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Leak Charge

  1. Jan 30, 2010 #1

    Pythagorean

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    I've noticed that some devices (when they're low on battery charge) can be shut off and will regain some charge while they're off (if you leave them off long enough).

    What's going on here, physically?
     
  2. jcsd
  3. Jan 30, 2010 #2
    Some capacitors can recharge slightly after apparently being completely discharged, due to "memory". The only safe large capacitor is one with the two terminals shorted together. Any unshorted capacitor should be treated as though it were (or was) charged. I consider it to be long-term time constants in the dielectric.

    Bob S

    [added] Look for phrase Dielectric absorption (or DA), sometimes called "soakage", "voltage retention", and other things. In high-voltage power cables it´s called "return voltage". A capacitor, once charged, stubbornly retains part of the charge, even after being discharged (shorted for some number of seconds), as if it had "soaked" into the dielectric. in

    http://my.execpc.com/~endlr/
     
    Last edited: Jan 30, 2010
  4. Jan 30, 2010 #3

    uart

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    Batteries don't deplete their reagents uniformly, particularly under heavy loading. They are depleted near the electrodes first, and as the reagent concentrations near the electrodes falls the battery appears to be flat. When left unused for some time the reagents will have time to diffuse from higher concentration regions deeper within the cell toward the lower concentration regions near the electrodes and the battery therefore appears to have spontaneously regained some charge.
     
  5. Jan 30, 2010 #4

    Pythagorean

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    Yeah, I have a giant capacitor I'm working with to make an EM cancrusher (peak 14 kV). I definitely keep the terminals shorted. Still, I'm curious of how the charge separation physically reoccurs (assuming that's what's happening in this case).
     
  6. Jan 30, 2010 #5

    vk6kro

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    With batteries, I've seen this at an elementary level.

    In that case, it was due to a collection of bubbles on the surface of one of the electrodes.
    When the bubbles cleared, more of the electrolyte liquid could contact the metal electrode and the voltage rose accordingly.
    Because this was happening in a glass beaker, the problem was easily visible.

    I guess something similar happens in commercial batteries. They do tend to recover slightly after being taken off load for a while.
     
  7. Jan 30, 2010 #6

    MATLABdude

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    I had the same problem on a Lithium Ion (?) I bought to replace the one I'd had in my Palm PDA (TIP: buy third party or even 'OEM' batteries from reputable sellers, eBay or otherwise). I think it's probably poor battery performance under load (and for extended duration, with warming, etc.). That or it doesn't match the expected charge profile. Either way, if this device is still under warranty, you may want to look into it.

    This is probably also dependent on what the 'restoration' is like. Mine would auto-shutoff and then, 10 minutes later, report 90% charge.
     
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