Converting between mAh and C

Discussion in 'Homework Help' started by OckhamsHammer, May 15, 2014.

  1. OckhamsHammer

    Thread Starter New Member

    Apr 25, 2014
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    0
    The problem asks:

    First part is simple. Low power USB port is given as ~100mAh. So 680mAh/100mA = 6.8hours.

    For the second part and also the reason I'm posting here today. Charge is units of Coulombs so I have (mili-Coulombs/sec)*hrs. So I figured if I just multiply by 3600 seconds (ans= 2.448kC) I would get the correct answer. But the answer given (for both is just below this) they multiplied by 60 seconds: can someone tell me why?

    Thanks in advance!
     
    Last edited: May 15, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    actually batteries take a little more to charge than they are rated for discharge so the charge time is a little more than just by mAh.
     
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  3. OckhamsHammer

    Thread Starter New Member

    Apr 25, 2014
    3
    0
    I get that this is all theoretical but I don't get how they are getting their answer without multiplying by how many seconds there are in a hour...I must be reading it wrong I just don't know..
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,737
    4,789
    This might just be a typo, but low power USB is rated for 100mA, not 100mAh.

    Because they messed up. But at least they are tracking their units and so the mistake is easy to spot and correct. The have (60 s/h) as one of the terms but, as you've already indicated, this is clearly wrong and it should be (3600 s/h).

    On a side note, it is not truly correct to say that this much charge has been stored in the battery because that implies that somewhere in the battery there is a charge separation comparable to what you would find in a similarly charged capacitor. But a battery is not a capacitor and the charge that is driven into it is almost instantly converted to chemical potential energy with very little charge separation remaining.
     
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