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Experiment about Discharging Batteries

  1. Feb 22, 2013 #1
    Right now I'm doing an experiment for my EE about battery capacity. How would I be able to monitor the battery's true emf while discharging the battery at a constant current?

    Also, when I connect the battery in a circuit and discharge it at a constant current, I notice that the voltage across the battery decreases. Since I am discharging at a constant current, will this difference in voltage from when its in a circuit and the true emf be constant?
     
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  3. Feb 22, 2013 #2

    mfb

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    Connect a voltmeter?
    Right.
    Which difference? The measured voltage IS the true voltage the battery delivers at that current.
     
  4. Feb 22, 2013 #3

    Simon Bridge

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    Also see:
    https://www.physicsforums.com/showthread.php?t=670059
    OP feels that the "true" emf of the battery is the open-terminal voltage.
    He wants to keep track of it for the discharge curve... most people seem happy just to use the terminal voltage on-load.
     
  5. Feb 22, 2013 #4

    mfb

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    Hmm, well, can you interrupt the discharging process for the measurements?
     
  6. Feb 22, 2013 #5

    Simon Bridge

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    That's what I thought - but doesn't the open circuit voltage for a battery take a bit to "relax" after the load is disconnected, like a hysteresis effect ... possibly altering the discharge curve? It would depend on the battery of course...

    I also thought that the early measurements could be used to work out the internal resistance and, after that, the open-circuit voltage would just get calculated.
    However, the internal resistance may change during discharge - it would certainly be affected by temperature.

    If the load resistor was very large compared with the internal resistance, then the loaded voltage would be close to the open-circuit...
     
  7. Feb 22, 2013 #6

    mfb

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    I would be surprised to see any effect on the scale of a human intervention (~1s), but that would be easy to test.
     
  8. Feb 22, 2013 #7

    Simon Bridge

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    I was thinking that repeatedly switching the circuit may be different enough from a continuous discharge, over the entire discharge time, to affect the shape of the curve - but like you say: easy to test.

    We don't know how accurate things are either... any effect could be small compared with the sensitivity of the instruments - especially seeing the battery in question is quite high tech.
     
  9. Feb 22, 2013 #8

    jim hardy

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    @ johnsmith

    That would be interesting to test. I never tried it with an exotic battery.
    Published curves are with battery under load.

    Tinkering with automobile batteries, indeed there is a time delay when you release the load. Voltage bounces back to 12.6 in about a minute.

    I've seen car batteries that'd indicate 12.6 volts but were too run down to illuminate even the dome lamp.

    So i think you should take your data with battery loaded.

    Then experiment with the voltage 'bounce' when unloaded from various states of discharge.
     
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