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Capacitors instead of batteries.

Discussion in 'Power Electronics' started by Dan Robbins, Feb 5, 2015.

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  1. Dan Robbins

    Dan Robbins

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    Feb 5, 2015
    Hey guys. Im just thinking aloud.
    Im wanting to hook up a small turbine/solar system to an aquaponics system that i have set up at home. I want water circulation via a small DC air pump but i do not require this to be continuous.
    My question is, because of the larger internal resistance of the batteries, it would be expected that i'd get more charge for the same voltage supplied by the solar cell.

    Would it be feasible to just hook up a couple of super capacitors to a my solar system and just allow it to charge/discharge running a small air pump? Is there a way that I could slow down the discharge from the capacitors to the order of a few minutes per discharge?

    My apologies for if this seems a stupid question, i am by no means experienced in electronics. Dan
     
  2. BobK

    BobK

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    Jan 5, 2010
    Hi Dan, and welcome to the forum!

    I don't quite understand what you are trying to do. Normally, a solar or turbine system would use batteries to store energy to use in periods where there is not energy available. This would typically be hours, or even days. But you are talking about using up the energy in a few minutes.

    The only way I can make sense of this is if the solar / turbine system does not produce enough power to run the pump directly. So you need to save up the power, for say 1 hour and then use it to run the pump for a few minutes. Is that what you are trying to do?

    To help you out with this we will need to know:

    The output of your solar / turbine system in volts and amps.
    The voltage and current requirements of your pump.
    How long and how frequently the pump is run.

    A capacitor is not likely to be the right solution, even though, as you point out, there would be less waste than with batteries. This is because the energy storage capacity of capacitors is far less than the same weight / volume of batteries.

    Bob
     
  3. Dan Robbins

    Dan Robbins

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    Feb 5, 2015
    I do not have the solar or turbine system yet, i will be making this in the coming weeks. There is no reason that i cannot just use a mains plug to power this as i do have access to it, however, I want to make it as self sustained as possible without the need to run off mains. It's just fulfilling my need to play with wires and capacitors, - just for me.
    This is partly reason for wanting to avoid the batteries. they're big, bulky, heavy and don't last that long, right? I understand that capacitors can be charged and discharded many many more times than batteries can. If I'm paying to constantly buy batteries, i may as well just plug in the AC air pump that i already have, it's only a few watts.

    I would just like it to run for about 2 minutes, and charge for 8minutes for example.

    So this is just more of an experimental build than anything.
    Dan
     
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    Capacitors do wear over time, and when subject to excessive temperature swing, electrical noise, charge or discharge rates. I would much rather replace a small Sealed Lead Acid (commonly used in home alarm system backups, and motorcycles) once every few years than to source and resolder Capacitors every season or two. You should also understand that capacitors can pack a decent charge, and can provide a lot of current... but they do not have the same capacity of batteries yet... This means you will either end up with a large capacitor array, or a smaller rechargeable battery.

    Ultimately, it's your call. If you treat your components nicely, they will last much longer.
     
  5. BobK

    BobK

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    Jan 5, 2010
    Okay you answered one of my questions, and apparently cannot answer the question about the solar / turbine.

    But you did not tell me what voltage and current your pump needs. I need to know this in order to calculate what you need in terms of capacitors.

    Also, you refer to batteries being large and bulky. Capacitors with the same energy capacity will be on the order of 10x larger and bulkier:

    http://en.wikipedia.org/wiki/Supercapacitor

    Bob
     
  6. Dan Robbins

    Dan Robbins

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    Feb 5, 2015
    My apologies.
    I do not have any of the turbine/solar or components of this system including the DC air pump. This is what i was thinking of however.
    http://www.ypump.com/mini_air_pump_ajk-b2701.htm
    And when i was talking about capacitors i was actually talking about super capacitors, my bad!
    so, feasible? Dan
     
  7. Gryd3

    Gryd3

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    Jun 25, 2014
    Yeah, it's feasible, but scroll up and read the quoted section from wikipedia... you will end up with quite a large capacitor bank in relation to just using a battery.
     
  8. BobK

    BobK

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    Jan 5, 2010
    Okay. 6V 400mA it is.

    The equation for a capacitor discharge is:

    dV/dT = I / C

    So, let's say you have charged your capacitor up to 6V and the pump will still run reasonably at 5V. This means you will discharge the capacitor from 6V to 5V over 2 minutes.

    dV / dT = 1 / 120

    So:

    1 / 120 = 0.400 / C

    C = 0.4 * 120 = 30F

    This is actually not that bad. Quite doable. Typical max voltage for supercaps is < 5V though, so you would need 2 or more in series and circuitry to balance them when charging, or you would need a boost converter to get from the capacitor voltage to the needed 6V.

    To recharge the capacitor over 8 minutes you would need 100mA + a bit for losses, say 125mA.

    For comparison, 4 AA batteries would run the motor for about 5 Hours without recharging, and would be roughly similar in size.

    Bob
     
  9. Dan Robbins

    Dan Robbins

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    Feb 5, 2015
    very helpful indeed. Thank you!
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
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