NIMH Memory effect

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?

On Sun, 18 Jan 2004 11:50:43 -0500, "Dave VanHorn"
<(E-Mail Removed)> 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

> 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.

http://vancouver-webpages.com/peter/nicad.faq
http://www.jlcnet.com/banner/nbp3.html
http://www.antonbauer.com/46-a-Terminology.htm
http://www.straightdope.com/classics/a990312b.html
http://www.k3emd.com/Technical%20References.htm
And so on..

In article <(E-Mail Removed)>,
(E-Mail Removed) 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

> 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.

In sci.electronics.design, "Dave VanHorn" <(E-Mail Removed)> 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" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In sci.electronics.design, "Dave VanHorn" <(E-Mail Removed)> 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.

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

In article <zPKdnRxhqYqz0pbdRVn-(E-Mail Removed)>, (E-Mail Removed)
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

> 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.