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Re: [Fwd: Re: [avr-chat] Li-ion cell phone batteries]

From: Joerg Wunsch
Subject: Re: [Fwd: Re: [avr-chat] Li-ion cell phone batteries]
Date: Sun, 30 Mar 2008 08:25:43 +0200 (MET DST)

Robert L Cochran <address@hidden> wrote:

> ... -- if it is safe to solder those wires, I mean...the heat from a
> soldering operation on the battery wires won't explode the battery,
> will it?

Of course, you can solder them unless you're soldering it with a
hydrogen torch. ;-)

I don't really understand why the danger of LiIon and LiPo cells is
often exaggerated.  Yes, they can burn when being handled improperly.
But about any kind of battery with enough stored energy in it is
dangerous, in one way or another.  Your 44 Ah car battery is probably
much more dangerous than an 800 mA LiIon cell.  It contains lead acid,
and its stored energy can easily arrange for an explosion if you
shortcut the battery e.g. by dropping a tool onto it that doesn't
evaporize within a few hundred milliseconds.  Yet, nobody talks about
banning lead acid batteries...

Li coin cells have been around for much longer than LiIon cells, and
they are *way* more dangerous.  Nobody really made much fuzz about
them during the 20+ years they are in use.  First, they contain
metallic Li (unlike LiIon cells) which is much more reactive -- after
all, that's the entire point of the LiIon rechargables to not contain
metallic Li anymore.  (There have been metallic Li rechargables for a
short term, but they were really, really dangerous.)  Second, they are
sealed to stand the high pressure which can happen at the upper end of
the usable temperature range (plus a safety margin), so *if* you
manage to increase the pressure high enough to explode them, already
the mechanical forces set free by that are already quite high.

Of course, Li coin cells are normally not charged, and discharging
them doesn't yield a very strong current, so in normal operation,
nothing happens.  However, a standard circuit for using them as backup
batteries contains of two diodes with a common cathode, and one anode
connected to the main power supply, the other one to the coin cell.
Now imagine what happens if the diode in front of the coin cell
breaks...  the cell gets charged from the main supply.  And it will
really explode by that.  There are photos of damaged computer
mainboards caused by such an explosion, and they really don't look
like you want to have that. ;-)

Back to the LiIons, when properly handled, their mass application in
cell phones, cameras etc. shows that they are pretty safe.  The RC
model people run them at the edge of their parameters, both when
discharging as well as charging -- because RC models always take the
last out of the batteries.  So well, these folks might have been a bit
surprised that while you can run a NiMH or Pb cell quite a bit beyond
its parameters, and it will not spit into your face, you'd better
avoid that with a LiIon cell.

One note of caution: usually, these battery packs are required to have
a secondary protection circuit inside the battery when the pack can be
removed from the device.  You can find that kind of devices e.g. when
you look into the product catalog of Texas Instruments (who acquired
Benchmarq which used to be a leading manufacturer of that kind of
devices).  The purpose of that circuit is to avoid just the severe
effects mentioned when you remove the battery pack from the device,
and accidentally shortcut the connector pads outside the device, and
and also from too high charging current.  When you change the
connector, just keep that protection circuit within the pack.

> I can set it up for not more than 800 mAh and 1C charging.

Do that, but monitor the actual capacity, e.g. by discharging it once
afterwards.  That way, you can adjust the parameters subsequently.

> I can put everything in a 9 mm ammunition container I happen to have.

Non-US citizens don't have that kind of devices around, and we're
still alive anyway. ;-)

cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)

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