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Homework Help: Charging A battery question.

  1. Mar 19, 2006 #1

    "A 12.0 V battery with an internal resistance of .02 Ohms is used to charge a battery with an emf of10.5 V and an internal resistance of .15 Ohms. What is the charging current?"


    I'm pretty much stuck on the set up of this problem. I know how to find the current if there was some resistor and these were hooked up in parrallel, but the way the problem is worded it makes me think these are simply just in their own loop. More over I'm not sure how I would set this problem up if they just created their own loop.... Any help would be appreciated.

    PS: To charge a battery the current across the battery points from negative to positive right?
  2. jcsd
  3. Mar 19, 2006 #2
    Start by drawing this as one loop, yes.

    Determine a direction for current flow, so start with the 12 V battery and determine initial direction for current, by convention.
  4. Mar 19, 2006 #3
    Alright, I have set the problem up. I have the first battery on the left side with the current moving up and then right (From the positive) to the second battery on the right side where the current moves down through the internal resister into the negative side of the second battery which then wraps around to the internal resister of the 1st battery.

    I'm assuming I use V=IR.

    Please correct me if I do any of the following incorrectly:


    Loop: -Ir1 + E1 - E2 - Ir2 = 0

    I = (E2 - E1) / -(r1+r2)

    I = 8.8 A

    Would this be the correct way of doing this?
    Last edited: Mar 19, 2006
  5. Mar 20, 2006 #4

    WHen we do loops, we mean that the total voltage is zero.. o_O

    so it be something like..

    0 = IR + Voltage(battery) - .....
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