NIMH Memory effect

Discussion in 'Electronic Design' started by Dave VanHorn, Jan 18, 2004.

  1. Dave VanHorn

    Dave VanHorn Guest

    For years, I've followed the conventional wisdom that memory was a myth.
    Having seen it documented in a Sanyo data sheet, along with specifics on how
    to reproduce it, I decided to try it myself. I had previously built a rig
    to test battery charge systems under development, so this just took a few
    minutes to alter the code to terminate discharge early, at a specific
    voltage point, which simulates a device like a digital camera designed for
    4x1.5V running on 4x1.25V.

    Well gentlefolk, I have seen it, and it is real.

    http://www.dvanhorn.org/NIMH/Index.php

    About midway down the page, you'll find the text, how the experiment was
    conducted, and the discharge plots showing memory effect after only 10
    cycles, and that it's gone, after a single normal discharge cycle.
    I'll grant that it's not all that significant after 10 cycles, but it's
    definitely there.

    I have a 30 cycle test in progress.


    --
    A: Because it messes up the order in which people normally read text.
    Q: Why is top-posting such a bad thing?
    A: Top-posting.
    Q: What is the most annoying thing on usenet?
    Dave VanHorn, Jan 18, 2004
    #1
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  2. Dave VanHorn

    Don Pearce Guest

    On Sun, 18 Jan 2004 11:50:43 -0500, "Dave VanHorn"
    <> wrote:

    >For years, I've followed the conventional wisdom that memory was a myth.
    >Having seen it documented in a Sanyo data sheet, along with specifics on how
    >to reproduce it, I decided to try it myself. I had previously built a rig
    >to test battery charge systems under development, so this just took a few
    >minutes to alter the code to terminate discharge early, at a specific
    >voltage point, which simulates a device like a digital camera designed for
    >4x1.5V running on 4x1.25V.
    >
    >Well gentlefolk, I have seen it, and it is real.
    >
    >http://www.dvanhorn.org/NIMH/Index.php
    >
    >About midway down the page, you'll find the text, how the experiment was
    >conducted, and the discharge plots showing memory effect after only 10
    >cycles, and that it's gone, after a single normal discharge cycle.
    >I'll grant that it's not all that significant after 10 cycles, but it's
    >definitely there.
    >
    >I have a 30 cycle test in progress.


    Welcome to the world the rest of us have known for years and years and
    years...

    d

    _____________________________

    http://www.pearce.uk.com
    Don Pearce, Jan 18, 2004
    #2
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  3. Dave VanHorn

    Dave VanHorn Guest

    Dave VanHorn, Jan 18, 2004
    #3
  4. In article <>,
    says...
    >
    > > Welcome to the world the rest of us have known for years and years and

    > years...
    >
    > Yeah well..
    > There's a whole bunch of sources out there, that say it's a myth, and
    > unfortunately, I believed them.


    IN the real world, it is a myth. That is, no one is ever likely
    to see any such problem, and if they do it's easily correctable,
    as you've stated. You've performed a very accuratee test
    demonstrating the effect on *one* cell type. Yawn!

    What most refer to as "memory" is the destruction of cell(s) by
    reverse charging a multi-cell pack or by cooking the poor
    bastards in a crappy charger. One is *highly* unlikely to ever
    see the dreaded "memory" effect. And not every cell shows even
    this.

    --
    Keith
    Keith R. Williams, Jan 19, 2004
    #4
  5. Dave VanHorn

    Dave VanHorn Guest


    > IN the real world, it is a myth. That is, no one is ever likely
    > to see any such problem, and if they do it's easily correctable,
    > as you've stated. You've performed a very accuratee test
    > demonstrating the effect on *one* cell type. Yawn!


    I don't see it as a "yawn", it was not something I'd expected.
    I intend to cycle through some more cell types and manufacturers, as soon as
    possible.

    > What most refer to as "memory" is the destruction of cell(s) by
    > reverse charging a multi-cell pack or by cooking the poor
    > bastards in a crappy charger.


    That certainly wasn't my understanding. The first is reversal damage, and
    the second is charge depression, both very different from memory.

    > One is *highly* unlikely to ever see the dreaded "memory" effect. And

    not every cell
    > shows even this.


    Hmm. I got lucky then? 1 for 1.
    Admittedly a small sample.
    I have a large number of the HR-AUC packs available for testing.

    I'm running a 30 cycle test now, which should be interesting, relative to
    the 10 cycle test earlier.
    In the future, I plan to run cycles to a specific discharged energy (kJ)
    rather than voltage, and to dithered points, to determine the effect of the
    variation of the discharge point on the total effect.

    The nice part is that it's pretty much automated, I just have to set it up,
    and let it run, and save the data at the end.
    Dave VanHorn, Jan 19, 2004
    #5
  6. Dave VanHorn

    Ben Bradley Guest

    In sci.electronics.design, "Dave VanHorn" <> wrote:

    >For years, I've followed the conventional wisdom that memory was a myth.


    Your subject says NIMH. For decades I thought it was NiCad cells
    that allegedly had this alleged mythcal memory effect.
    Are you claiming these are the same, or is there a separate alleged
    memory effect myth for NIMH cells?

    -----
    http://mindspring.com/~benbradley
    Ben Bradley, Jan 19, 2004
    #6
  7. Dave VanHorn

    Dave VanHorn Guest

    "Ben Bradley" <> wrote in message
    news:...
    > In sci.electronics.design, "Dave VanHorn" <> wrote:
    >
    > >For years, I've followed the conventional wisdom that memory was a myth.

    >
    > Your subject says NIMH. For decades I thought it was NiCad cells
    > that allegedly had this alleged mythcal memory effect.
    > Are you claiming these are the same, or is there a separate alleged
    > memory effect myth for NIMH cells?


    I haven't personally tested NICAD yet, but I suspect it will be same, or
    worse.
    NIMH manufacturers disagree on wether there is, or is not memory in NIMH
    cells, Sanyo, who makes the particular cells I tested, says there is. I
    intend to test some Panasonic cells soon, which come from the "no memory"
    camp, under identical conditions.

    Wether the electrochemistry is the same or not, I can't say for sure, but
    since they are both nickle based chemistries, I suspect it's the same, or
    very similar.

    The 30 cycle test on the Sanyo HR-AUC cells is nearly done, I should have
    the data online in a couple hours.

    I might start the Panasonics off next, as I have a pack made up already.
    Dave VanHorn, Jan 19, 2004
    #7
  8. Dave VanHorn

    Mac Guest

    On Sun, 18 Jan 2004 22:30:37 +0000, Dave VanHorn wrote:

    >
    >> IN the real world, it is a myth. That is, no one is ever likely
    >> to see any such problem, and if they do it's easily correctable,
    >> as you've stated. You've performed a very accuratee test
    >> demonstrating the effect on *one* cell type. Yawn!

    >
    > I don't see it as a "yawn", it was not something I'd expected.
    > I intend to cycle through some more cell types and manufacturers, as soon as
    > possible.
    >
    >> What most refer to as "memory" is the destruction of cell(s) by
    >> reverse charging a multi-cell pack or by cooking the poor
    >> bastards in a crappy charger.

    >
    > That certainly wasn't my understanding. The first is reversal damage, and
    > the second is charge depression, both very different from memory.
    >
    >> One is *highly* unlikely to ever see the dreaded "memory" effect. And

    > not every cell
    >> shows even this.

    >
    > Hmm. I got lucky then? 1 for 1.
    > Admittedly a small sample.
    > I have a large number of the HR-AUC packs available for testing.
    >
    > I'm running a 30 cycle test now, which should be interesting, relative to
    > the 10 cycle test earlier.
    > In the future, I plan to run cycles to a specific discharged energy (kJ)
    > rather than voltage, and to dithered points, to determine the effect of the
    > variation of the discharge point on the total effect.
    >
    > The nice part is that it's pretty much automated, I just have to set it up,
    > and let it run, and save the data at the end.



    I for one always appreciate knowing the complicated truth rather than a
    simplified version of it. That is, if there is a mild and easily overcome
    memory effect associated with a particular cell, I would rather know the
    details than just have someone tell me "memory effect is a myth."

    What I have always read and believed was that NiCad cells really do have
    a memory effect, but that it is only detectable when a battery is
    discharged very regularly to a specific level over and over again. In
    practice, this is not what most people do. This is why knowledgeable
    people often say that memory effect is a myth, I guess.

    Also, there are lots of people out there who ruin their batteries by
    overcharging or discharging too far (so that the weakest cell is
    reversed), and they mistakenly call it "memory effect." This is another
    reason why people like Keith go around saying memory effect is a myth.

    Mac
    Mac, Jan 19, 2004
    #8
  9. In article <>,
    says...
    >
    > > IN the real world, it is a myth. That is, no one is ever likely
    > > to see any such problem, and if they do it's easily correctable,
    > > as you've stated. You've performed a very accuratee test
    > > demonstrating the effect on *one* cell type. Yawn!

    >
    > I don't see it as a "yawn", it was not something I'd expected.
    > I intend to cycle through some more cell types and manufacturers, as soon as
    > possible.


    You specifically went looking for trouble, and found it. Yawn.

    Under normal operating conditions one would not repeatedly discharge
    the cell to precisely the problem level and then recharge. Also, this
    is one cell type and one operating condition. Other cells may or may
    not have this characteristic, or may at a different discharge point.
    It goes into the "don't do that *again*" basket.

    > > What most refer to as "memory" is the destruction of cell(s) by
    > > reverse charging a multi-cell pack or by cooking the poor
    > > bastards in a crappy charger.

    >
    > That certainly wasn't my understanding. The first is reversal damage, and
    > the second is charge depression, both very different from memory.


    Sure, I know that and you know that. ...but that's not what the non-
    technical people call it. Whatever happens to reduce a NiCd or NiMH
    cell capacity is "memory".

    > > One is *highly* unlikely to ever see the dreaded "memory" effect. And

    > not every cell
    > > shows even this.

    >
    > Hmm. I got lucky then? 1 for 1.
    > Admittedly a small sample.


    No, you have a cell type that is known (it's manufacturer tells you) to
    exhibit this phenomenon. If it hurts when you do that, don't do it
    again.

    > I have a large number of the HR-AUC packs available for testing.


    Try the specific discharge/charge profile for another manufacturer's
    cell. You'll likely find a big difference. Cells are optimized for
    different things. Perhaps Sanyo didn't see this as a major issue and
    went for maximum capacity (or some such) accepting the "memory" at a
    repeated specific discharge.

    > I'm running a 30 cycle test now, which should be interesting, relative to
    > the 10 cycle test earlier.


    Why? The manufacturer said that it was a problem. You're simply
    verifying the datasheet. In my book, that rates a "yawn". OTOH, your
    statement about fully recovering after one complete discharge is
    interesting.

    > In the future, I plan to run cycles to a specific discharged energy (kJ)
    > rather than voltage, and to dithered points, to determine the effect of the
    > variation of the discharge point on the total effect.


    That would be interesting. Now you're gathering information perhaps
    not available in the datasheets. Try it on other manufacturer's cells
    and the results could be quite interesting.

    > The nice part is that it's pretty much automated, I just have to set it up,
    > and let it run, and save the data at the end.
    >


    Want to share your charger/discharger design?

    --
    Keith
    Keith R. Williams, Jan 19, 2004
    #9
  10. Dave VanHorn

    Dave VanHorn Guest


    > Under normal operating conditions one would not repeatedly discharge
    > the cell to precisely the problem level and then recharge.


    Actually, you do, in many devices.
    Pretty much anything with electronics in it, is going to have a shutdown
    point.
    If the device was designed for alkalines, but you're using NIMH in it, then
    the shutdown point will be wrong, and result in short cycles like this.


    > Also, this is one cell type and one operating condition.


    Granted, more data as I have time to collect it.



    > Try the specific discharge/charge profile for another manufacturer's
    > cell. You'll likely find a big difference. Cells are optimized for
    > different things. Perhaps Sanyo didn't see this as a major issue and
    > went for maximum capacity (or some such) accepting the "memory" at a
    > repeated specific discharge.


    I will.

    > > I'm running a 30 cycle test now, which should be interesting, relative

    to
    > > the 10 cycle test earlier.

    >
    > Why? The manufacturer said that it was a problem.


    Because they give no guidance on the magnitude of the problem.
    It helps a lot to know when a limit is a brick, or a rubber wall.


    > That would be interesting. Now you're gathering information perhaps
    > not available in the datasheets. Try it on other manufacturer's cells
    > and the results could be quite interesting.


    All it takes is time.

    > > The nice part is that it's pretty much automated, I just have to set it

    up,
    > > and let it run, and save the data at the end.
    > >

    >
    > Want to share your charger/discharger design?


    I can share the discharge/monitor stuff.
    The charger is proprietary, but isn't really relevant to this. It was just
    what was convenient to use. I've turned off a number of it's interesting
    features for this test, so it's very vanilla, though it still senses end of
    charge very nicely.
    Dave VanHorn, Jan 19, 2004
    #10
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