12v battery charging mod for efficiency

Discussion in 'Gen 3 Prius Accessories and Modifications' started by Velo781, Aug 3, 2020.

  1. Velo781

    Velo781 Junior Member

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    The idea is to improve efficiency by reducing the load on the HV battery by keeping the 12v battery at full charge during driving.
    To do this, I would use a 72v 4ah lithium battery (from my lawnmower/ebike) connected to a dc step down converter to output hopefully 14v or so.
    The 72v battery has about 250 usable watt hours. So if I drain that whole battery in one hour that would be the same as saving about 250wh from the HV battery. That equates to about 2 miles (considering ev state is only really used at low speeds the 125wh/mile is realistic.) So an extra 2 miles could result in an improvement from 55mpg to 56.8mpg if my average speed is 60mph or 55mpg to 60.5mpg if my average speed is 20mph in town.

    A 5mpg improvement from using a tiny battery that I already have.... Seems like a good deal. Thoughts?
     
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  2. Grit

    Grit Senior Member

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    An Extra 2 miles to the tank? You might as well get on the road earlier and drive a bit slower to get to your destination on time and still get similar results.
     
  3. Velo781

    Velo781 Junior Member

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    2 extra miles per hour of driving.

    If I go slower I just piss off drivers which I don't want to do. I typically keep around speed limit.

    Its not about money savings. Its about satisfaction from playing the mpg game. And that lithium battery seldom gets used. Its going to age out of use before its capacity drops from cycles. Seems like a waste.
     
  4. ASRDogman

    ASRDogman Senior Member

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    ??? Maybe I missed part of this post... Or you didn't post it here?
    Anyway...
    I don't believe there is much of a "load" of the HV 240 +/- volt to charge the 12 volt battery and
    run all the electronics. It's designed to do so, and very efficiently.

    Seems like a lot of work, and expense, for extremely minor results...

     
  5. fotomoto

    fotomoto Senior Member

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    Add 10 psi to all 4 tires. You may not care for the ride quality but it will be a lot cheaper and achieve the same results or better. Since you're a bicyclist, you're probably VERY familiar with this concept.

    Also, folks with sharp pencils will ask you to account for the energy to charge the second battery in your calculations whether it was "free" or not.
     
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  6. sam spade 2

    sam spade 2 Senior Member

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    BAD idea.
    The amount of energy used to keep the 12 V charged is very small.
    The improvement won't be anywhere NEAR what you estimated. More like .05 MPG maybe.
    The cost of your parts probably wouldn't be recovered over the course of 30 years.
    And then there is the very real chance that your modifications would FRY something.
     
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  7. bisco

    bisco cookie crumbler

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    this is brilliant. i have a bunch of these batteries and a charger. can you please post the set up details?

    i think you're on to something spectacular here. all the best!(y)
     
  8. sam spade 2

    sam spade 2 Senior Member

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    No it isn't. :rolleyes:
     
  9. bisco

    bisco cookie crumbler

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    it's all about perspective, and frame of reference
     
  10. PriusCamper

    PriusCamper Senior Member

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    The charging system for the 12v is efficient, this additional effort doesn't improve gas mileage as far as I can tell... Adding air to your tires will improve MPG and so will pulse & glide driving technique, but not this...
     
  11. fuzzy1

    fuzzy1 Senior Member

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    I'm also not a believer in using only gasoline MPGs as an efficiency metric when additional energy is piped in from a wall socket. The plug-in electric energy is not free, so must be included in any serious calculation. That is why I pay no attention to 200+ MPG claims from PHEVs, other than as playful curiosities.

    OTOH, if that e-bike battery is already sitting around under-used, and more likely to expire from age than from use, I can't object to some projects just for the fun of it ...
     
    #11 fuzzy1, Aug 4, 2020
    Last edited: Aug 4, 2020
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  12. Grit

    Grit Senior Member

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    Ah now you tell us, I was going to say 2 miles extra to the tank for all that work. Will the step down converter be able to handle power going to the other direction; from prius dc-dc converter upstream to the step down converter to the 72v battery? Or will some type of circuitry be implemented to convert and invert power back and forth simultaneously?
     
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  13. Grit

    Grit Senior Member

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    I know a feller here who has a prime and solar array at home plus free charging at work so he’s free of charge when it comes to plug in electricity.
     
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  14. PriusCamper

    PriusCamper Senior Member

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    Ran into a guy at the gas station back in January and he was filling up his Prius Prime and I asked him how good his gas mileage is and he said, "I have no idea, I haven't filled up the tank since last Summer and yet I drive it everyday." :)
     
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  15. Grit

    Grit Senior Member

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    You have to wonder if the gasoline sits in the tank for that long will do harm to the engine. I heard the volt will detect current fuel in the tank for a time period, if too long ICE will come on to burn the gas in case it goes bad after so and so months.
     
  16. fuzzy1

    fuzzy1 Senior Member

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    Toyota merely recommends adding 20 liters per year. If there isn't enough empty room in the tank to add 20 liters, then go burn off enough to make room.

    See also:
    How desiging the Chevy Volt's sealed gas tank brought automakers, CARB together | Autoblog
     
  17. bisco

    bisco cookie crumbler

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    the pip runs every 124 miles, come rain or come shine. t must have learned something to remove it from prime
     
  18. PriusCamper

    PriusCamper Senior Member

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    Lol... that's funny... I bet Prius doesn't offer that bad gas burning feature... But it's a good reason to buy no-ethanol gas which is stable for longer than gas with ethanol, which can absorb water.
     
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  19. Raytheeagle

    Raytheeagle Senior Member

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    Do tell:p.
     
  20. Velo781

    Velo781 Junior Member

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    A blocking diode to keep current from flowing from the 12v battery to the dc step down converter would be a good idea.

    Has anyone actually measured how much 12v power is used per hour? Obviously its more if the lights are on. But even if its really efficient, the drain has to be at least 200 watts to power everything. Power steering, brake pumps, all the electronics, brake lights.... That's 200watt hours that could used for a couple miles of rolling at low speed. Maybe 2 bars equivalent on the HV battery.
    I already have my tires at 42psi and do the p & g when possible.
    The cost is a $16 dc converter. I said its not about money, I'm not counting how long it would take to break even from a 3mpg increase. Its purely the satisfaction of using less fuel. The 3 cents it cost to charge my 72v 4ah lithium battery could get me 2 miles. That's 1.5 cents per mile. Its really not a big project at all. My only concern is getting the exact 14.5v I need to keep the current flowing from my small dc converter not the HV battery converter.
    Some testing to see how the stock system operates would be the first step. Such as what voltage does the 12v battery reach at max during driving. I'm not new to this sort of stuff. Been building my own small ev's for years.
     
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