Battery Basics 101

Discussion in 'General Electronics Chat' started by Denesius, Oct 24, 2014.

  1. Denesius

    Thread Starter Member

    Feb 5, 2014
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    Ok, another late night bar question that I found perplexing, and here's hoping someone on this forum can address:
    I think I'm accurate in stating that a single cell example of a battery is a chemical reaction producing electrons- A reduction rxn at the cathode & oxidation at the anode. The power capacity is dependent upon the volume of reagents present, the current capacity a function of the internal resistance (itself based on the surface area & size of the electrodes & the proton conducting electrolyte, etc).
    What determines the voltage of a single battery cell- the number of electrons produced by each unit of the reaction, the equilibrium point of the reaction, the activation energy, or some other factor?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the materials the battery is made of.
     
  3. #12

    Expert

    Nov 30, 2010
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    Yes, it's really that simple. Every combination of electrodes creates a voltage peculiar to that exact combination of materials.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    It's the difference between intrinsic (voltage, dependent on thermodynamics, independent of scale) and extrinsic properties (current, dependent on surface area, geometry, fouling and other properties of scale).
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Each reduction half reaction (ion plus electron) has a unique potential

    The reverse reaction, oxidation (ion or metal minus electron) has, ideally the negative of the reduction potential.

    Simple algebra determines the total voltage (add the reduction potential to the oxidation potential and get the total voltage of one cell.

    A list of "reduction potentials" should be on Wikipedia.
    You can also look up "half cell reactions" and "electrochemistry" for more detail.

    [​IMG]
     
    Last edited: Oct 25, 2014
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  6. Denesius

    Thread Starter Member

    Feb 5, 2014
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    Very interesting & enlightening answer - So if I could build a Flourine-Lithium battery, I'd have almost 6 volts per cell? I'm sure there some other factors restricting this battery design!
     
  7. Denesius

    Thread Starter Member

    Feb 5, 2014
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    I think what you refer to as 'extrinsic properties' are current limiting, not voltage determining. A battery's voltage is constant at zero current flow- that in turn is determined by the reaction undergoing in each cell. The question was: what property of the reaction determines the voltage of each cell. I think GopherT answered that nicely
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    In theory you are right, in practice, building a battery that can handle the highly exothermic reaction of lithium metal and fluorine gas without breaching will be a trick. Also, some applications are difficult enough (and dangerous enough) to not use lithium, now add fluorine to the mix and you have real potential for disaster.

    Also, these are in aqueous solvent at 25 C. Notice that lithium is more difficult to reduce than it should be according to periodic table trends (it should be K > Na > Li). However, lithium holds a pretty big solvent sphere of water around it to essentially protect it from reduction. Also, lithium would spontaneously react with aqueous mixtures without the completed circuit to discharge the battery. Changing solvents can change the reduction potential. Propylene carbonate is a comment electrolyte solvent in lithium batteries.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Yes, that's why I said exactly that.

    Note that the potentials of those half reactions are at defined conditions. Those values change somewhat with ambient conditions, such as temperature.
     
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