Re: Laptop Battery Problem


"budgie" <me@xxxxxxxxxxx> wrote in message
news:b1ou42pm8v4151o93k820l7mab9p0vbdrb@xxxxxxxxxx
On Wed, 26 Apr 2006 09:40:36 +0100, "The Electric Fan Club"
<ian.shorrocks@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:


"budgie" <me@xxxxxxxxxxx> wrote in message
news:ho5u42pi3tot40hqqank6p4seai2tv6gqn@xxxxxxxxxx

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.

Possibly my attempts at trying to make the explanation simple. There is a
specific charge level below which the cell sustains damage due to
liberating
copper from the chemistry. This precise level is less than 3.0 volts
(lets
forget the 2.5 volt chemistries for now - but similar arguements apply).
The precise level is also dependant on the chemical formulation which
varies
from one manufacturer to another and indeed on variations in formulation
within the same manufacturer.

In general, most manufacturers specify 3.0 volts as the minimum permitted
charge level so as to keep clear of the real minimum charge level.
Appliance manufacturers generally follow this specification (though some
set
it higher at 3.2 volts or any other arbitrary figure that leaps to mind
(Windows lets you pick your own to some extent)). The level at which
charging is refused is set to a little less than this, typically 2.8 to
2.9
volts. Thus if a cell is allowed to discharge to the point where the
appliance shuts down, the charger will recharge it if it is charged
immediately, and it can be charged at full permitted current.

So far I agree although we will probably never agree on the two key
voltages, as
that depends very much on the source of the cells and their chemistry.


I think I said that. The voltages were presumably selected to encompass the
worst case.

If, however,
the cell is left in place, then there is a real risk that any small
discharge (including self discharge) discharges the cell to below the
nocharge point and the charger should not charge the cell at all (not even
a
very low current).

I will comment from the point of "dumb" pack protection modules (those
that do
not communicate with a host via serial communications), as those are the
only
ones we have incorporated into industrial packs to date.

If the PPM has interrupted discharge due to reaching the LVCO point, the
resulting high impedance IS *high*. Except for rebound (cured by
hysteresis in
the LVCO sensing mechanism) the available discharge current is uA at most,
while
self-discharge is equally insignificant. Your use of the term
"immediately" is
overly dramatic - I couldn't envisage combined discharge dropping 200mV in
six
months.


Maybe. It depends on the both the cells and the equipment. I have a piece
of test equipment that will completely discharge its battery pack in just 2
weeks if never switched on. Fortunately, the battery is of a design where
it cuts its output off.

If charging is initiated before this nocharge point is
reached, it can be carried out at the full rate, because the cell
integrity
has not yet been compromised. It should be noted that the tolerances on
what can be tolerated are, in many cases, very tight and there is often a
very thin dividing line between normal operation and cell abuse.

The commercial PPM's we used only allowed effectively a trickle charge
until the
LVCO point was reached. Similarly - but separately - the charge
controller also
ensured that "normal" charging (in our case ~0.5C) did not commence until
the
LVCO threshold had been crossed.

Although I refer to 'cells' in all this, it makes life simple because the
charge monitoring can be made a function of the charger alone. If
batteries
are required that have a specification higher than 3.7v (nominal) and
around
1.5 AH, then multiple cell constructions are necessary (though the latter
figure is rising all the time). The higher AH capacities are simple
enough
because the voltage/charge characteristic permits simple parallelling of
cells to achieve this - something NiCd and Ni-MH cannot do.

However, higher voltages require series connection of either single cells
or
banks of parallelled cells. Once again the parallelled bits are no
problem,
but the individual cells in the series (the paralleled bits can be
considered as individual cells in this context) must be individually
monitored for charge - in particular, the charge monitor will signal if
any
part of the chain reaches 3.0 volts, or in a few designs, may even cut the
battery off itself. The charge monitor will also signal if any part of
the
chain is below the nocharge level or again in a few designs, actually
inhibit the charge itself.

The PPM's we used monitor cell voltage differentials in series strings.
Once
this exceeds a predetermined amount, the pack is isolated. In fact this
is a
known cause of reduction in usable pack capacity in many appliances such
as
laptops, unfortunately attributed by many as simply the cells "wearing
out".
Because any differences in (charging) coulomb efficiency will still effect
the
SOC when charge terminates - unlike NiXX types where a continuous trickle
charge
is typically applied - the cell differences not only appear cycle after
cycle
but increase as cycle count mounts. This can only be properly addressed
by
intrusive means - charging cells individually to restore balance at full
charge.

One battery design that we have seen (not a laptop), actually monitors the
individual cells of the paralleled parts of the circuit. If an individual
cell loses capacity or discharges below the nocharge level, the monitor
circuit isolates it and allows the battery pack to continue to
charge/discharge albeit with a reduced capacity. The degree of
sophistication is, I suspect, a function of the pereived profitabilty of
your replacement battery business -v- the required reliability of the
original equipment.

It is difficult to conceive a mechanisn to effectively monitor individual
cells
which are in parallel, without introducing significant series impedances.


Well somebody clearly has.

This narrative is by no means exhaustive on the subject.

but I suspect we are probably the only ones still reading ...

At least one other person is still reading, but I shall now go to see what
he says.

Have a nice day...


.

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