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Testing mAh of a battery?

Discussion in 'Power Electronics' started by tomadom, Oct 29, 2016.

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  1. tomadom

    tomadom

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    Jan 18, 2013
    I wanted to test some battery packs and want to determine their true mAh rating. How do I do this?

    Is there a unit I can buy to test this or is it a matter of using a Multi-meter and a clock?

    If someone could point me in the right direction that would be great. Thanks
     
  2. HellasTechn

    HellasTechn

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    Apr 14, 2013
    There is no way you can measure battery capacity with a multimeter. There are devices though "micro controller based" that would charge the battery to 100% and then discharge it under specific load and calculate the capacity based on the load value and discharge time.

    Something like this:
    http://www.ebay.com/itm/150W-Consta...743421?hash=item281f4ba6bd:g:8SwAAOSwHMJYELBp

    but i am not sure if they work good or not.
     
  3. tomadom

    tomadom

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    Jan 18, 2013
  4. Bluejets

    Bluejets

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    Oct 5, 2014
    First question would be what type and size of battery packs?
     
  5. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
  6. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Not really. The one in your post can only monitor current output.
     
  7. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    Set the battery up under a known load and see how long it lasts, You will find you get a different mA-Hr result at different currents. As a general rule for most common batteries figure an 8 hour life. Divide mA-Hr rating by 8 to calculate load resistance.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Well, divide it by 8 to get the 8 hour discharge current in mA.

    e.g. 1600mAh equates to 200mA discharge current for a projected 8 hour discharge.

    Then to calculate the load resistance, get the battery voltage and divide by the current in AMPS to get the load resistance in ohms.

    Let's say it's a nominally 7.2V battery, the load resistance will be 7.2/0.2 = 36 ohms.

    If you're testing a battery, the voltage will fall during discharge. This presents 2 problems.

    The first is that as the voltage falls, the load draws less current. In addition, the battery may well start at a voltage higher than its nominal rating. Thus the load current is not constant and this introduces errors into your calculation. The answer to this is to use a constant current load, but that's a little more complex.

    The second problem is where to stop? The best measure is to find what the manufacturer recommends as the discharge voltage, however this may be too low for a typical device to operate at, and thus you may choose a higher voltage.

    Preferably you have some means of detecting the stop point, and this will stop the clock and remove the load.

    The two problems mentioned above don't really happen if you're testing one of those USB power sources because they have a regulated 5V output and will automatically shut down when the battery is flat (or at least they should). SO your load will always be "correct" and all you need do is wait until the output voltage suddenly drops to zero.
     
  9. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016

    Good observation. I use Excel and note batter voltage and current, calculating amphours on a per hour basis, then totally them as it progressed.
    When to stop? consult the battery manufacturer.
    How precise do you imagine the battery rating is? Is 10% an acceptable error? 20%?
    If you test four batteries would you expect them all to be within 1% of each other and the rated value?

    What does the manufacturer suggest?
     
  10. tomadom

    tomadom

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    Jan 18, 2013
    Just to let you know why I want to do this, I also want to test some smartphone batteries to see if they are actually the 1800mAh rating they are claimed to be.

    I have bought a china made grey import phone and would like to determine if the battery has the mAh rating it's claimed to have. They seem to run out a little fast so I'm dubious.

    Will also test some battery packs as previously mentioned.
     
  11. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    The meter I suggested can do that measurement for you. Start with a completely discharged phone, plug the meter and charger in and read the mAh from the meter when the phone reaches a fully charged state.
     
  12. Gryd3

    Gryd3

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    Jun 25, 2014
    I don't have an easy answer for you, but I have some resources for you ;)
    Depending on what you are testing, you may be able to find test results already.

    There has already been a comment about the current draw... the mAh rating will vary when you compare a a heavy to a light load on the battery. The resources I've attached will show this as well.

    Take a peek and see if you have more questions.

    http://denishennessy.com/2012/04/08/measuring-battery-capacity-with-an-arduino/
    http://www.batteryshowdown.com/
    http://www.lygte-info.dk/info/indexBatteriesAndChargers UK.html
     
  13. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    Attached is a pdf version of an Excel file similar to what I used the last time did a similar test. It is not a real representation. Battery voltage is measured every hour. Current is calculated. Amp-hours are calculated and summed up as you go along.

    Stop whenever you feel like it. I was comparing rechargeable AA batteries. My concern was relative, not absolute as yours is. As long as I stopped at the same time in each comparison my test was acceptable. Amp-hours we pretty close to advertised.
     

    Attached Files:

  14. Andy Kay

    Andy Kay

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    Sep 26, 2016
  15. mww

    mww

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    Mar 7, 2016
  16. Andy Kay

    Andy Kay

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    Sep 26, 2016
    @mww:

    Don't need to mess around with spreadsheets using the meter I indicated, since it has a mAh measurement on the readout that is zeroed when the meter is powered up and when it is reset with the reset button.

    Regarding overdischarging at the bottom end... yeah, done that... got distracted and forgot about it. Cell went to zero for maybe an hour, but it showed me that these 18650 Li-Ion cells are more resilient than many people would have you believe... after a full charge and a new capacity test it showed no damage, but I now make sure it doesn't happen again by using the following device (which is much cheaper if you want more than one and buy from China and don't mind a long delivery time). As well as looking after the charging process, the device switches off the discharging current when the cell falls below 2.5V and re-connects when the cell recovers to some overdischarge release threshold (can't remember what that is but around 2.9V or 3.0V, and notably a feature that would make bottom balancing simple if you're inclined that way):

    http://www.ebay.co.uk/itm/TP4056-WI...213092?hash=item3ad0902e24:g:QXsAAOSwU-pXrhEe

    I've also pulled the protection circuit out of a dead protected LiPo cell and successfully used that for the same purpose.

    Just a few more ideas to add to your suggestions.

    Edit: see this vid:
     
  17. Protoman

    Protoman

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    Dec 30, 2020
    A multimeter will give a voltage reading of the battery, but not the mAh capacity.
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009

    This is a VERY old thread, please try and avoid responding to ones like this :)


    thread closed
     
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