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Homework Help: Non-Ideal Battery Voltage with few data points

  1. Mar 6, 2016 #1
    1. The problem statement, all variables and given/known data
    The problem includes a graph. All I have is a current to external resistance graph, with 20 A coming at 10R of external resistance. I am to find the EMF and internal resistance of the battery.

    Here is the problem and my two attempts.

    2. Relevant equations
    DeltaV = E - I * Rinternal

    3. The attempt at a solution

    I included two of my attempts at a solution. In both cases I simplify the circuit such that I can find the R equivalent from Rinternal and Rexternal. While that can give me a ratio, I still don't know what the EMF of the battery is.

    One idea I had was to assume the max current on the graph (60 amps) is the max current the system can take. If this is correct, then I can reformulate the earlier formula to find a 'max possible I'. With this, I can find E in terms of Rin and I do find a solution, though I am highly unconfident if this is allowed.
  2. jcsd
  3. Mar 6, 2016 #2


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    I believe your idea is right. This is the only approach to solve this problem.
  4. Mar 7, 2016 #3


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    They have been kind to you by plotting the current at no external resistance. More generally you would not have such a datapoint, so it's worth knowing how to proceed in that case.
    Consider the points (5, 30) and (10, 20). If the battery voltage is V and internal resistance R, what equations do those two datapoints allow you to write?
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