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Voltmeter with hold reading

Discussion in 'Electronic Basics' started by mg, Jun 20, 2005.

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  1. John Fields

    John Fields Guest

  2. Guest

    If I understand the criterion correctly, everything between 1)-6) must
    have passed the test.; i.e a known load for 5.5 hours did not discharge
    below 15.0V. All others failed. Consider result 7), at best the loaded
    battery dropped to 15.0V in 5.5 hours then lost an additional 0.1V with
    no load on it in 8 hours. Clearly this is not a battery for long term
    standby application.

    I agree that he wants to minimize uncetrainty. The test I outlined is
    not very uncertain regarding ability to deliver a particular charge in
    a particular time and having a low self-discharge rate.
  3. ehsjr

    ehsjr Guest

    Ok, readings 1) through 6) range from 15.5 to 15.0

    So he measures at ~8 hours and his battery reads 15.5.
    How does that answer his post:
    "We would like to add one additional feature to our discharger. Some
    sort of voltmeter that we could hold the last reading. This would
    allow us to double check that we stop the discharge process at 15v."

    Does the 15.5 volt reading mean that: a) the discharger was not
    finished, or b) the discharger was finished, but shut off at
    other than 15 volts or c) the discharger shut off at 15 volts,
    but battery voltage without load rose to 15.5?

    The same questions apply for the other voltages posted.
    He ends up where he started - uncertain as to whether the
    discharger is turning off at 15 volts.

  4. ehsjr

    ehsjr Guest

    Change R4 to 560 ohms. This will ensure that the
    discharger can still turn on, even if the battery
    that is to be discharged is almost fully discharged
    (say 15.1 volts) when it is put in the discharger.
    With the 1K for R4, it is highly probable that it
    will turn on at 15.1 - with the 560 ohm for R4, it
    is definite. The change of value does not affect
    the 15 volt shut off voltage.
  5. Guest

    You are correct if a voltage comparator is used to terminate the test.

    I suggested that the comparator isn't necessary. Just see how much the
    voltage falls after some fixed time under load (which is essentially
    equivalent to what the originally proposed test does.)

    Voltage rebound creates uncertainty if the open circuit voltage
    measurement takes place too soon after the load is turned off. This can
    be mitigated by waiting "long enough" or better yet by reinstating the
    load during the voltage measurement.

  6. ehsjr

    ehsjr Guest

    No it is not, by a long shot. See below.
    A NiCd should not be discharged below 1 volt per cell.
    Terminating the discharge based on time rather than
    voltage is a damn fool idea that could damage the batteries,
    by discharging them well below that level.

    It is nothing like terminating the discharge at a specific
    voltage level (~1 V per cell). An 18V (nominal) pack subjected
    to a discharge of 5.5 hours could end up ruined by that. Its
    terminal voltage could be anywhere between 0 and 18 volts.
    The same pack, subjected instead to a discharge that terminates
    at 15 volts, will end up close to 15 volts.

  7. ehsjr

    ehsjr Guest


    This idea had great promise, but I got real disappointing
    results. I ordered a 10 uf polypropylene and received it
    yesterday. (Aerovox ARPM 10uF 175 VAC 9833)
    It is cleary NOS (August 1998 never used)
    The maximum insulation resistance is 500,000 meg per the
    spec sheet. That seems to contradict the curve on the sheet
    on page 5. It looks like the actual is 1,000,000 meg.

    I charged it to 30.2 volts for 5 minutes, then disconnected
    the test leads. I came back 2 hours later and connected
    the DMM - 15.xx volts! It was so low (ie losing 1/2 the
    voltage in 2 hours) I didn't even bother to record the decimals.

    Then I thought a super cap and a 5:1 voltage divider might
    work for the OP. So I charged one up to 4 volts (without a
    divider) and measured it 3 hours later - 3 volts. I tried it
    again, charging it to 5 volts, and measured 3 hours later -
    4 volts.

    In all cases, I charged for 5 minutes, then completely
    disconnected. All voltage readings after the elapsed time were
    made in under 5 seconds. I even tried charging and leaving
    the DMM connected to the super cap (.1 farad, 5 V) - the DMM
    does not discharge the cap rapidly enough to screw up the
    readings made after elapsed time.

    Whatinthehell is going on? The polypropylene idea seems to
    make sense. So does the super cap. Actual results do not
    bear this out. Both caps I tried came from the same supplier,
    and they might be seconds, rejects, call them what you will.
    I am beginning to suspect that. I hope someone else will
    try this out. It will be a while before I need to order
    more parts - when I do it will be from Digikey, where I
    won't suspect the parts as I do with these. In the meantime,
    it sure would be nice to hear the results others get.

  8. John Fields

    John Fields Guest

    I don't wanna sound insulting or anything, but I think you're wasting
    your time as far as zeroing in on what the OP wants is concerned.

    ISTM that what he's really concerned with is the elapsed time between
    when the discharger starts and when the battery gets to 15V, the
    reason being that if the battery discharges to 15V in _less than_ some
    specified time with a given load on the battery, then the battery's
    capacity is starting to wane and the battery needs to be replaced.

    To that end, then, a clock which starts accumulating time when the
    battery starts being discharged and stops when the battery voltage
    falls to 15V seems to fill the bill in that all that's required is to
    read the clock to determine the time from start to finish and,
    therefore, to determine the condition of the battery.
  9. Roy Lewallen

    Roy Lewallen Guest

    It's very tough to get an extremely high resistance. Even with a perfect
    capacitor, you end up with leakage around the outside. I'm not an expert
    at this, but a friend who is tells me that what you need to do is wash
    the surface very well with deionized water. You can use alcohol or
    something first if it's grungy, but the DI water rinse is essential.
    Then don't touch it!

    I assume that you could use distilled water from the grocery store in
    place of the DI water, but I'm sure some of the chemists who frequent
    this group could tell you for sure.

    Roy Lewallen
  10. ehsjr

    ehsjr Guest

    Doesn't sound insulting at all. I believe your understanding
    (below) is correct. At least I understand it the same way.
    But I think he has already implemented the clock idea, so
    recording the elapsed time isn't a problem. I sent him
    a circuit design back in January which included an AC socket
    switched on by a relay as long as it was energized. I think
    he used the relay/AC socket idea, but the TL431 & relay driver
    circuit were not suitable for his level of familiarity with

    I think the problem is that the OP doesn't trust his discharger
    to shut off at 15 volts each time. He can't wire up much more
    than a relay circuit, so I was hopeful that Sue's idea would

    As far as wasting my time with regard to designing a solution
    for him - at this point, I have to agree with you. I'm out of
    ideas he would be able to implement. )-:

  11. ehsjr

    ehsjr Guest


    Thanks. That probably is the killer. The humidity
    is very high, and no effort was made to clean the
    parts, let alone wash them with de-ionized water.
    They have my fingerprints all over them.

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