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How build a decent battery tester?

Discussion in 'Electronic Components' started by Alex Coleman, Jul 1, 2007.

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  1. Alex Coleman

    Alex Coleman Guest

    How can I build myself a decent battery tester for domestic use? Are
    there any circuits? Any info about off-the-shelf equipment? (I am in
    the UK).

    Of course I don't want to spend more cash than I save in batteries,
    so there is a limit budget.

    At the moment I use an ordinary digital multimeter which has a
    setting to put the cell under a load of approx (a) 10mA or (b) 150
    mA.

    At present the *main* cells I would like to test are AAA and AA cells
    made of NiMH, NiCd, manganese-alkaline or zinc chloride

    I would like to have a device which simultaneously shows the current
    and voltage of a cell under load. And then it would be useful to
    simulate different loads - perhaps using a variable resistor?

    If it's not too hard to also get a display of the internal resistance
    of the cell that I'm told that can be useful. I gather that internal
    resistance of rechargeables can show the battery's state of health.
    However int resistance increases as the theoretical max capacity
    increases and I guess this means that absolute calibration is not
    possible, however perhaps internal resistance varies during charge
    and is an indicator of capacity?

    Thanks for any info.

    --
     
  2. a DVM, note pad, and a resistor you likely already have !
     
  3. JeffM

    JeffM Guest

    "Testing" rechargable batteries is a waste of time.
    http://groups.google.com/group/sci....ht-until-the-battery-is-discharged+recharable
    http://groups.google.com/group/rec....r+rechargeable+flat-discharge-curve+NiCd+NiMH

    To test a disposable battery:
    1) Measure the no-load voltage of the cell.
    2) Measure the voltage when loaded with a resistor (~10 ohms).
    3) Make a chart of what is a "good" reading
    (correlated to real-world failure of your battery-powered devices).
     
  4. mike

    mike Guest

    What's your objective?
    The whole idea of batteries is that when the thing stops working, you
    replace them. For critical items, you use NEW batteries and change them
    on a schedule determined empirically.

    I use a computer, GPIB programmable power supply and programmable load.
    I've found two useful measurements.
    Lithium ion rechargeable batteries fail with high internal resistance.
    The electrons are still in there, but the device shuts off due to low
    voltage and won't let you have them.

    I built a tool for sorting lithiums at the surplus dealer.
    8 AA nicads in series. A car tail light and switch in series to the
    test leads.
    DVM across the test leads.
    Measure the cell voltage. Under two volts, probably scrap.
    Close switch and charge the cell until the dv/dt slows down.
    Measure the voltage, let go of the switch, measure the voltage again.
    Difference is related to internal resistance. You can do the math and
    get a number, sorta, but experience over a few dozen cells compared to
    a new cell is your best teacher.

    Second useful thing for nicads is to plot the voltage while charging.
    The curve shape of a good cell is unmistakable. Ditto for discharge.
    mike
     
  5. Franc Zabkar

    Franc Zabkar Guest

    I use Bob Parker's ESR meter.

    See the table of battery ESRs at the bottom of this page:
    http://www.flippers.com/esrkthnt.html

    - Franc Zabkar
     
  6. Damit Spock

    Damit Spock Guest


    A $80 *kit* to test dead batteries.

    Capitalism at it's best ...
     
  7. One thing that rechargeable and alkalines have in common that they
    have high impedance when close to discharged state.
    So cheap and easy (partially distructive though) test is to use
    a multy-meter, switch it to current measurement mode and using a
    high-current path (10A), short the cell momentary.
    If you see a sustained current (usually 3-4A but it is not
    important) than cell is OK. If you see current drop very fast to below
    1A, cell is close to dead state.
    Yes, you will lose some charge while doing this test, but for that
    you don't need any additional equipment, even no resistor. If you happen
    to have a 1 Ohm resistor, you can use it instead of the short.
    Procedure is the same but current will be much less.

    Regards,
    Yevgen
     
  8. A variant on that that can be revealing is a very brief short-circuit,
    about a tenth of a second, then watch how the voltage recovers immediately
    afterwards. It might not reveal much, but probably enough to allow less
    time at heavy current.
     
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