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9V battery testing; Thevenin equivalent; car headlamps.

Discussion in 'Electronic Repair' started by Adam Funk, Aug 1, 2006.

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  1. Adam Funk

    Adam Funk Guest

    I recently tested a 9V alkaline battery by measuring its open-circuit
    voltage (9.0 V) and then measuring it with a car headlight lamp (R = 1
    Ohm) across the terminals (4.0 V). The lamp lit up brightly and got
    warm, but from the significant voltage drop I conclude that the
    battery is basically dead. Correct?

    From those measurements I get a Thevenin model of the circuit as
    follows, where Rb is the battery's internal resistance and Rl is the
    load (lamp).

    - Vb + Rb
    | |
    | |
    o o
    | |
    | Rl |

    With the load removed, and assuming the voltmeter is an open circuit,
    Vb = 9.0 V. With Rl = 1 Ohm in place and the voltage across o-o
    measured as 4.0 V, the loop current is 4 A. So Rb is 1.25 Ohm.

    Is there a rule of thumb for judging a battery as "still OK" or
    "dead" based on the calculated Thevenin resistance?

    I measured the headlamp as 1.0 Ohm, which in a 12 V car circuit
    (assuming a negligeable series resistance) should have a power of
    144 W. Does that sound reasonable?
  2. Tom Biasi

    Tom Biasi Guest

    Nice experiment and good math practice but that battery was not designed to
    supply that much current. You really can not tell much about the battery
    from that test. Get the test specs on the battery and load it properly and
    repeat your experiment.
    BTW: The resistance of the lamp filament will not stay the same as it heats
  3. No, 1,25 ohm ESR (Equivalent Series Resistance) for a 9V battery is
    quite good. For a 1,5V "D" cell i would consider it too high.
    Yep. But remember that the 1 ohm of the lamp is measured in cold state.
    At 9V it is probably a lot more.
    It depends of the size, technology and voltage. A large "D" cell will
    have in its new state an Rb of about 0,1 ohm or even less.
    A new 9V battery might give you somathing like 1 ohm.
    No, the resistance will increase by a factor of 2 or 3 as the lamp
    gets hotter. Connect it to a decent power supply (or car battery) and
    measure the current. That will give you the corrent operating wattage
    and resistance.
  4. Greg Neill

    Greg Neill Guest

    If this is a typical 9V battery as in:

    then the usual load is in the neighborhood of a few tens
    of mA up to maybe a couple of hundred mA at greatly
    shortened service life. If your load current was really
    as high as 4A, then if the battery wasn't finished before
    it probably is now.

    Also, the car headlight will increase in resistance as
    its filament gets hotter, so your load might have started
    at 1 ohm, but would have risen considerably very quickly.
  5. Phil Allison

    Phil Allison Guest

    "Joop van der Velden"

    ** More like 10 or 12 times, actually.

    Depends just how white hot the filament becomes.

    Halogen projector lamps with 50 hour rated live increase by a factor of
    about 16.

    ........ Phil
  6. Your calculations are correct but they are on paper and your testing
    arrangement is in real life with real components so:
    a) R1 can and does change depending on brightness produced by ~1-10 or
    20. (Read the Watts rating at 12- 13.7V --> car battery on charge).
    b) Battery has I dependant on rate where the production of current has
    some upper limit and then the voltage drops independant of Rb

    So to test your measurment introduce in this circuit an ampermeter and
    do your calculations again.

    Have fun

    Slack user from Ulladulla.
  7. lsmartino

    lsmartino Guest

    Adam Funk ha escrito:
    Problem is that a headlamp isn´t an adequate load for a 9V battery. A
    5W tail lamp would be a better load, so the test you made is esentially
    meaningless, even if the math model is perfect.
  8. Adam Funk

    Adam Funk Guest

    I haven't got an ammeter but "fun" would be a good excuse!
  9. Adam Funk

    Adam Funk Guest

    Interesting. I though a 44% voltage drop sounded like a lot, but as
    you and others have pointed out, the load resistance I've used is very
    low. What sort of resistance do I really need for this sort of test?

    Right. I measured the lamp's resistance with an ohmmeter, which of
    course puts very little current through it.

    But I took the measurements by clipping the voltmeter (actually it's
    the same meter) leads onto the battery terminals, reading the
    open-circuit voltage, then pressing the lamp's terminals against the
    battery terminals (the spacing was convenient --- that's where I got
    the idea from) and immediately reading the loaded voltage (before the
    lamp heated up).

  10. Adam Funk

    Adam Funk Guest

    So a load in the area of 270 to 620 Ohm (the examples in that data
    sheet) would be much more suitable.

  11. redbelly

    redbelly Guest

    It might be dead now that you've run it at 4 amps, but 9.0 V open ckt
    is perfectly fine. When the o.c. voltage drops down to 7 V or so it
    might be considered dead, but that would really depend on what you're
    using it for (i.e. how much is the load).

    Next time, try testing it with 470 to 1k ohms. Even better would be if
    you know the load resistance of your intended application.


  12. Guest

    By the way, some 9 volt batteries are made up using cells that are not
    welded together, and only touching each other, which can lead to higher
    ESR and unreliability. I don't know if there is another battery which
    has been made like this. Beware.

  13. Adam Funk

    Adam Funk Guest

    How about the ASCII art?
  14. mc

    mc Guest

    No. A 1-ohm load is way too low a resistance to use with a 9V battery; the
    excessive current that flowed (over 4 amps) will damage the battery. The
    battery may be dead *now* because you killed it. Or, if the abuse was only
    momentary, it may be OK.

    I would have tried maybe a 100-ohm load, drawing 90 mA, which is a typical
    (rather heavy) load on a 9V battery.
    If you're sure the load was 1.0 ohm, then the total resistance in the
    circuit is 9/4 ohm, so indeed, Rth is 1.25 ohms.

    That is a rather low Rth for a 9-volt battery. As much as 5 or 10 ohms
    might be OK.

    Indirectly, yes. At the current that the battery is designed to supply, the
    voltage should be at least 90% of full voltage.

    Actually, in this era of digital voltage meters, I measure Vth, not Rth.
    Vth drops from well over 9.0 V for a new battery to 8.8 V for one that is
    showing its age.

    Yes. Headlamps normally draw something like 10 amps at 12 V, which would
    imply about 1.2 ohms. And that is approximate.
  15. redbelly

    redbelly Guest

    For the handful of batteries I've measured, I get 9.4 V when they are
    brand new.

  16. Adam Funk

    Adam Funk Guest

    I was under the impression that the open-circuit voltage wasn't a
    reliable measure of how much "juice" is left in a battery.
  17. Sofie

    Sofie Guest

    Adam Funk:
    You are absolutely correct about open circuit testing of batteries is
    useless.... many otherwise almost dead batteries will have a surprisingly
    high open circuit terminal voltage......
    .....BUT the test LOAD (car headlamp) that you are using is not much more
    than a dead short compared to the design capabilities of the 9V battery.
    These batteries were designed with practical average load currents of 20ma
    and occassionally will be found in devices using 100ma or possibly a little
    A good and practical load that I have used for actual evaluation and
    comparison with other 9volt batteries of questionable freshness is a 270 ohm
    , 1 watt resistor, which gives a load current of about 33ma.... this will
    "test" the battery terminal voltage with a suitable load and will not ruin
    or drain the battery while doing it.
    Best Regards,
    Daniel Sofie
    Electronics Supply & Repair
    - - - - - - - -
  18. mc

    mc Guest

    Actually, in this era of digital voltage meters, I measure Vth, not Rth.
    It wasn't back in the days of analog meters with 10% accuracy. It is now
    that we have digital voltmeters with very high input impedance and 1%
  19. John Larkin

    John Larkin Guest

    Shorting this kind of battery for some seconds won't do permanent

    At high loads, the ESR of the battery will change with time, too.
    Connect a 9v alkaline battery to an ammeter. It will start at, say, 2
    amps and drop off as internal polarization kicks in; the decay and
    recovery time constants are very roughly in the area of a minute. At
    lower currents, ESR is pretty steady.

  20. Old Mac User

    Old Mac User Guest

    Damn, that's hard on a little 9 v. battery!!
    Killed it dead.
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