Beware of 9v batteries that can't supply 12mA!

Discussion in 'General Electronics Chat' started by THE_RB, Mar 28, 2014.

  1. THE_RB

    Thread Starter AAC Fanatic!

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    I was fixing a consumer electronics RF doorbell type transmitter for a friend and got a weird fault.

    When it was activated the voltages went haywire and nothing seemed to make sense.

    The first thing I tested was the 9v battery, this measured 8.94v which I assumed was "good enough".

    When I pulled the product apart and separately powered the internal modules from my bench supply everything worked. The total device drew 12mA at 9v DC. Then when reassembled and running from the 9v battery it was all haywire again.

    After an embarassingly long time checking all the voltages and trying to diagnose what was wrong I finally checked the voltage on the 9v battery when it operated! The 9v battery dropped to less than 5v!

    I subsequently tested the battery with a resistor load etc and with a resistor to cause 12mA at 9v the battery voltage dropped to 5v! The battery itself was only capable of providing about 5mA of current into any load. Wow.

    It was a "Battery World" brand 9v battery and my friend said it was expensive, supposed to last "5 years". I'm not sure if it was alkaline etc but it lasted less than one year and then developed this weird fault.

    I've seen lots of high internal resistance faults with lead acid batteries but never before with a 9v battery. They are normally good, or flat. I wonder if the special "5 year" type of battery has some weird internal chemical makeup? There was no sign of corrosion, leakage or bulging and it measured 8.94v on the multimeter.
     
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  2. KeepItSimpleStupid

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    My story: Beware of the intermittent fuse.

    It happened to me about 3x in my lifetime so far, A regular 3AG class or ceramic fuse that opened in circuit, but tested fine with an ohmmeter out of circuit.
     
  3. crutschow

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    Perhaps it was a lithium 9V battery. They could possibly exhibit strange behavior when nearly discharged, different from a typical zinc or alkaline battery.
     
  4. t06afre

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    A new and "fresh" 9 volt battery (common zinc–carbon variants) should show about 9.6 volt with no load. A 9 volt zinc–carbon battery showing below 9 volt without any load. Will most often be on the worn out side in my experience. Due to the high internal resistance.
    [​IMG]
     
    Last edited: Mar 28, 2014
  5. KMoffett

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    And not all diodes are created equal. I was throwing together a circuit that included a fu llwave bridge I put together on a breadboard from (new)1N4002 diodes. Didn't work. After much head scratching I checked each diode. They were all installed properly, but one showed 0.5Vf with the +lead on the cathode (banded end) and open when reversed.
    Doubted my readings but after several confirmations it was wrong. They were all from the same batch, but this one had the cathode band on the wrong end. ?????

    Ken
     
  6. RichardO

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    In my first job as a test technician I saw many reverse marked signal diodes. I have wondered if they were all from the same bad batch...
     
  7. #12

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    Another good reason to attach a load to the volt meter leads when testing batteries. It is especially true when checking the CR2032 batteries in computer memory circuits.

    Some people call the voltage of an unloaded battery, "surface charge" because it is, "superficial". Essentially, an unloaded battery will return the voltage inherent to the chemistry used in manufacturing it. A load as small as C/100 can reveal the problem.
     
  8. THE_RB

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    Sorry but I will argue that. In my experience the bulk of the normal operating life of a 9v battery occurs between 9v and about 7v;

    [​IMG]

    And regarding internal resistance I have not seen significant internal resistance issues in 9v batteries, just reduced voltage as the battery becomes more discharged.

    I've never seen a 9v battery that read 9v with no load and 5v with a 5mA load!


    Thanks for that! I did not think it was a lithium battery, there was no marking to that effect just a lot of company branding and a "long life" tag.

    I did a quick google and did not see any info on the no load terminal voltage of a dead lithium 9v battery, but here is a 9v lithium discharge curve;

    [​IMG]

    Looking at the green "1mA" line, it is possible right at the end of the curve (when the battery is close to dead) that the no-load (0mA) voltage might be close to the 8.94v that I measured.

    Very interesting! :)
     
  9. #12

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    It worries me that you seem to want to find the explanation of this behavior in a continuous use graph. A remote control handle is not a continuous use, and taking the remote control handle to get repaired is not a continuous use. Batteries in intermittent use do not have smooth discharge curves, especially when they get near the end of their life.
     
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  10. THE_RB

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    No need to worry, I was checking Crutschow's suggestion that it might have been a lithium battery, and I wondered if a lithium 9v battery could display this weird symptom of reading 8.94v with no load, and 5v with a 5mA load. A symptom which I have never seen on a 9v alkaline or carbon battery.

    The graph was found from a very quick google, and shows that with a lithium 9v battery it looks possible that it could have a noload voltage close to 9v even when the battery is close to dead. Unless you interpret that graph differently?

    Regarding the original intermittant use of the battery, I can't see why you think that is relevant? I stated that the continuous noload voltage was 8.94v, and the continuous load voltage with a resistor attached was 5v at 5mA. Nothing intermittant about that.
     
  11. #12

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    The mystery seems to be about why a battery would show nearly its best voltage at no load when it can not sustain a very reasonable load. The answer is not in a continuous use graph, but in the way a fairly used up battery will show a rise in voltage when it has no load. The chemical reaction is so used up that it can not produce even a few milliamps, but when given some time, the output voltage slowly rises to a level that appears to be good.
     
  12. crutschow

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    That may be true for many battery chemistries but I have not noticed that with alkaline dry cells. My experience is that their open circuit voltage seems a pretty good indicator of their remaining charge. The voltage seems to steadily decline (perhaps not linearly though) with use and when close to a volt per cell they are pretty well dead.
     
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  13. Wendy

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    I have several battery testers scattered through my toolboxes. One of my better investments. I can state a fresh alkaline 9V will provide 1A through a short, details are not important (and I ain't gonna tell).
     
  14. crutschow

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    What does an intermittent fuse have to do with turning on the fog lights? :confused:
     
  15. THE_RB

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    Well I didn't happen to see THAT graph in my "very quick" google search! ;)

    But th egraph I DID find for a lithium 9v battery has a 1mA curve, which is close enough to no load to make some assumptions.

    If you check the green curve for 1mA you can see the battery voltage is just under 9v, right at the end of the graph when the battery is almost dead. The same almost-dead battery at 5mA would be well into the voltage drop range, providing only a few volts.

    Since the "very quick" google search showed a graph which seems to back up the weird symptoms I measured that seemed good enough at the time.

    For now I will assume it was a lithium 9v battery, right at the end of its life.

    Something to watch out for if you are in the habit if testing 9v batteries with a voltmeter! They may read "9v" but be a dead lithium battery! :eek:
     
  16. t06afre

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    For 9 volt batteries my tongue never fails me:p The lick test is a good indicator
     
    Last edited: Mar 31, 2014
  17. #12

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    Exactly what I was saying. Relatively used up batteries often measure as fairly near their labeled voltage at no load.
     
  18. THE_RB

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    Which type?

    I've tested 9v batteries with a voltmeter for decades, all carbon or alkaline ones.

    By the time the battery has used up most of it's chemicals and is near the end of its life the no-load voltage is usually quite low; 7v or so.

    Like I said I've never seen a dead 9v battery measure 9v before! :eek:
     
  19. Little Ghostman

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    Some people dont like Lithium batteries because of the scare stories etc, you would think that any battery would clearly state what the chemistry was?
    Considering the rules in the UK over food labels I am surprised batteries dont have to have some kind of poison's sheet and all the over forms the safety people insist on!
     
  20. THE_RB

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    I agree, the chemistry should be learly labelled on the battery, even if just for dispoal. Also things like Ah rating are nice too.

    This battery was sold by "Battery World" store and just has their branding and crap all over it. Rebadging at its worst. :(
     
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