Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Experiment about Discharging Batteries

  1. Feb 7, 2013 #1
    I am trying to find the capacity of a AAA battery by discharging it. How would I be able to discharge the battery at a 1A current so that I dont have to wait for a long period of time?
     
  2. jcsd
  3. Feb 7, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    ... pick a low capacity battery.
     
  4. Feb 7, 2013 #3
    I was actually inquiring more about the constant 1A current discharge bit.
     
  5. Feb 7, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Ah - not the time part.
    Use Ohm's law and pick an appropriate resistor.
     
  6. Feb 8, 2013 #5
    You realise that the discharge current affects the capacity?
    If it's discharged at just a few mA, a AAA alkaline battery can deliver around 1200mAh. However at a current of 400mA the capacity is only 400mAh and at 1A it will be even much less than that. That's why alkaline batteries are really bad for digital cameras.
    http://data.energizer.com/PDFs/l92.pdf
     
  7. Feb 22, 2013 #6
    Well I'm actually using a NiMH battery. It happens to be one of those eneloops so looking from the data sheets on their website, the capacity doesn't decrease a lot with an increase in the discharge current.

    The only problem I am facing now is that I can't measure the true emf acrosss the battery at the same time I am discharging it. Does anyone have a solution?
     
  8. Feb 22, 2013 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    What do you mean by "true" emf?
     
  9. Feb 22, 2013 #8
    The emf of the battery without connecting it to any resistors.
     
  10. Feb 22, 2013 #9

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Well, there is no way to measure the open circuit emf without opening the circuit.
    You could just throw a switch.

    What do you need to know that for though?
    You may want to see how the internal resistance varies as the battery loses energy.
     
  11. Feb 22, 2013 #10
    Well I need it so that I can get enough data about the open circuit emf to plot a discharge curve.
     
  12. Feb 22, 2013 #11

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    http://www.powerstream.com/AA-tests.htm
    http://www.mpoweruk.com/performance.htm
    ... I don't see anyone using open-circuit voltage for the discharge curves - but you can just open the circuit periodically to measure it. If the internal resistance is fairly constant, you can calculate the open-circuit voltage from the closed circuit one. You'll want to keep the battery at a reasonably constant temperature though.
     
  13. Feb 22, 2013 #12

    NascentOxygen

    User Avatar

    Staff: Mentor

    I think you'll find that the open circuit emf is generally not of much interest. (Though maybe you have a niche application where it is.) You should check that you are required to plot the no-load voltage, as that would be unusual. Most common would be a plot of load voltage vs. time while the load current is maintained at some specified constant level.

    I think for common types the no-load voltage changes very little, once past the initial drop.
    Immediately after the cell has its load removed the voltage takes some time to rise and reach a steady value. Do you intend allowing your cell time to recover before measuring your no-load voltage?
     
  14. Feb 22, 2013 #13

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Experiment about Discharging Batteries
Loading...