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Battery voltage spike

Discussion in 'Power Electronics' started by shiekh, Oct 13, 2010.

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  1. shiekh

    shiekh

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    Oct 11, 2010
    I have noticed something quite interesting; when one releases the load from a lead-acid battery one gets a heavy over-voltage spike which I do not think is of an inductive nature at all; anyone any idea what might be causing this?

    I am using it, despite my lack of understanding, to desulfate old batteries as the the over-voltage can drive the sulfate back into solution, albeit rather slowly.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It's inductive.

    Even if you just have some short leads, they will have some inductance. As you break the load, that inductance wants the current to continue flowing. An arc can form briefly, and there may be a large voltage spike.
     
  3. shiekh

    shiekh

    77
    1
    Oct 11, 2010
    Some inductance, sure, but enough to get a 20V over-kick at the battery terminals? I even used a low inductance resistor as load in my experiments.

    Other approaches use real inductors to achieve the same effect, but with the same sort of kick resulting. If the mechanism were inductive, then one might expect smaller kicks when just stray inductance was responsible.
     
  4. Resqueline

    Resqueline

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    Jul 31, 2009
    When you change the inductance (from "real" to stray) the only thing that changes is the frequency components involved, not the voltage (or current) levels.
     
  5. shiekh

    shiekh

    77
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    Oct 11, 2010
    So if I open a switch with a resistive load (only stray capacitance) it suffers the same voltage kick as one carrying a heavily inductive load?
     
  6. trobbins

    trobbins

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    0
    Jun 15, 2010
    shiekh - imho, you will be lucky to obtain any tangible recovery of battery capacity with any such technique - at best you will see some capacity retention, but it will soon revert to being bad again. A lot of snake-oil has be made of such techniques.
     
  7. shiekh

    shiekh

    77
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    Oct 11, 2010
    I agree, a LOT of snake oil; but I think there may be something to it.

    For example, it is known of old that if one empties the battery and fills it with distilled water, the resultant weak electrolyte allows for a high voltage which is said to remove sulphation. This is just an attempt to apply a high voltage without the need to weaken the electrolyte; but I agree, the subject has gone the way of witchcraft... but it does make for a fun project.

    I monitor progress by watching the peak value of the kickback; it just takes a small rectifier circuit and one can use a DMM to measure this peak. The peak falls as the battery internal resistance supposedly recovers, but we are not talking about overnight progress here, more like months. If it works (and that is a big if) I conceive of a small cheap unit hooked permanently to the working battery where waiting months is not an issue. However, subjecting the connected circuits to high voltage high frequency pulses may be a problem...
     
    Last edited: Oct 14, 2010
  8. Resqueline

    Resqueline

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    Jul 31, 2009
    Stray capacitance is not the same as stray inductance...
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Consider that it is very hard to make a circuit where the current through the battery, and through the connecting leads DO NOT form the boundary of some area in which a magnetic field can form. When the current stops, the magnetic field collapses giving an inductive 'kick'.

    If you think about it, it's awfully hard to avoid. (As is stray resistance and capacitance, not sure about memristance though.)
     
  10. shiekh

    shiekh

    77
    1
    Oct 11, 2010
    I don't deny there will be stray inductance, but enough to double the battery voltage?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Only double?

    If you have faster test equipment you should be able to see something much higher!

    Check out the operation of a boost switch-mode power supply and see if there is a limit to the increase in voltage. Then read Resqueline's first post in this thread again.
     
  12. shiekh

    shiekh

    77
    1
    Oct 11, 2010
    I would upload my results but the PDF files exceed the size limit.
     
    Last edited: Oct 15, 2010
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Try uploading it here.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Post your documents and then tell us exactly which part you don't understand. I suspect that it's a question of how the circuit in figure 3 produces the waveform in figures 4 and 5 (in the second document).

    I suspect that the answer is as I first stated -- Inductance. I also suspect that it's the most ineffectual method of achieving your objective since the power available in each spike is determined by both the voltage and the current. In this case the current (when the circuit is opened) is rather limited.

    As an academic issue, yes it illustrates that the stray inductance and capacitance is enough to cause ringing. However alternatives with much larger inductors and capacitors have the ability to store and then release significant amounts of power.

    Your paper has a number of weaknesses, and I'm sure that people would be pleased to assist you by pointing out where you have failed to cite references, or where you have accepted as fact, things which are either unproven, or where you have not offered evidence of efficacy.
     
  15. shiekh

    shiekh

    77
    1
    Oct 11, 2010
    Thank you for your help.
     
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