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Charging an super/ultracapacitor from AAA batteries

  1. Feb 10, 2013 #1
  2. jcsd
  3. Feb 10, 2013 #2

    mfb

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    It is possible, but it will take some time (probably of the order of seconds to a minute with a single battery, depending on your setup) to fully charge it.
     
  4. Feb 10, 2013 #3
    that is a perfectly acceptable time scale.

    I am designing a device that will carry a payload of around 100grams vertically up a pipe for around 2 metres. the motor will power a wheel.

    The reason for the involvement of capacitors, is that i am limited to the 'energy' supplied by two AAA batteries.

    Obviously capacitors are literally more powerful as they can discharge the same energy faster. the device needs to climb as quickly as possible, so a higher power is better.

    how could i best choose a capacitor to run a small DC motor?
     
  5. Feb 10, 2013 #4

    mfb

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    I would expect that the motor or the mechanical part afterwards limit the speed. The mass just gains 2J of potential energy (+some friction), something even those AAA batteries could deliver in ~1-2 seconds, I think. If you aim for something significantly quicker than 1 second, you need more energy to accelerate your mass.

    Charging a big capacitor is possible, of course - connect it to the battery via some resistor (not too small, but its maximal size depends on the repetition rate of your lifting process).
     
  6. Feb 10, 2013 #5
    Lithium batteries are quite powerful. You can easily draw 1A from a AAA.
    http://data.energizer.com/PDFs/l92.pdf
    If 2 AAA are connected in series you need 2 resistors, each rated for 1.5 Ohm and 2W connected in series to limit the charging current to 1A.
    Theoretically 1F would be more than enough. 1F * (2.5V)^2 / 2 = 3.125J
    But you have friction and also you want it to move as fast as possible. Therefore you need more. Maybe 3F. But remember that those capacitors are only rated for 2.5V so don't connect 2 AAA batteries directly to it. You need something to limit the voltage. For example the LM350 voltage regulator.
    http://www.onsemi.com/pub_link/Collateral/LM350-D.PDF
    Only costs between 1 and 2$.
    You could also try to do it without a capacitor. The maximum discharge current for AAA lithium batteries is 2A and you have two of them. 2A * 1.5V * 2 = 6W. That should be enough to lift 100g by 2 metres in half a second.
    Do you need to use a motor? What about a coilgun? Place copper coils around the pipe and then you shoot the weight up the pipe by discharging a capacitor though the coil.
     
  7. Feb 10, 2013 #6
    before i get excited about a coil gun, does the pipe have to be ferrous for that to work?

    its a copper pipe
     
  8. Feb 10, 2013 #7
    Actually for the coil gun idea to work, the pipe should not be conductive. A plastic pipe would be best.
    Although it may also be possible with a copper pipe but with lower speed and several coils wound at different places around the pipe. If there is a magnet attached to the weight you just have to turn on the coil that is closest to the weight and the magnet will be pulled into the coil. But since a current is induced in the copper pipe during the first few milliseconds after a coil gets turned on, the whole thing is slowed down.
     
  9. Feb 10, 2013 #8
    i think i will have to stick to the motor driving a wheel as the load isnt dense, it is a large container and will be difficult to incorporate into acoil gun design.

    I have been thinking...

    more powerful motors operate at higher voltages with higher current (P=VI)

    two AAA in series = 3v and ~2A = 6W as you said...

    If a 1F cap. holds 3J then it will charge in ~1.5 seconds? (Q=CV Q=IT) charging at 2A.

    is there a way to increase the voltage from 3v to say 6 or 12v, supply it to a 6 or 12v capacitor which will take longer to charge because of the reduced current...

    this capacitor could then power a more powerful motor which would propel the device up the tube faster...


    I hope this makes sense.

    Anthony

    edit:

    the load is on the outside of the pipe not inside
     
    Last edited: Feb 10, 2013
  10. Feb 10, 2013 #9
    If the load is outside the pipe that makes it easier. You could use 3 coils positioned around the base of the pipe and 3 magnets each placed in one of the coils. If you discharge a capacitor through the coils, the magnets will shoot out.
    If you want to increase the voltage you can use a step up converter, e.g. this one
    http://cds.linear.com/docs/en/datasheet/1073fa.pdf
    But I don't think that it will make much of a difference. 12V motors are not automatically more powerful than 3V motors.
    A 1F cap at 2.5V holds a charge of 1F*2.5V = 2.5As. At 2A that would charge in 1.25s.
     
  11. Feb 10, 2013 #10

    mfb

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    If you get 2A from an AAA cell, the voltage might go down a bit. In addition, I would add some resistor to avoid a short circuit, which leads to a slower charging process. This gives some seconds for the charging process.

    It is possible, but I would try to avoid it.
     
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