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'[EE]: Lithium Ion Battery Packs'
2001\05\07@182820 by Josh Koffman

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Does anyone know why lithium ion battery packs fail so easily? I've had
two die on me in under a month, two separate types of devices too. I've
been told that it's not the cells, but the little circuit in the pack
that dies, which renders the pack useless. Anyone have any idea why this
happens, what causes it, and why they aren't a bit more robust? Also a
general question, should I be keeping the battery packs charged when not
in use (i.e. does allowing them to self discharge over a period of
months adversely effect their life)?

Thanks for any help,

Josh
spam_OUTjoshyTakeThisOuTspammb.sympatico.ca

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2001\05\07@194614 by Barry Gershenfeld

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Li-ion has this really neat runaway failure mode which can make
them catch fire or explode.  Thus they are very careful about
their treatment and no one will sell packs without that circuit.

The circuit will disconnect from the battery if it is overcharged. It
will also disconnect if overly discharged; this also protects
the battery.  Despite all this if you charge a discharged pack or
put a load on the overcharged pack the battery should reconnect.
I played with one of these myself on a lab power supply.

Of course yours might really be broken in which case I don't
know.  But check the circuitry for a fuse.

Barry


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2001\05\07@213124 by Alan Beeber

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Lithium Ion battery packs are not for the faint of heart. Sony and Apple lost
facilities to fires caused by Li-ion failures, and the company I used to work
for had several small fires, contained by elaborate engineering controls.

The protection circuit usually consists of a voltage comparator chip and one
or two FETs used to interrupt the battery current during overcharge and
underdischarge conditions. The voltage comparator senses each cell in the
battery stack separately. You have to disconnect the cells and
charge/discharge the cell that is out of balance before you can reconnect the
circuit. Permanently disconnecting the circuit is asking for trouble.

Barry Gershenfeld wrote:

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2001\05\07@213801 by David VanHorn

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At 09:26 PM 5/7/01 -0400, Alan Beeber wrote:
>Lithium Ion battery packs are not for the faint of heart. Sony and Apple lost
>facilities to fires caused by Li-ion failures, and the company I used to work
>for had several small fires, contained by elaborate engineering controls.

Reminds me of my "battery box" for experimental circuits.

3/8 lexan "coffin" 10 x 4 x 4 inches inside, with something like 10 or 12
screws holding down the lid.
Sides had 1/4 inch vents, with an outer vertical wall so that NOTHING could
fly out of the box without hitting a wall, but there wouldn't be enough
overpressure to pop the lid.

I never needed it, but I often used it :)

Don't know where it finally ended up.
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2001\05\07@234939 by M. Adam Davis

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Lithium Ion Battery packs are extremely sensitive.  You can't buy lithium
ion batteries bare from the major manufacturers.  If you are using the pack
in a way that makes the circuit break, you are using it incorrectly and
should be glad the protection is there.  These things WILL explode
violently, unexpectedly, and will always land butter-side down.

It sounds like you don't know much about lithium ion rechargeable batteries
(otherwise you wouldn't be asking us about the protection circuit).  You
cannot charge them the same way you charge NiMH, never mind NiCd.  You'd
best leave the job to a dedicated Li-Ion battery charging IC such as those
available from linear technology.  You'd probably be better off getting a
charger from the firm that sold you the packs and integrating that into
your product/project until you have a very good understanding of how to
deal with Li-Ion.

As far as your question, trickle charging is NOT recommended for these
packs, their self discharge rate is quite low compared to NiMH or NiCd, and
allowing them to self discharge will not affect them adversely.  Keep in
mind that they can only be recharged (reliably) 300-500 times as compared
to NiMH's 400-600 and NiCd's 800-1200 times.  They are getting
better.  While trickle charging itself does not damage the cell when done
right, it is unnecessary when the cells are charged and used correctly.  A
proper recharging circuit will charge the cells only when needed, and not
at any other time.

Oh, and please keep in mind that there are currently two major chemistries
for Li-Ion batteries, and the charging for the two chemistries is different
enough to cause problems if you try and charge one like you'd charge the other.

Finicky, finicky, finicky little creatures.  If you can manage with NiMH
I'd recommend it for now.  Besides, all the Li-Ion production is being
sucked up by cell phone and laptop industries, so your production lines are
going to be idling a lot unless you have some amount of clout in the
industry (or deep pockets).

-Adam
(returning from the dead when he figures out how to import Eudora mail into
netscape...)

At 05:24 PM 5/7/2001 -0500, you wrote:
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2001\05\08@005751 by Josh Koffman

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Hi Adam, thanks for the reply. Perhaps I misrepresented myself in asking
the questions that I asked. I in fact know nothing about LiIon
batteries. I don't intend on using them in any product or anything.
However, they are used in two devices I own, a Sony MZR30 Minidisc
recorder, and a Yaesu VX-1R ham radio. Both battery packs died within a
few weeks. Rather, I left both devices idle for awhile, then when I went
to charge them, on two unrelated matters, but within a few weeks of each
other, both devices refused to charge their pack. I was (am) quite
annoyed because of how much it will cost me to replace the packs, and I
was wondering why they fail, and how to keep them from failing. As far
as I can tell, the only bad thing I have subjected this packs to is my
inattention. Any ideas?

Thanks,

Josh
EraseMEjoshyspam_OUTspamTakeThisOuTmb.sympatico.ca

"M. Adam Davis" wrote:
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2001\05\08@010414 by Josh Koffman

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How safe is this disconnect/discharge/recharge/reconnect operation? I'd
rather spend the money than risk well...me. Also, I haven't opened this
packs up, but what if there is only one cell? Is it possible for the
cell to simply die?

Thanks,

Josh
@spam@joshyKILLspamspammb.sympatico.ca

Alan Beeber wrote:
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2001\05\08@013629 by Bill Westfield

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While I'm certainly not an expert in Li-Ion batteries, I've read by share of
data sheets and have concluded that a lot of the paranoia is 1) overstated,
and 2) comes into play mainly when you're aiming at maximum charge, maximum
lifetime, and minimum charge time.  I've been taking apart discarded battery
packs to individual cells for quite some time now.  Usually most of the cells
still have 4+ V on them and will make a credible attempt at melting your
tools if you're careless and short them out.  The additional electronics
varies a great deal in complexity, and I haven't analyzed any of them for
failure analysis.  I charge (occasionally - I should stress that I haven't
done this for more than a dozen or so cells) the cells individually using a
lab-style CCCV power supply set to 4.1V and 0.5A or so (this is very close
to the idea charging method, assuming that my voltages is accurate to 1% or
so.) I've yet to see sign of overheating or other stress (well, except for
that cell I connected backward for a couple hours.)  If my cells are 4.2V
chemistry, well, then I'd be consistantly undercharging them a bit, so I
wouldn't get full capacity out of them and I might affect their overall
lifetime.  [Since these are "free" cells, I don't much care on those points.]

I HAVE discovered that you can apparently make a Li-ion cell VERY dead by
over-discharging it.  If I turn off the CCCV power supply, it leaves a
low-impedence meter across the outputs.  Leave a cell on there for 24 hours
or so before you notice, and it will have discharged to 0V and will NOT allow
itself to be recharged again.

I did have a conversation once with someone who had worked at a LiION
manufacturer, and they admitted under some pressure that some of the more
spectactular results that had been mentioned occured when a cell was
punctured, shorted, AND overheater (ie external fire.)  This is "modern"
cells, of course.  I'm also given to understand that older cells were
more exciting...

BillW

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2001\05\08@091347 by David VanHorn

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At 10:33 PM 5/7/01 -0700, William Chops Westfield wrote:
>While I'm certainly not an expert in Li-Ion batteries, I've read by share of
>data sheets and have concluded that a lot of the paranoia is 1) overstated,
>and 2) comes into play mainly when you're aiming at maximum charge, maximum
>lifetime, and minimum charge time.

Or trying to keep the average of $20/cell retail price afloat.


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2001\05\09@183142 by Peter L. Peres

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> Or trying to keep the average of $20/cell retail price afloat.

I believe that the price is credible (end user wise). You do not want to
know what needs to be done to keep a LiIon factory from catching fire. And
then you need to pay for the missed attempts too ;-)

Peter

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