Re: Laptop Battery Problem


"budgie" <me@xxxxxxxxxxx> wrote in message
news:n30r42p7s3gp8e2bev8h6vvt2lh4vjsq04@xxxxxxxxxx
On Mon, 24 Apr 2006 14:27:49 +0100, "The Electric Fan Club"
<ian.shorrocks@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:


"budgie" <me@xxxxxxxxxxx> wrote in message
news:pf7p42pokck4g7oge82d97eufk76m2ggoa@xxxxxxxxxx
On Mon, 24 Apr 2006 09:04:16 +0100, "The Electric Fan Club"
<ian.shorrocks@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:

This sounds exactly like a dead battery. The charging circuit has
detected
that the battery is discharged below the minimum limit and has
(correctly)
refused to charge the battery. There is no method of recovering the
battery
and indeed, it is extremely dangerous to try.

Don't know where you get that extremely dangerous stuff from.


When a cell is discharged below 3.0 volts (2.5 volts for some early
cells),
the cell chemistry deposits copper on the internal structure of the cell.
If a cell is charged in such a condition, the copper will shunt the cell
and
pass an unwanted discharge current. It also bypasses the PTC element in
the
cell, if there is any (the cell's in-built protection). This discharge
current, if large enough can cause the cell chemistry to then liberate
oxygen gas. As soon as the highly flammable electrolyte and the oxygen
find
themselves in the same place (along with any lithium that has been
liberated
during use), the cell ruptures, shooting out quite a large flame, igniting
anything near it. We've tried it, so we know, bu then we have blown up
Ni-Cd batteries albeit deliberately.

You must be referring to extremely early (Lithium) cells,. The "standard"
18650
cells we worked with had a factory recommended "user-no-go-below" voltage
of
2v5, and our charger designs were based on a more conservative 3v0 low
voltage
cutoff. This aligned well with the majority of available pack protection
chips.
There is also limited capacity delivered at those lower voltages, as the
terminal-voltage-vs-time curve droops (under constant current) below about
3v3.


No I am refering to current technology cells. The 2.5v lower limit applies
to early technology cells that used carbon anodes. There is still current
production that uses this early technology. Your cells sound like these.

There are published schemes for recovering Li-ion cells that have been
allowed to reach this condition for short periods of time but the very
thinly plated out copper reduces the cell's capacity (having been removed
from the chemistry) and increases the cell's self discharge rate. These
schemes recommend (or in some cases, *should* recommend but don't) that
they
are carried out in a fire proof area.

Lithium-ion cells are extremely dangerous items if abused in any way.
Used
properly with correctly designed charging circuits, there is little
danger.

Completely agree with that, although there are authenticated cases of
spontaneous conflagration in cellphones. User abuse has been ruled out in
most,
but abuse by a feral phone has not.

No charger that I know of will bring an over discharged battery back into
the minimum charge zone and indeed should not do so. Having said that, it
may be that you are aware of a specific charger that leaks enough current
to
do so. If so it is either a badly designed charger, or specifically
intended to perform the recovery outlined in the last paragraph.

We may be discussing apples and oranges. You use the term
"over-discharged".
I am referring to a cell/pack that has been locked out by going
under-voltage,
meaning under that low voltage cutoff point. In the case of the cells we
used,
3v0 was the cutoff point for the pack electronics while 2v5 was the
manufacturer's recommended low voltage shutoff, below which was the onset
of
deleterious and non-reversible changes.


The 3.0 volt cutoff point may be designed so that cells can be used from
more than one source without having to have 2 versions of the management
circuitry. In this case 'overdischarged cells that are below 3.0 volts but
above the chemistry minimum of 2.5 volts can obviously be charged without
danger.

Once a cell/pack went hit that 3v0 threshold, the PPM went high impedance
and
limited further discharge to uA or nA. On test, packs that reached
3v0/cell
were still about 2v95 when we gave up monitoring their condition several
months
later, so within that half volt range there was every opportunity to
recover the
cells back into normal operation.




The PPM provided a high - but sensible - impedance to charging when in the
UVLO
state. Remember that UVLO is a protective state to prevent further
discharge -
it is NOT intended to be a terminal state. Chargers that normally operate
on a
current-limited constant-voltage will simply present their (say) 4v2
potential
to the pack, usually through their own *additional* limiting impedance as
they
do for below-safe-temp cells, to provide a trickle charge. With the low
rate of
self-discharge, this trickle will progressively bring an UVLO-ed pack back
into
the non-locked-out state and normal recharge will commence.


This is not (IMO) a *special* or *trick* charger behaviour. In service,
packs
are going to be discharged until either the load appliance (typically in
single-cell loads such as cellphones) or the pack electronics decides the
low
voltage threshold has been reached and UVLO occurs. If the charger wan't
able
to recover these packs they would be single use items, not rechargeables.



I stand by my statement that: chargers should make no attempt to charge any
cell that is below its minimum voltage. This should be independant of the
circuitry on the cell pack. The charge monitor on the cell pack should cut
out at the minimum voltage (unless that functionality is built into the
appliance), but it should not prevent charging as charging from the minimum
voltage is perfectly OK. I would regard any cell pack that refused to allow
you to charge it once the minumum voltage had been reached as badly designed
(effectively a primary cell as you state). The refusal to charge from below
this point should be a function of the charger *not* the cell pack (though
IMHO there would be no disadvantage in a belt and braces approach).


Bare Lithium-ion Polymer cells are a particular fire risk, because if
anything penetrates the thin heat shrink sleeve, the cell will burn.

I find that particularly "interesting". The manufacturers of the cells we
used
had a nail penetration test, whereby a cell at nominal voltage (3v6
usually) had
to survive a complete transverse penetration by a steel nail without
explosion
or fire. And no, we haven't felt enticed to reproduce these results!


Owners of iPods and their derivatives have repeated the demonstration on
many occasions - usually destroying the iPod and a sizeable area of floor in
the process (many photographs have even turned up on numerous blog sites).
These devices use Lithium-ion polymer cells which are supposed to reduce the
hazard because they (allegedly) lock the flammable electrolyte in the
polymer. However, they still seem to burn, especially well if over-charged
or recharged from over-discharge.. We have to check these things out as we
are in the aerospace business and there are considerable liability issues
with our products. Generally once you enclose the cell is something
substantial, it ceases to be a problem, though this does not solve the
problem of diyers who are unaware of the dangers.


It is interesting to note, that we have recently received some Lithium-ion
Polymer cell samples that are claimed to be able to sustain quite heavy
discharge rates (like in about 10 to 15 minutes) without damage. The
claim
may be true as they haven't exploded - yet. The jury is still out on the
number of cycles.

That's really about tradeoffs in the construction and the chemistry. I
wouldn't
expect to see 200 cycles from them.

The manufacturer claims 500, but we will make our own minds up once we get
that far.


.

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