Battery life: how to measure

Discussion in 'General Electronics Chat' started by chuyenle, Jun 12, 2013.

  1. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I did some calculation to show how long my batteries can last. Now I need to prove in real life how long it actually last.

    What I want to do is figure out a way to measure how much the battery discharge and how much it has left. So I can calculate or estimate how much longer the battery can last, which I don't know how yet.

    Any ideas?
    I appreciate any help.
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    You do that by capturing the discharge current and integrating that over time. That gives you the quantity Amp-hours. Monitor the voltage as you discharge the battery. When the voltage is low to indicate the battery has fully discharged, then the amp-hours at that time is the battery capacity.
     
  3. Mike33

    AAC Fanatic!

    Feb 4, 2005
    349
    25
    Hmm, good question. You'll have variation based on the individual battery's construction, of course. How about making a test circuit, and measuring the current and voltage over time. When you get to the point that the battery is considered 'dead' (I think that is maybe 8.3V? Someone correct that), you will have your answer!

    You might try graphing the discharge. Thing is - you have to know/find out at what point your circuit stops working dependably. For most purposes, looking up a data sheet and computing your mAh's vs. the battery rating is good enough...
     
  4. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
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    Get an electronic load, it will do what you want.

    Or for a constant loading device, make a constant current load + counter/timer + comparator.

    connect the constant current load to your battery, start your counter/timer, your comparator is to stop your timer/counter when the battery voltage drops below your preset voltage.

    expect less from your calculation.
     
  5. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    This pop another question that I can't measure the current for some reason. Using the Meterman brand multimeter, I connect the multimeter in series between the batteries and LED and resistor. And the LED isn't on; current always show 0. Any idea what might cause the problem?
    I could measure the current couple time and now it just stops working:confused:
     
  6. bug13

    Well-Known Member

    Feb 13, 2012
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    did your check the fuse of your DDM? Is LED still working?
     
  7. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    LED is working.
    If I change from mA port to 10A port on the multimeter, the LED is on but the multimeter show 0 for current
     
  8. bug13

    Well-Known Member

    Feb 13, 2012
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    Good chance that your fuse on mA range is blown, I did that many times myself.
     
  9. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I changed to a smaller size multimeter and it can measure the current to the LED. So I guess it has something to do with the resistor inside the multimeter?
     
  10. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
    38
    Have you check the fuse yet?

    The 10A range shows 0 current is likely because your current is too small for the 10A range to measure, that's why you have 10A and mA on your DDM, mA is for measure small current.

    There is nothing to do with the resistor inside the DDM in your situation.
     
  11. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    Why do I need to check the fuse?
     
  12. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
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    Refer to post #8
     
  13. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I don't know where the mA fuse is :(
    I saw one fuse but it's good still; the value on the fuse is 200mA/250V
     
  14. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
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    I would check the fuse with your other DDM in ohm range, it should read no more than 1 or 2 ohm, but if it still good, I am out of idea, you might want to wait for someone who is more capable.
     
  15. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    0
    I do think the fuse is blown but it just doesn't look like it and I don't have a spare fuse to replace.
    Back to the battery measure, I can measure when my load is connected. It takes 0.25A. Using 8 D batteries, does that mean my batteries can only last 48hrs (12000mAh capacity of D battery). After 48hrs, my batteries voltage should be around 0.8V (according to the battery datasheet, it continues discharges to 0.8V). Is that correct?

    After running my load for 12hrs, is there anyway to measure how much left inside the battery?
     
  16. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
    38
    Your load is 250mA, the rating on your fuse is 200mA, I am now quite sure your fuse is blown.

    it's correct in theory, but your LED won't work at 0.8V, you will need the forward voltage drop of your LED (minimum working voltage), if your batteries drops below your LED's forward voltage, it will simply turns off.

    so expect less that 48hrs battery life.

    For a rough estimate, I think you can measure the voltage of your batteries and cross check the datasheet of your batteries.

    Battery life is also effect a lot by temperature for some type of batteries as well.
     
  17. johannheb

    New Member

    Jun 19, 2013
    2
    0
    Hi,

    Yes indeed you can use a multimeter to do it. A multimeter will show how "charged" is your battery accordingto the numbers that are dispalyed on the screen.

    You can can find tutorials out there, with illustrations and everything to make your life easier when it comes to use a multimeter.

    Here is a good one: http://www.alertelectrical.com/32/tool-guides
    Another good one: http://learn.adafruit.com/multimeters/

    Hope that helps
     
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