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'[EE]: NiMH batteries'
2001\05\08@123343 by Olin Lathrop

face picon face
> One further wrinkle, it's NICAD that has the negative droop.
> NIMH just flattens out.

Not according to Panasonic, who is one manufacturer of these batteries.


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2001\05\08@130727 by Olin Lathrop

face picon face
> I've built up a 12V stack of them that is theoretically 10 Amp-Hours.
>
> I find the voltage floating all over the place while these batteries are
> charging, and I can't imagine finding the negative voltage slope after
> watching the voltmeter wander up and down so much while charging with
about
> 2 amps.

I think you are charging them with much to high current.  The fast charging
current for NiMH batteries is usually about 1C, which is probably 1/2 amp
for these batteries.  I also wouldn't charge them as a 12V stack.  Too many
cells end up in series and variations between cells can cause problems since
they are all getting the same current.

By the way, I changed the topic to EE.  AD is not appropriate anymore.


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2001\05\08@130941 by Peter Tiang

flavicon
face
NiMH has only a small -deltaV (<20mV),
using dT/dt, a sharp rise in battery pack
temperature, is a much more accurate detection
of full charge. This method used in most
"smart" notebook batteries.

But this might not prove practical if
the NiMH pack does not have built-in thermistor.
Don't bother measuring temperature outside the pack.

I read somewhere (a couple of years ago, so don't ask
for detail) that manufacturer intentionally design
in -deltaV into NiMH packs so that they
can be charged on the plenty NiCD chargers out
there and that -deltaV is not true characteristic
of NiMH.

So you are right and wrong.

Cheers,
Peter Tiang

{Original Message removed}

2001\05\08@133203 by Alan Beeber

picon face
Over the practical life of the battery pack, the -delta V will flatten even
further, making voltage detection even more difficult. The best way is to use
dT/dt next to the cells, and add a absolute maximum temperate charge cutoff.
Check the data sheets for this value, and derate it a bit. I guessing, but
the dT/dt should be about 1-2 deg. C/minute, and the maximum T should be
about 50C. Double check these numbers, it's been a little while....

Peter Tiang wrote:

{Quote hidden}

> {Original Message removed}

2001\05\08@142013 by Lawrence Lile

flavicon
face
Too late to not charge as a 12V stack, they are all wired up in
series/parallel to get a 10 amp-hour 12V battery.

Theoretically, 1C charging rate would be 10 amps for this stack, but I was
only charging at about 1-2 amps.  Somebody else suggested I need to use a
controlled current source.  I could also bury a temp. sensor in the battery
stack, as it's about 6" cube.

Unfortunately I can't always charge at 1C.  I'll often need to charge at
lower currents, around 1/10 C.  I am afraid this won't heat up the battery
pack sufficiently to allow delta Temperature charging to work OK.  If deltaV
charging also doesn't work, then this pile of batteries might o' been a
waste of time.

I've also heard advice to use zero deltaV slope as the cutoff point.
Doesn't put quite as much charge in the battery, but might extend battery
life a little.  Maybe that's the only scheme that will really work with
these niMH's.

-- Lawrence Lile

{Original Message removed}

2001\05\08@145249 by Peter Tiang

flavicon
face
> Too late to not charge as a 12V stack, they are all wired up in
> series/parallel to get a 10 amp-hour 12V battery.
>
> Theoretically, 1C charging rate would be 10 amps for this stack, but I was
> only charging at about 1-2 amps.  Somebody else suggested I need to use a
> controlled current source.  I could also bury a temp. sensor in the
battery
> stack, as it's about 6" cube.

   You absolutely need a constant current source.
   Only then will you be able to observe
   the characteristic charge curve (V) for
   -deltaV or 0deltaV termination.

   I'm wondering how you are going to get/build
   a 10A constant current source... ;-)

   I might have missed some early posting,
   but at this capacity, would'nt a lead acid
   battery be a better (and cheaper) choice.
   Unless weight is a concern.

>
> Unfortunately I can't always charge at 1C.  I'll often need to charge at
> lower currents, around 1/10 C.  I am afraid this won't heat up the battery
> pack sufficiently to allow delta Temperature charging to work OK.  If
deltaV
> charging also doesn't work, then this pile of batteries might o' been a
> waste of time.

   dT/dt method is observation of the "rate" of
   increase in temperature, so the absolute value
   of the battery temperature is of no concern.

   What you want to do is to observe a sudden
   jump in the "rate" of temperature increase.
   I'm sure it'll still work at 1/10C.

   Also the max T cutoff is used for failsafe
   instead of full-charge termination.

> I've also heard advice to use zero deltaV slope as the cutoff point.
> Doesn't put quite as much charge in the battery, but might extend battery
> life a little.  Maybe that's the only scheme that will really work with
> these niMH's.

   Yes, 0deltaV termination can also be used, just
   that is not exactly accurate. Sometime you undercharge,
   which is still OK, but sometime you might overcharge
   which lead to shorter lifetime for your battery.

   I would still go with the dT/dt method.

>
> -- Lawrence Lile

Regards,
Peter Tiang

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2001\05\08@151347 by David VanHorn

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At 12:18 PM 5/8/01 -0400, Olin Lathrop wrote:
> > One further wrinkle, it's NICAD that has the negative droop.
> > NIMH just flattens out.
>
>Not according to Panasonic, who is one manufacturer of these batteries.


Everyone else (including Maxim and BQ, who make the charge ICs)
Says that NIMH should be terminated on flattening, and NICAD on droop.

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2001\05\08@152400 by David VanHorn

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At 12:16 PM 5/8/01 -0400, Olin Lathrop wrote:
> > I've built up a 12V stack of them that is theoretically 10 Amp-Hours.
> >
> > I find the voltage floating all over the place while these batteries are
> > charging, and I can't imagine finding the negative voltage slope after
> > watching the voltmeter wander up and down so much while charging with
>about
> > 2 amps.
>
>I think you are charging them with much to high current.  The fast charging
>current for NiMH batteries is usually about 1C, which is probably 1/2 amp
>for these batteries.  I also wouldn't charge them as a 12V stack.  Too many
>cells end up in series and variations between cells can cause problems since
>they are all getting the same current.
>
>By the way, I changed the topic to EE.  AD is not appropriate anymore.

4C is workable, if the cells are designed for it, and the charger is right.
But you're right, 1C is much safer, and without a controller, I'd be
nervous at that.

Also, I agree on the voltage limit. You don't see it discussed very often,
but notice how few applications use higher voltage NIMH or nicad stacks.


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2001\05\08@153021 by David VanHorn

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face
>
>     I'm wondering how you are going to get/build
>     a 10A constant current source... ;-)

No problem. This is how I did my first Nicad charger.
I was charging those racing packs.
10A SMPS constant current source, switched down from 24V.
I worked out the negative slope cutoff from reading NASA tech notes on
charging satellite batteries.
After all, they are VERY interested in long battery life, but yet making
use of every watt that they can.
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2001\05\08@174841 by Alexandre Domingos F. Souza

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>I worked out the negative slope cutoff from reading NASA tech notes on
>charging satellite batteries.
>After all, they are VERY interested in long battery life, but yet making
>use of every watt that they can.

       Where did you find it? Seems to be an interesting subject! ;oD

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2001\05\08@180404 by David VanHorn

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>
>         Where did you find it? Seems to be an interesting subject! ;oD

This was almost 20 years ago, I don't remember.

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2001\05\08@184224 by Alexandre Domingos F. Souza

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>>         Where did you find it? Seems to be an interesting subject! ;oD
>This was almost 20 years ago, I don't remember.

       Ops... :o(

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2001\05\09@021646 by V sml

picon face
How fast can the high temperature kill the batteries, I am about to
try to Solder together a few NiMH batteries to replace a dead and
outdated laptop battery.  Should I try it?

Cheers, Ling SM

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2001\05\09@025710 by Peter Tiang

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face
Use high wattage soldering iron and solder
at high temp but short duration.

Also, does your old battery pack comes with
built-in electronics (Smart battery)?
If so, you need to match the capacity of the
replacement battery, otherwise there will be
a mismatch as the electronics thinks that it
is X capacity while your battery is Y capacity.

Regards,
Peter Tiang

{Original Message removed}

2001\05\09@085648 by V sml

picon face
>Use high wattage soldering iron and solder
at high temp but short duration.

Thanks peter, Guess I would also need to use low-temperature solder
since I don't have a spot-welder.  Normally the thermal mass of the
battery head would means the battery need to be heated up first before
normal soldering is possible.

>Also, does your old battery pack comes with
built-in electronics (Smart battery)?
If so, you need to match the capacity of the
replacement battery, otherwise there will be
a mismatch as the electronics thinks that it
is X capacity while your battery is Y capacity.

Luckily there is no electronics for this one, just a therminister and
4 AA NiCD cells.

Cheers, Ling SM

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2001\05\09@095326 by David VanHorn

flavicon
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At 02:16 PM 5/9/01 +0800, V sml wrote:
>How fast can the high temperature kill the batteries, I am about to
>try to Solder together a few NiMH batteries to replace a dead and
>outdated laptop battery.  Should I try it?

That's what the straps are for.
Soldering directly to the cells isn't good for them.

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2001\05\09@111952 by Peter Tiang

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> >Use high wattage soldering iron and solder
> at high temp but short duration.
>
> Thanks peter, Guess I would also need to use low-temperature solder
> since I don't have a spot-welder.  Normally the thermal mass of the
> battery head would means the battery need to be heated up first before
> normal soldering is possible.
>

I think I would avoid slowly heating up the
battery terminal.

This is because the slow transfer of heat
from your solder iron to the terminal
had a chance of heating up the cell.

As typical cell are designed with a vent
to relieve internal pressure, the heating
up of the cell means you potentially lose
some amount of chemical due to the vent
release.

This chemical loss can never be recovered,
therefore battery capacity is lost for good.

Regards,
Peter Tiang

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2001\05\09@112918 by O'Reilly John E NORC

flavicon
face
>As typical cell are designed with a vent
>to relieve internal pressure, the heating
>up of the cell means you potentially lose
>some amount of chemical due to the vent
>release.


This just reminded me of the time I put a vent in a battery.  I was about
14, and had heard something about rechargable batteries.  Well, I had a few
D cells (alkaline) sitting around, so I thought I would try it.  So, I tape
a couple of wires to the ends, and proceeded to plug it in.  ZAP - POP,
instant vent hole about .5mm in diameter.  Scared the _ _ _ _ out of me.

John

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2001\05\09@114811 by James Paul

flavicon
face
You reminded me too of a funny thing that happened when I was in
Tech School.  We were going to have a test in theory class, and as
luck would have it, we had a lab first.  Anyway, this guy in
the lab found out his batteries in his calculator were low.  So low
in fact that they probably wouldn't last throughout the theory class.
(It lasted for 2 hours).  So he decided to charge the batteries while
he was in the lab.  So he pulls them out of the calculator, and
holds them together with his hands, while holding one wire to each
the top and bottom of the 2 cell stack.  All goes well for about 5
minutes or so.  He sitting there talking to the rest of us.  When
all of a sudden we hear this loud pop.  Next thing we know is the
batteries have basically destructed.   The tops popped out of both
of them.  And this guy has black powder (carbon) all over his face.
Along with plastic and the paper label.   Everyone had a good laugh
over that one.  Except Bart (the guy).  He was rather upset, and
embarrased, and trying not to laugh too, all at the same time.
He wound up leaving the lab and going to the store to get new
batteries for his calculator.   And yes, he did pass the test in
theory class.
It was rather simple, and didn't even require the calculator.  But
at least he did replace the batteries, and he was ready for the next
test.   But as long as I live, I will never forget the look on his
face when those batteries blew.  Or the little black specks of
carbon, plastic and paper label all over his face and shirt.
He was a sight.

                                                Regards,

                                                  Jim






On Wed, 09 May 2001, O'Reilly John E NORC wrote:

{Quote hidden}

jimspamKILLspamjpes.com

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2001\05\15@083413 by Olin Lathrop

face picon face
part 1 714 bytes content-type:text/plain; (decoded 7bit)

I ran another test with a different virgin battery pack.  This time the
current was higher than before.  It started out around 130mA and ended up
around 40mA, which is .26C to .08C if these packs are made of 500mAH cells.

This test ran for over 13 hours.  A plot of the measurements is attached.  I
was hoping to see something different happen when the pack got fully
charged, but the voltage just kept going up.  I guess I wasn't patient
enough.  I'm about to start another test, and this time I'll let it run much
longer.  I don't care if the pack gets destroyed in the process, I want to
see what these things do as they get fully charged and beyond.

I'll keep you posted.


part 2 13350 bytes content-type:image/gif; (decode)


part 3 335 bytes

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2001\05\15@101436 by Peter Tiang

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face
How many cell per pack ?

I assume the standard 4-cell pack.

In that case, in most likelyhood you'll just reach
an equilibrium point where the charge current
equals the self-discharge current and the
voltage will almost (but not quite) approach 5V.
It's like an open circuit with pack voltage at
(close to) 5V.

I doubt that it will be destroyed as well,
since full charge is at 1.8V/cell, which
for a std 4-cell pack comes to around 7.2V.
This can never be arrived at, given your 5V source.

I really don't see what you can gain from this
experiment if you don't use constant current
AND a higher voltage source.

Lawrence Lile has some good suggestions in
his summary.

Regards,
Peter Tiang

{Original Message removed}

2001\05\15@110631 by David VanHorn

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>
>I doubt that it will be destroyed as well,
>since full charge is at 1.8V/cell, which
>for a std 4-cell pack comes to around 7.2V.
>This can never be arrived at, given your 5V source.

That 1.8V is an absolute maximum cutoff point to keep things from being
destroyed, it's NOT the full charge voltage.  You can try 1.8, but I'll sit
behind something solid while you do.

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2001\05\15@113019 by Peter Tiang

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Sorry, misread the spec, but at ~1.5V per cell
at full charge at ambient temp (from Panasonic
website info, full charge V tends to vary with
temperature and rate of charging),
you still get about 6V, which can never be
arrived at with Olin's 5V-sourced charge circuit.

Maybe, full charge is not Olin's intention ?

{Original Message removed}

2001\05\15@120533 by David VanHorn

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At 11:33 PM 5/15/01 +0800, Peter Tiang wrote:
>Sorry, misread the spec, but at ~1.5V per cell
>at full charge at ambient temp (from Panasonic
>website info, full charge V tends to vary with
>temperature and rate of charging),
>you still get about 6V, which can never be
>arrived at with Olin's 5V-sourced charge circuit.
>
>Maybe, full charge is not Olin's intention ?

It does sound like the charger needs a little more headroom.
OTOH, assuring that the cells can never be overcharged is probably going to
extend their life in terms of number of cycles, at the expense of run-time
per cycle.

Engineering is the art of compromise. :)

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2001\05\15@121204 by michael brown

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----- Original Message -----
From: "Peter Tiang" <KILLspampetertiangKILLspamspamPD.JARING.MY>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Tuesday, May 15, 2001 10:33 AM
Subject: Re: [EE]: NiMH batteries


> Sorry, misread the spec, but at ~1.5V per cell
> at full charge at ambient temp (from Panasonic
> website info, full charge V tends to vary with
> temperature and rate of charging),
> you still get about 6V, which can never be
> arrived at with Olin's 5V-sourced charge circuit.

FWIW They are three(3) cell packs so 4.5V would be full charge.

{Quote hidden}

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2001\05\15@121814 by Bill Westfield

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THey're three-cell packs...

BillW

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2001\05\15@131800 by Olin Lathrop

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> How many cell per pack ?

These packs have 3 cells.  I guess that means their discharged voltage is 3V
and charged voltage a bit over 4V.

> I doubt that it will be destroyed as well,
> since full charge is at 1.8V/cell,

I thought 1.8V/cell was the absolute maximum never exceed voltage, and that
the normal fully charged voltage was less.

> I really don't see what you can gain from this
> experiment if you don't use constant current
> AND a higher voltage source.

I little more understanding, I hope.  The constant current source will come
in due time.  I've got a day job keeping me busy.

> Lawrence Lile has some good suggestions in
> his summary.

Yes, they may be useful when I try to build a charger for real.  However, as
I keep having to say, that's not what I'm trying to do right now.  There is
a lot of information about these cells out there, but some of it conflicts,
even from the same source.  Much of what is available is just "how to" info
on charging.  It doesn't give much insight what happens if the recipe is
deviated from a little bit or how it could be modified to suite particular
needs.


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2001\05\15@131811 by Olin Lathrop

face picon face
part 1 1439 bytes content-type:text/plain; (decoded 7bit)

> Charging at less than 0.1 C, I expected the charge time to be over 10
hours.
> So I think you're right -- the pack still isn't fully charged; you just
weren't
> patient enough.
>
> Just so you have all the possible variables covered, consider recording
room
> temperature.
>
> While Peter Tiang seems to be jumping to conclusions based on assumptions
that
> you have not yet confirmed, I agree with his recommendation to go to a
higher
> voltage source. (You can always compensate by increasing the value of your
> current-limit resistor).

Yes, I plan on doing that.  I have a regulated +12V source available in this
setup.  I wanted to do one more apples to apples comparison before changing
the setup.  I also don't have the right power resistors hanging around, so I
have to kludge something.  In the next modification I'll probably go
straight to a regulated current source from the +12.  Yes, I know none of
this is hard, but it's still enough work that it's something I have to "get
to".  The last test with the existing setup will be done tomorrow morning.

> (p.s. Your last GIF was great. This GIF seems to be slighly goobered up.
Problem
> on my end ?)

The GIF file itself is fine on my system.  It was garbled when I got my own
message back, so something happened to it during the email transport.  I've
attached it again to this message.


part 2 13274 bytes content-type:image/gif; (decode)


part 3 335 bytes

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2001\05\15@134632 by Peter Tiang

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I'm not trying to discourage your experiment,
but the reason I've been insisting on constant
current is because it's like doing an experiment
without a control, with every variables freely
running it's course.

The only variable that is constant is the 5V
supply. But after the drop across the current
limiting resistor and transistor, it's no longer true.

Not sure you can make much conclusion under
these conditions.

Perhaps David VanHorn (who seems to be very
knowledgeable on battery charging) can enlighten us.

Rgds,
Peter Tiang

=====================================================

>
> > I really don't see what you can gain from this
> > experiment if you don't use constant current
> > AND a higher voltage source.
>
> I little more understanding, I hope.  The constant current source will
come
> in due time.  I've got a day job keeping me busy.
>

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2001\05\15@141945 by David VanHorn

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>
>I thought 1.8V/cell was the absolute maximum never exceed voltage, and that
>the normal fully charged voltage was less.

1.8V is the "grenade" voltage.

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2001\05\16@021628 by Chris Cox

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Your gif image was badly garbled. Send it to yourself and see if it's
OK...

Olin Lathrop wrote:
{Quote hidden}

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2001\05\16@090626 by Olin Lathrop

face picon face
> Your gif image was badly garbled. Send it to yourself and see if it's
> OK...

Somehow it got chewed up in the mail transport.  I got it garbled too when I
got my message back from the PIC list.  I already sent it again yesterday,
and I got that message back intact.  Let me know if you didn't get the
second copy and still want it.  I'll send it to you privately.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspamEraseMEembedinc.com, http://www.embedinc.com

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2001\05\16@100831 by Martin Wehner

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face
I was looking around for information on how to re-charge NiMH batteries apart
from everything that has been posted so far.

I found a pretty interesting guide at:
www.duracell.com/OEM/Rechargeable/Nickel/nickel_metal_tech.html
It describes charging methods along with charging graphs.

Also, check out www.mahaenergy.com/products/consumer/battery.htm
and look at the MH-AA155 Product Technical Data Sheet at the bottom of the
page for information on capacity testing.

I realize that none of this information applies 100% directly to the packs
some of us received, but should be helpful nonetheless.

       Martin



-------------------------------------------
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2001\05\16@104607 by Olin Lathrop

face picon face
part 1 2374 bytes content-type:text/plain; (decoded 7bit)

I just completed another test on a virgin NiMH battery pack, and this time I
let it run for over 21 hours.  The interesting thing about this test is that
it did catch a dip in the battery voltage, probably as it got almost fully
charged.  Two GIF files are attached.  The first one provides an overview of
the data, and the second zooms into the section with the dip.  This allows a
few interesting observations about the data:

1  -  The event itself lasted about 1/2 hour.

2  -  The current for the 2 hours surrounding the event was essentially
constant, always between 45.5 and 46.0 milliamps.

3  -  The effect was very small, and the change in voltage was only a
fraction of the A/D LSB.  It was caught only because there was some random
input noise and many samples were filtered together for exactly this reason.

4  -  The negative voltage blip was detected in the charging voltage only,
and not in the open circuit voltage.

5  -  Even though the effect was small, additional low pass filtering on the
charging voltage would have provided very adequate signal to noise ratio for
detecting the event in software.

6  -  The negative blip event was surrounded by a long period of essentially
0 dV/dT.  This long term average dV/dT was sufficiently different from the
charge voltage before and after the flat region that this region could have
been detected by software even if there was no negative blip in the middle
of it.

So, what conclusions can we draw from this about designing a charger?  None
really since this is a single sample and there is therefore no information
about variance between packs and how pack characteristics change with age
and charge/discharge cycles.  The current used was also much lower than
would be desirable in a useful charger.  However, I find these results
encouraging.  What I've read so far leads me to believe that the effect seen
in this test gets stronger at higher currents.  It would not have been
difficult to design an algorithm that would have terminated the charge at 16
hours at the peak of the blip, at about 16.3 hours after the blip was over,
or at about 17.2 to 17.5 hours after the charge voltage started going up
again long term.

Next I will give the PIC a means of controlling the charging current and run
more tests at higher and constant currents.  I'll keep you posted.


part 2 14681 bytes content-type:image/gif; (decode)


part 3 14232 bytes content-type:image/gif; (decode)


part 4 304 bytes

********************************************************************
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(978) 742-9014, TakeThisOuTolin.....spamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\05\16@105418 by Roman Black

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Olin Lathrop wrote:
>
> I just completed another test on a virgin NiMH battery pack, and this time I
> let it run for over 21 hours.  The interesting thing about this test is that


Olin, aren't you supposed to charge and discharge
new batteries a couple of times before expecting
normal use and performance? I know you must do this
with NiCds or you dont get full capacity.
-Roman

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2001\05\16@125016 by Lawrence Lile

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One of the things I'm looking for in conjunction with the battery charger is
a good low-side (preferably) or high side current measuring schematic.  I'm
envisioning an op-amp capable of rail-to-rail inputs, across a small sense
resistor between batt- and ground, maybe with a little low pass filtering.
Tried to whip this up in Spice and can't get it to work.  Anybody seen a
good schematic for this application?  I'm SOOOO rusty on analog...


--Lawrence Lile

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2001\05\16@130632 by Chris Cox

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face
GIF came in perfect the second time, thanks...

Olin Lathrop wrote:
>
> > Your gif image was badly garbled. Send it to yourself and see if it's
> > OK...
>
> Somehow it got chewed up in the mail transport.  I got it garbled too when I
> got my message back from the PIC list.  I already sent it again yesterday,
> and I got that message back intact.  Let me know if you didn't get the
> second copy and still want it.  I'll send it to you privately.
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, spamBeGoneolin@spam@spamspam_OUTembedinc.com, http://www.embedinc.com
>
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2001\05\16@141513 by Olin Lathrop

face picon face
> Olin, aren't you supposed to charge and discharge
> new batteries a couple of times before expecting
> normal use and performance? I know you must do this
> with NiCds or you dont get full capacity.

These packs are all discharged.  I assume this is from self discharge due to
sitting in a warehouse for many months.  I hadn't heard about them requiring
multiple charge/discharge cycles the first time, only that it might be
required to get around the memory effect.  Do you have a reference for this?


********************************************************************
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2001\05\16@150631 by Olin Lathrop

face picon face
> One of the things I'm looking for in conjunction with the battery charger
is
> a good low-side (preferably) or high side current measuring schematic.
I'm
> envisioning an op-amp capable of rail-to-rail inputs, across a small sense
> resistor between batt- and ground, maybe with a little low pass filtering.
> Tried to whip this up in Spice and can't get it to work.  Anybody seen a
> good schematic for this application?  I'm SOOOO rusty on analog...

I was going to do this on the high side.  The current sense resistor is
going to double to also limit the maximum possible current and reduce the
dissipation in the current regulator.  All you need to to turn this into a
ground-referenced signal proportional to the current is a diff amp.


********************************************************************
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(978) 742-9014, EraseMEolinspam@spam@embedinc.com, http://www.embedinc.com

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2001\05\16@153233 by Spehro Pefhany

picon face
At 02:35 PM 5/16/01 -0400, you wrote:
>> One of the things I'm looking for in conjunction with the battery charger
>is
>> a good low-side (preferably) or high side current measuring schematic.
>I'm
>> envisioning an op-amp capable of rail-to-rail inputs, across a small sense
>> resistor between batt- and ground, maybe with a little low pass filtering.
>> Tried to whip this up in Spice and can't get it to work.  Anybody seen a
>> good schematic for this application?  I'm SOOOO rusty on analog...
>
>I was going to do this on the high side.  The current sense resistor is
>going to double to also limit the maximum possible current and reduce the
>dissipation in the current regulator.  All you need to to turn this into a
>ground-referenced signal proportional to the current is a diff amp.

Or create a current source from the postive rail, controlled by the
voltage across the sense resistor. That allows the signal to be shifted
to the low side without requiring precisely matched resistors. You do
need an opamp with rail-to-rail inputs, a PNP BJT and a few resistors.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spamBeGonespeffEraseMEspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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2001\05\16@170617 by Alan Beeber

picon face
No reference, but this is something my old (battery) company did routinely.
Cycle twice before collecting data.....


Olin Lathrop wrote:

{Quote hidden}

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2001\05\17@060831 by Roman Black

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Olin Lathrop wrote:
>
> > Olin, aren't you supposed to charge and discharge
> > new batteries a couple of times before expecting
> > normal use and performance? I know you must do this
> > with NiCds or you dont get full capacity.
>
> These packs are all discharged.  I assume this is from self discharge due to
> sitting in a warehouse for many months.  I hadn't heard about them requiring
> multiple charge/discharge cycles the first time, only that it might be
> required to get around the memory effect.  Do you have a reference for this?


Olin, I confess ignorance regarding NiMH batteries.
But as a TV industry shop we sell a lot of batteries,
both NiCd and NiMH, and they come with the written
notice: "allow one or two full charge/discharge cycles
before expecting full capacity".

I charge all our NiCd and NiMH batteries at 10% C
overnight, about 15 hours. Many we have used for
years in regular use. New ones I charge at 10% C
for 15 hours, then run them flat, then charge again
for 15 hours, then put them to use.

I think using constant current chargers at 10% C
must work alright because it is all we have ever done
and never had a problem. I still don't see battery
charging as rocket science, and our constant current
sources are usually "transformerless" Xc type supplies,
basically a capacitor and a couple of diodes.
:o)
-Roman

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2001\05\17@094747 by Barry King

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Olin,

Just wanted to pass along thanks for your making the fruits of your
labor on this battery charging problem available to all of us.

This is exactly the "bench" data that makes a real prodcut from a
theoretical design.  I look forward to seeing the evolution.

Thanks,

Barry.
------------
Barry King
NRG Systems "Measuring the Wind's Energy"
http://www.nrgsystems.com
Check out theList Archive!  Start at:
PIC/PICList FAQ: http://www.piclist.com/faq

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2001\05\17@140756 by Peter L. Peres

picon face
>current measure

The easiest way for low current is a small value resistor in the GND wire
of the battery. For higher current I use the wire itself as a shunt. Plan
for 50-100mV across the shunt at max current (ex. 0.1 ohms for +/- 1A). By
using a couple of resistors and a single opamp you can scale the readout
to read 0-5V for unidirectional or bidirectional current (2.5V with I==0),
all with single supply. You need an opamp that includes GND in its common
mode range and drives output to GND. Like a LM324 or LM358. Of course rich
people pick something better ;-) The required opamp gain is low and there
are no problems except if a very noisy load is driven (like a motor with
brushes).

Peter

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2001\05\18@102338 by Lawrence Lile

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part 1 1545 bytes content-type:text/plain; (decoded 7bit)

Op Amp low-side current sense:

I finally found a good schematic for the low-side current sensing op-amp, in
a MAXIM app note.  It simulates OK in BSpice.  My problem was, I started out
with a microchip op-amp, supposedly a "rail-to-rail" op amp (????? ) and
simulating it with a small input signal hugging one "rail" it would not work
at all.  When I popped the maxim Op amp in there, with "real" rail-to-rail
capability, it simulated OK.

Perhaps the problem is only in the Microchip spice model (many spice models
are defective, even if you get them from the MFR.)  But I won't be bothering
to use any Microchip Op Amps for a rail-to-rail application if they won't go
rail-to-rail!

I'm sure this would work OK on the high side too, just swap the inputs.

The simulation outputs 1 volt per amp, a simple ammeter.  In a real
application it might be better to scale it for full scale (5V) corresponding
to full battery current (1C or whatever the system's maximum charge current
is) .

1 volt per amp works really well for my system, I'm planning to charge at
about 2 amps, right in the middle of the PIC A/D input range, approximately.
In my system, the sense resistor only needs to sense charging current, not
load current.  Load current's sensed by a fuse ;-).

I needed a 0.025 ohm resistor as a sense resistor.  This could be built up
from 29 Cm of 24 AWG solid copper wire, if such a resistor can't be found.

Onward to the constant current circuit!

-- Lawrence Lile





part 2 8176 bytes content-type:application/octet-stream; (decode)

part 3 144 bytes
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2001\05\18@122145 by Lawrence Lile

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face
> If you are going to have a constant current source then why do you need to
> measure the (presumably constant) current?


It's the feedback portion of the constant current source.  Just now I was
trying to incorporate it into a linear feedback circuit with a BJT pass
transistor and see how well it regulates in Bspice.  Eventually it is going
to be part of the charger, that has to supply constant current to a battery.

-- Lawrence.



{Original Message removed}

2001\05\18@124842 by David Cary

flavicon
face
Dear Lawrence Lile,

Lawrence Lile <.....llileSTOPspamspam@spam@TOASTMASTER.COM> on 2001-05-18 09:22:56 AM wrote:
> I needed a 0.025 ohm resistor as a sense resistor.  This could be built up
> from 29 Cm of 24 AWG solid copper wire, if such a resistor can't be found.
> Onward to the constant current circuit!

Why don't you a big resistor (10 Ohms ?) as a sense resistor ?

I imagine you're going to use a cheap linear regulator -- would the
constant-current circuit in the LM317 data sheet work ? I think I downloaded my
datasheet from
 Philips http://www-us.philips.com/

.

Given a constant voltage input, a battery voltage output, and a linear
constant-current regulators, you're forced to waste power *somewhere*. I think
it's simpler and cheaper to waste it in the sense resistor than in your
constant-current regulator transistor. Is there a way to do something clever
like Olin Lathrop and make the same resistor do triple-duty as a current sense,
a voltage sense, and part of the constant-current regulator ?

Um... some resistors change their resistance significantly when they warm up.

--
David Cary

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2001\05\18@143248 by Lawrence Lile

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face
I made a 0.025 ohm resistor in the shop over lunch hour.  It ended up being
14" of #24AWG tinned solid copper wire at 250C.  I measured it with a
recently calibrated $3000 HP 4 wire ohmmeter just to make sure it was what I
needed. For anyone else trying to build a sense resistor, if you don't have
a $3000 ohmmeter handy, this is a good recipe.  Wound around a 1/4" dowel,
it ends up about 1" long.

FYI: Yes, I am in the process of designing a battery charger for NiMH
batteries.  This is step 3 of 5000 steps.  The sense resistor and low-side
amplifier are going to feed current info into a PIC.

To answer your question, David:

1.  Using the smallest sense resistor I can get away with is a way to get
more efficiency out of my system.  I don't want to waste power anywhere.
Besides, I've got motors to run, don't want to reduce the voltage to them
with an oversized sense resistor.

2.  Right now I have started designing a linear current regulator, and
already have realized it is only 50% efficient.  Very bad!  I will probably
end up with a switching regulator before I am done, to get the efficiency
high.  Never having designed one before, this should be fun!

3.  I don't want to use a cheap linear regulator (LM7805), because it is
hard to find a 2 amp regulator, souping them up for higher current is almost
as hard as building a regulator from scratch, and I need to also be able to
turn OFF the regulator, and linear regulators are going to be ~~50% to 75%
efficient.  Bad!  All these functions(feedback, battery voltage sensing,
battery temperature sensing, shutting down the current regulator, trickle
charging) are eventually going into a PIC.

4.  A copper resistor is going to change resistance as it warms up.  It is
also inductive, being an air core wirewound resistor.  Copper coefficient of
thermal resistance is 3930 ppm/Degree C.  My resistor is dissipating 0.1
watts, and it's as big as a 3 watt resistor, so I don't imagine I'll have to
worry about self-heating too much.  Ambient temperature changes are going to
make a bigger difference.  If it's a big deal, I'll compensate for them
later.  Good Point, David!

Aha!  Nichrome wire (the stuff they use in toasters!) is only 150 ppm/0C
Mucho Better!  I'll wind my next resistor out of nichrome.  150ppm/0C X 120C
is only .0018 ohms, a change of only 7% between 250C and 370C (75F to 100F).
Not too bad.

5.  Yes, this is part of the constant current regulator for an NiMH battery
charger, eventually part of a mowbot.


I'm sort of designing this thing online, and people keep coming in in the
middle of the discussion and asking me "What the hell are you doing?  Why
don't you just design a battery charger?"   It's getting kind of funny.

I'll keep posting more design details as I go.  Hopefully this will benefit
somebody else doing the same thing!Pictures are on the way!

--Lawrence Lile

{Original Message removed}

2001\05\18@144658 by David VanHorn

flavicon
face
At 01:31 PM 5/18/01 -0500, Lawrence Lile wrote:
>I made a 0.025 ohm resistor in the shop over lunch hour.  It ended up being
>14" of #24AWG tinned solid copper wire at 250C.  I measured it with a
>recently calibrated $3000 HP 4 wire ohmmeter just to make sure it was what I
>needed. For anyone else trying to build a sense resistor, if you don't have
>a $3000 ohmmeter handy, this is a good recipe.  Wound around a 1/4" dowel,
>it ends up about 1" long.

You did wind it non-inductively, right?


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2001\05\18@154605 by Lawrence Lile

flavicon
face
No, just wound it crudely.  I always try the crudest, simplest way first.
;-)

I've since come up with a much better sense resistor material.  Contantan
(the red wire in a Type J thermocouple) has a nearly zero temperature
coefficient (+/- 20 ppm/0C), and high resistivity.  I just happened to have
a type J thermocouple laying around.  0.357" of 24 gauge Constantan gives a
nice 0.025 ohm resistor, just a straight wire, no coils.  I soldered it into
a 4-wire resistor configuration, so's I could measure it with a 4 wire
ohmmeter, and I think I'll use it that way, too, in the real circuit.


So how do you wind non-inductively?  Reverse the direction of winding
halfway through?

-- Lawrence Lile
P.S.  This is a sense resistor for a current measurer for a constant current
supply for a NiMH battery charger for a mowbot with PIC for brains, if
anybody is just tuning in.


{Original Message removed}

2001\05\18@155746 by David VanHorn

flavicon
face
>  I soldered it into
>a 4-wire resistor configuration, so's I could measure it with a 4 wire
>ohmmeter, and I think I'll use it that way, too, in the real circuit.

That's the only way to fly in circuits like this.



>So how do you wind non-inductively?  Reverse the direction of winding
>halfway through?

Yes. The fields, to the degree that they are coupled, cancel.
You can't get total non-inductance, but it's a lot better than the other way.
L = KN^2, where N is the number of turns, and K is some other variables I
lumped together :)

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2001\05\19@105818 by Peter L. Peres

picon face
Next time when you run out of $3000 meters for making shunts do it my way:
Set benchtop PSU to current limit 1.00A, clamp 1 crocodile to start of
shunt, clamp DVM probe to same crocodile, do the same with the other croco
and DVM probe and then measure out wire until you read as many mV as
mOhms you need. And since you are at it pick heater wire (usually robbed
from new 500W space heater elements bought on purpose). This is rated 2+A
(220V countries) and fairly high R*l. You only need a few cm to make a
shunt usually and the temperature coefficient is near zero (!). In case of
need twiddle several strands together (but by this time you can use a
beefier PVC clad cable as is). A 20A shunt for a SLA battery is mostly a
20 cm long battery cable with a thin measuring wire attached at either end
just at the PVC sleeve.

If you worry about the precision, get a 0.1% 1Ohm resistor put it in
series with your $3000 (x 10^-4) shunt meter and set 1.000V with the
current limiter control on it (using the SAME DVM after disconnecting the
probes). You want to check before and after because benchtop PSUs are not
known for being precision sources (they drifet as they heat up). If you
plan to do this often replace the current setting pot on the PSU with a 10
turn precision Spectrol or Bourns pot.

If you really worry about precision buy a Vishay or other four-point
shunt. These come in a special case with four terminals (so you need to
define a footprint for your layout). Not very expensive but hard to get at
short notice in small quantity.

Peter

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2001\05\21@095126 by Lawrence Lile

flavicon
face
Thanks Peter!

The heater wire you mention is probably Nichrome (also available from used
toasters) with a thermal resistance gradient of about 150 ppm/0C.  Not too
bad.  As I mentioned I ended up using Constantan wire (the red wire from a
Type J thermocouple) which is probably called constantan because it's
resistance is nearly constant with temperature (+/- 20ppm/0C).  Copper
actually makes a pretty good thermistor(or a really bad shunt), having a
coefficient three orders of magnitude higher than constantan and two orders
higher than nichrome.

Dan says he is shipping Pic-O-Botboards today, so the brains part of the
robot is soon on it's way!

Lawrence Lile

{Original Message removed}


'[EE]: NiMH batteries'
2001\06\24@114356 by Bob Ammerman
picon face
Olin,

I have also been experimenting with these batteries.

While I haven't been directly logging the charge curves with a PIC I have
been watching them by eye.

Here is my understanding so far. Some of this is based on research into the
technology, the rest on my experiments.

You can charge them fully in about 12-15 hours at about 50ma constant
current charge. As far as I can see they can then be left on this 50ma rate
indefinitely without damage. In this case they will get  a little warm but
as the cell generates oxygen (IIRC) it will chemically recombine within the
cell and no venting or damage will occur.

The faster the charge rate the more pronounced the terminal voltage drop
will be at full charge. At about a 2/3 C rate it is very noticable.

So, based on this I am thinking of building charger that charges at 2/3C or
1C until it sees the drop, then dropping down to 30ma or so to peak the
charge and hold the battery at full charge.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)



{Original Message removed}

2001\06\24@125621 by goflo

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Olin Lathrop wrote:
> I finally got a chance to experiment with the NiMH batteries again...

Bob Ammerman wrote:
> I have also been experimenting with these batteries.
> Here is my understanding so far...

Perhaps limiting heat transfer out of the pack (insulation)
would exaggerate dT at low C.

Jack

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2001\06\24@150553 by Olin Lathrop

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> You can charge them fully in about 12-15 hours at about 50ma constant
> current charge. As far as I can see they can then be left on this 50ma
rate
> indefinitely without damage. In this case they will get  a little warm but
> as the cell generates oxygen (IIRC) it will chemically recombine within
the
> cell and no venting or damage will occur.

I've seen conflicting statements about this.  I've looked at a lot of
manufacturer info, so I don't remember exactly who said what, but at least
one manufacturer (Panasonic ?) explicitly says not to do this.  Others
explicitly say it's OK.  Unfortunately the Chinese company that made these
batteries provides virtually no useful information.

> So, based on this I am thinking of building charger that charges at 2/3C
or
> 1C until it sees the drop, then dropping down to 30ma or so to peak the
> charge and hold the battery at full charge.

I would have an alternate terminating condition in case the blip doesn't
occur.  I've been looking at recommendations from various manufacturers, and
some of them come right out and say that the blip might not happen
sometimes, even at the high charge rates.  My conclusion is that it's a good
indicator when it's there, but you can't count on it.  I've seen the blip
once at .1C, and several times not at .2C.  Apparently the blip is also less
prounounced as the cell ages.


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2001\06\24@151215 by David VanHorn

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>
>I would have an alternate terminating condition in case the blip doesn't
>occur.  I've been looking at recommendations from various manufacturers, and
>some of them come right out and say that the blip might not happen
>sometimes, even at the high charge rates.  My conclusion is that it's a good
>indicator when it's there, but you can't count on it.  I've seen the blip
>once at .1C, and several times not at .2C.  Apparently the blip is also less
>prounounced as the cell ages.

The info that I used to design a couple commercial projects, is that NIMH
dosen't roll over at full charge, but rather that it's delta goes to zero.

I used an SMPS constant current source, and low side sense resistor to set
the charge rate, then ADC to read the battery voltage, with diodes in
series to expand the scale.
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2001\06\24@180633 by Olin Lathrop

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>  Perhaps limiting heat transfer out of the pack (insulation)
>  would exaggerate dT at low C.

Perhaps, but heat can damage the battery.  Also it seems the voltage blip
can't be relied on, so an alternative termination strategy needs to exist
anyway.


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2001\06\24@190348 by goflo

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Olin Lathrop wrote:

> >  Perhaps limiting heat transfer out of the pack (insulation)
> >  would exaggerate dT at low C.
>
> Perhaps, but heat can damage the battery.
> Also it seems the voltage blip can't be relied on, so an
> alternative termination strategy needs to exist anyway.

Quite so. I'm suggesting that the temp "bloom" as full-chg
is reached would be easier to detect at fractional C if T
is allowed to rise somewhat more than it otherwise would,
Tmax < spec.

It's possible that dV/dT would be similarly affected.

regards, Jack

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2001\06\25@045443 by Roman Black

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Olin Lathrop wrote:
>
> > You can charge them fully in about 12-15 hours at about 50ma constant
> > current charge. As far as I can see they can then be left on this 50ma
> rate
> > indefinitely without damage. In this case they will get  a little warm but
> > as the cell generates oxygen (IIRC) it will chemically recombine within
> the
> > cell and no venting or damage will occur.
>
> I've seen conflicting statements about this.  I've looked at a lot of
> manufacturer info, so I don't remember exactly who said what, but at least
> one manufacturer (Panasonic ?) explicitly says not to do this.  Others
> explicitly say it's OK.  Unfortunately the Chinese company that made these
> batteries provides virtually no useful information.


Hi Olin, we have always trickled charged our NiCds
at 10% C (constant current source).
Usually overnight, often for a couple of days
if we forget. We get excellent use from all our NiCd
powered test equipment, for many years. My personal
opinion is that this is the best charging method,
provided you don't need a quick charge. :o)
-Roman

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2001\06\25@141004 by Olin Lathrop

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> Hi Olin, we have always trickled charged our NiCds
> at 10% C (constant current source).
> Usually overnight, often for a couple of days
> if we forget. We get excellent use from all our NiCd
> powered test equipment, for many years. My personal
> opinion is that this is the best charging method,
> provided you don't need a quick charge. :o)

This seems to be less clear for NiMH.  Here are two direct quotes from
Panasonic literature about their NiMH batteries:

"Trickle current: 0.033 to 0.05CmA.  When the trickle current is set higher,
the temperature rise of the batteries is increased, causing the battery
charactersistics to deteriorate."

"Total timer: 10 to 20 hours.  The overcharging of nickle-metal hydride
batteries, even by trickle charging, causes a deterioration in the
characteristics of the batteries.  To prevent overcharging by trickle
charging or any other charging method, the provision of a timer to regulate
the total charging time is recommended."

On the other hand, I've seen other manufacturers explicitly state that
trickle charging at .1C indefinitely causes no ill effects.  Perhaps there
are slightly different chemistries.

My conclusion is not to do long term trickle charging unless you know it's
OK for the particular batteries you have.


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2001\06\26@032403 by Vasile Surducan

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Roman said:

> > powered test equipment, for many years. My personal
> > opinion is that this is the best charging method,
> > provided you don't need a quick charge. :o)

Olin reply:
>
> My conclusion is not to do long term trickle charging unless you know it's
> OK for the particular batteries you have.
>
Vasile said again:

 Olin, from my experience ( more than 20 years involved in doing
electronics ) there is no battery ( from standard acid battery, half-wet
acid batery, NI-Cd, NI-MH or other, which don't like long term low current
charge algorithm. This phrase is based on experience and not on data
sheet. Most of the worlwide producers want to sell their
products. Remember the first moment when NI-MH was launch on the
market...there were rummors this batteries aren't rechargeable.
For the producers, there is no reason to keep in use too many years the
same batteries. For a noticed user, there are many reasons...
Cheers, Vasile

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2001\06\26@040448 by Roman Black

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Thanks for the info Olin, but I want to question
something in Panasonic's literature; even at 0.05C
the battery won't charge fully in 20 hours.
Averaging their current and time figures you get
0.0415C for 15 hours. This is only 0.62C and allowing
for normal losses the battery is not even HALF charged!
You normally need to allow 30% to 40% overhead with
any rechargable battery...

Vasile said their job is to sell batteries, and that
means making you wreck them. :o) I have had batteries
fail in 4 months from using the "proper" smart fast
charger with the appliance, then started using the
10% trickle charge method and had years of use. I
encourage our video camera and mobile phone customers
to toss their fast chargers and use trickle chargers.

Mostly we put them on charge at 5pm when leaving work
and take them off at 9am the next day, at 10% C which
is constant-current regulated for about 14 hours.
But sometimes they get left on for two days or even a
weekend. No problems getting over 1000 charge cycles.
-Roman


Olin Lathrop wrote:
{Quote hidden}

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2001\06\26@090618 by Olin Lathrop

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> Thanks for the info Olin, but I want to question
> something in Panasonic's literature; even at 0.05C
> the battery won't charge fully in 20 hours.
> Averaging their current and time figures you get
> 0.0415C for 15 hours. This is only 0.62C and allowing
> for normal losses the battery is not even HALF charged!
> You normally need to allow 30% to 40% overhead with
> any rechargable battery...

That's because I snipped that quote from a description of a complete
charging algorithm which included charging at much higher rates for part of
the time.


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2001\06\26@093945 by Roman Black

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Olin Lathrop wrote:
>
> > Thanks for the info Olin, but I want to question
> > something in Panasonic's literature; even at 0.05C
> > the battery won't charge fully in 20 hours.
> > Averaging their current and time figures you get
> > 0.0415C for 15 hours. This is only 0.62C and allowing
> > for normal losses the battery is not even HALF charged!
> > You normally need to allow 30% to 40% overhead with
> > any rechargable battery...
>
> That's because I snipped that quote from a description of a complete
> charging algorithm which included charging at much higher rates for part of
> the time.


Fair enough. If you charge it at high C rates first
then permanent trickle charging afterward becomes
a different issue.

One more point, most of the "problem" they mention
with overcharging seems to be related to the
"excessive temperature rise" causing destructive
chemical changes in the battery.

At 0.1C and 1.4v at full charge you only have about
60mA into a AAA size battery, only 84mW will be
dissipated into a metal object of that mass will only
rise 2 or 3 degrees celcius. I can't believe that
will have any effect on the battery, and our real
world experience backs that up.
-Roman

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2001\06\26@105245 by miked

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I have always understood 1/10C to be standard charge(at least for NiCd) for
10-16 Hrs., and trickle to be 1/50C which is usally safe to leave on all the
time. It just compensates for self discharge.
> > Hi Olin, we have always trickled charged our NiCds
> > at 10% C (constant current source).
> > Usually overnight, often for a couple of days
> > if we forget. We get excellent use from all our NiCd
> > powered test equipment, for many years. My personal
> > opinion is that this is the best charging method,
> > provided you don't need a quick charge. :o)
>
<SNIP>
>
> On the other hand, I've seen other manufacturers explicitly state that
> trickle charging at .1C indefinitely causes no ill effects.  Perhaps there
> are slightly different chemistries.
>

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