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Battery theory

  1. Aug 5, 2007 #1
    In battery, after finishing of its construction and positive & negative electrode creation, why we will need an external electrical lead (for example copper wires) for transmission of energy between two poles?

    Why no short circuit occurs between positive and negative electrodes through battery electrolyte and why electrical potential difference is stable in battery?
     
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  3. Aug 5, 2007 #2

    NoTime

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    Batterys are sometimes constructed with multiple internal cells that have no exterior connections.
    However, what would be the point of having a battery if you can't connect something to it?

    Batterys do have a certain amout of internal short circuit.
    One of the design criteria for batterys is to minimize this intenal leakage.

    The electrical potential difference is determined by chemical properties of the electrodes and electrolyte.
     
  4. Aug 5, 2007 #3
    Batteries are surprisingly complex. Simple answers, like those given here are as likely to mislead as inform, nevertheless...

    1. Short circuits don't develop internally because it is electron transfer reactions that occur at each electrode. This means the reaction occurring at one electrode will liberate an electron (donor) and the other electrode will consume an electron (acceptor). So a short circuit requires electron transport between electrodes, in addition to ion transport. Electrolytes do support ion transport but do not support electron transport directly. That's the role the external connection (metals) plays.

    2. The potentials are stable because the active materials deposited at the electrodes produce stable potential until they reach extreme states of charge. You may want a more in depth explanation than that, but trust me that's where things become complicated quickly.
     
  5. Aug 5, 2007 #4

    NoTime

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    2. I think you meant discharge.
    A battery becomes discharged when either all the active material gets converted to the inactive form (in which case the potential goes to 0) or the active material gets shielded by the inactive form dramatically increasing the internal resistance (the potential will stay the same but you get no or very little power from the cell).
    In any event, the potential is determined by the chemistry involved and not the state of charge or discharge.
     
  6. Aug 6, 2007 #5

    Gokul43201

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    Short answer: Because there is a fixed chemical potential associated with each oxidation state of any given element.
     
  7. Aug 6, 2007 #6
    In the battery, the positive and negative polarities are relative concepts. In deed, in battery system one of the two poles is for example negative and the other is more negative in comparison with the electrolyte material.
    In fact, in a separated battery bank after electrochemical stability, the charge transmissions between electrolyte media and both of two electrodes will be stopped and opposite transmission current will be impossible.
    A similar mechanical phenomenon is heat transmission between two metal rods with different thermal conductivity coefficiency that are connected to a very hot metal plate. The thermal difference at the end of rods can cause the heat transfer through a metal wire between them, but the heat transmission between two rod is impossible through the hot metal plate because the plate is warmer than both of them.

    What is your opinion?
     
  8. Aug 6, 2007 #7
    Well not to split hairs but...I was using the term "state of charge" mathematically, as is done in the lit. Values close to 0 describe the state of full discharge and values close to 100 describe the fully charged state. The potential does vary a good bit near both extremes (0 and 100).

    At a deeper level, the potential is determined by thermodynamics, or more specifically statistical mechanics. That's where it gets complicated, and where the state of charge, as a variable in the model, plays a decisive role. Despite the mathematical complexity of the models, the potential turns out to be relatively stable over a very wide range of the state of charge.
     
  9. Aug 6, 2007 #8

    NoTime

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    You have to be very careful interpreting what statistical models are saying.
    As Gokul stated each and every individual reaction event that contributes an electron and ion to the battery has a fixed chemical potential.
    Treated statistically the sum of no event + event by time is going to show up as an average potential different from the electrochemical potential.
    Practically useful but not necessarily a good description of what is happening.
     
  10. Aug 6, 2007 #9

    NoTime

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    If what you mean by this is that the two charged surfaces in an unconnected battery will develop a barrier potential that will stop additional electrochemical events from happening then this is more or less correct. It can get a little fuzzy at the detail level.

    Heat will flow from the central plate down, both rods and into the wire until the entire system has the same temperature. It does not resemble an electrical system at all, in my opinion.
     
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