Re: Laptop Battery Problem

On Tue, 25 Apr 2006 08:01:11 +0100, "The Electric Fan Club"
<ian.shorrocks@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:


"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:

(snip)

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 our case, the 3v0 cutoff was chosen as the cells have little left to give
below that. As I previously posted, the voltage curve at constant discharge
current starts to droop noticeably below 3v3. If I had my "druthers" the LVCO
would have been around 3v25.

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).

Your use of the term "minimum" is confusing.

You state: "chargers should make no attempt to charge any cell that is below its
minimum voltage" yet in the same breath you follow up with: "I would regard any
cell pack that refused to allow you to charge it once the minumum voltage had
been reached as badly designed" and then: "The refusal to charge from below this
point should be a function of the charger".

There are two voltages of importance - the LVCO point and the "no-go-below"
safety-driven limit. Between those voltages, packs MUST be able to be
recovered/recharged. If not by a charger, then by what?

Sensible chargers (and I include in that my commercial designs) monitor the
cell/pack temp and terminal volts. If the voltage is below the no-go point,
they shut down. Simple. If above the no-go and below the LVCO point, and not
over-temp, proper CV charging output is applied with a low current limit. When
the cell temp is above the low temp lockout, if the cell voltage rises past the
LVCO point normal CLCV charging resumes.
.

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