Ah out of the 20 Ah-rated LiFePO4.

    LiFePO4 is just better at everything except for price and being charged in sub-freezing temperatures.
     
  14. CR94

    CR94 Senior Member

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    There's some truth in that, but lead-acids designed as deep-cycle batteries tolerate deep discharges better than the essentially ordinary car batteries some manufacturers are selling as Prius batteries.

    For people who make mostly shortish trips in cold climates, recharging the battery in sub-freezing temperatures happens to be important.
     
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  15. neez

    neez Member

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    "BETTER" is a relative term. I think it should always be in the context of the end performance gains of the vehicle, but as i explained earlier, is pretty much none for most people. The prius has a very low demand on the 12v battery. The standard battery is large enough to power the 12v system during stop lights. After which the engine will come on during acceleration and begin to recharge everything. There is a negligible gain for adding more capacity to the 12v system.

    Also, there are different types of lead acid batteries. Marine/RV batteries can take severe discharge and be fine. The optima yellowtop was designed specifically for this as well. "Starting" type lead acid batteries, found in most traditional automobiles, are what you're referring too and have thinner plates and more plate surface area. This is required to offer much higher amperage for cold starts which may exceed several hundred amps for a short burst. This isn't necessary on the prius since it uses one of the MG's to start the engine.

    The only time i see needing more 12v capacity is if you sit in your prius for very long periods of time, like heavy rush hour freeway traffic in mild weather(no HVAC), or sit in parking lots on patrol as mall security or something without the HVAC turned on, or use your prius as an on-demand plug out generator. You could always just wire a much more generously sized battery in parallel for these cases, having problems in freezing weather is not worth the Li-ION in my opinion.
     
  16. Easy Rider 2

    Easy Rider 2 Senior Member

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    Here we go off on another tangent.
    NONE of those things will cause a problem IF you are actually in the "ready" or "run" mode so that the system can keep the 12V charged as needed.
    The problem is when you drain the 12 V by doing something when the system is not "ON" or if you repeatedly turn it ON and OFF without enough time in between. There IS some extra drain booting up the computers and running the lights after the system is OFF.
     
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  17. neez

    neez Member

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    I never referred to anything as being a "problem", i'm just talking about performance gains. The prius Inverter/converter charges the 12v battery from the traction battery, so having more capacity could help decrease the engine runtime duty cycle.
     
  18. Redpoint5

    Redpoint5 Senior Member

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    One other benefit of using LiFePO4 over lead-acid is charge efficiency. Lead-acid batteries can be less than 50% efficient at accepting a charge when the battery is near full capacity. This is unfortunate because lead-acid batteries need to be maintained near full capacity or they become damaged. That means it's common to loose half of the energy that goes into keeping the 12v battery charged.

    The LiFePO4 chemistry is about 90% efficient at accepting a charge, even as full capacity is being approached.

    While these differences in efficiency won't make a huge impact on fuel economy, it will make some difference.
     
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  19. RobH

    RobH Senior Member

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    The 12V battery is a hold-over from conventional cars. I think it should be replaced with a voltage conversion circuit fed by the traction battery. The circuit could shut down if the large battery goes too low, just like a dead 12V battery.
     
  20. GregP507

    GregP507 Senior Member

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    The 12V battery is not a performance component, it's a maintenance component. It has nothing to do with any performance aspects of the car. It maintains a charge to provide 12V power to accessories and lights when the car isn't running. It also activates the computer
    Common sense would suggest that, but significant revenue is generated by the dealers from service calls due to dead batteries.
     
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