What happens when charged capacitors are swapped from parallel to series connected?

Discussion in 'Power Electronics' started by Mark Flint, Aug 2, 2017.

  1. Mark Flint

    Thread Starter Member

    Jun 11, 2017
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    I am reading about capacitors and have a few blocks in my understanding...

    If I have 3 supercaps, 2.7v 400F, and I charge them in parallel I will have 1200F at 2.7v, which is 4,374 joules.
    If I have 3 supercaps, 2.7v 400F, and I charge them in series I will have 133.33F at 8.1v, which is 4,374 joules.

    1) does it take more input power to charge caps in series than the same caps in parallel?
    2) If I now swap the 3 caps that were charged in parallel TO a series configuration, do I loose joules?
    3) If I loose joules WHERE do these joules go?
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    As you have calculated the stored energy is the same whether they are in series or parallel.
    Therefore:
    1) No
    2) No
    3) See 2)
     
  3. Mark Flint

    Thread Starter Member

    Jun 11, 2017
    52
    1
    So therefore I can charge the caps at 2.7v in parallel, swap them to series and use them at 16.2v with no loss of energy. Makes sense to me, but I've been advised that capacitors "don't work like that".

    Cheers,
    Mark
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    How did you plan to do this swapping? What do you think you are going to use them for?
    The reason I ask is that you may have some surprises in store for you.
    If you are not familiar with RC circuits, I recommend the following background material.

    https://en.wikipedia.org/wiki/RC_circuit
     
  5. Mark Flint

    Thread Starter Member

    Jun 11, 2017
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    I plan to run a DC motor from the capacitor bank. The caps would be charged in parallel, disconnected from the circuit, swapped to series, reconnected to the circuit.
     
  6. dl324

    AAC Fanatic!

    Mar 30, 2015
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    A conservative designer wouldn't operate components at their maximum ratings.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    There's the theoretical aspect and then there's the practical aspect. You can stack charged capacitors in series and the resulting voltage is the sum of all of them, in theory. But there are practical problems in doing this, one of which is that there is no simple way to insure that the voltage across each cap will stay below its rating. And while you get more voltage, you lose capacitance compared to the parallel arrangement. So energy is conserved but all you gain is voltage. There are other, more practical ways to get a DC-DC voltage boost.

    The advice may have been related to these practical issues, rather than the "on paper" analysis.
     
  8. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Incidentally, it is important that the capacitors all have the same value or bad things will happen when they discharged in series.
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    Running a motor from capacitors is a bad business. To start a motor requires a large initial current. The capacitor bank will provide this, but the cost is that the voltage will decline exponentially. What you have left may or may not keep the motor running and there will be less and less current as the voltage declines further. Where did you get the notion that this was a good idea?
     
  10. Mark Flint

    Thread Starter Member

    Jun 11, 2017
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    True.

    Noted.

    Yes, I think so.

    I just like finding different solutions - and don't let that stop me asking the next dumb question :)
    When someone says "bad idea" - I mostly hear "design challenge".
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    Sometimes bad idea means just that. By all means -- run the experiment and find out for yourself. Just don't be surprised, and don't say nobody told you so.
     
  12. Mark Flint

    Thread Starter Member

    Jun 11, 2017
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    You said it was a bad idea without any knowledge of my circuit.
     
  13. dl324

    AAC Fanatic!

    Mar 30, 2015
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    You said you want to run a motor from some capacitors.

    Do the experiment and see for yourself what's wrong with your idea.
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    That is because I know what will happen when you connect any kind of load to a capacitor bank. It's not rocket science, but it is freshman calculus and physics. If you missed them, then you get a pass.
     
  15. Mark Flint

    Thread Starter Member

    Jun 11, 2017
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    Here's an example of a similar motor, but smaller than mine, hence I am looking at bigger caps than this guy is using.



    I'm currently running my motor on batteries.
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    What is the load on the motors: his and yours? If there is no load then they're just toys.
     
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