Re: Laptop Battery Problem
- From: budgie <me@xxxxxxxxxxx>
- Date: Tue, 25 Apr 2006 10:22:25 +0800
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.
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.
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.
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!
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.
.
- Follow-Ups:
- Re: Laptop Battery Problem
- From: The Electric Fan Club
- Re: Laptop Battery Problem
- References:
- Laptop Battery Problem
- From: Roger Mills
- Re: Laptop Battery Problem
- From: The Electric Fan Club
- Re: Laptop Battery Problem
- From: budgie
- Re: Laptop Battery Problem
- From: The Electric Fan Club
- Laptop Battery Problem
- Prev by Date: Re: cicero laptop
- Next by Date: T30 Bios Update
- Previous by thread: Re: Laptop Battery Problem
- Next by thread: Re: Laptop Battery Problem
- Index(es):
Relevant Pages
|
