Re: Battery suggestions
- From: "PDR" <peter.rieden@xxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 1 Feb 2008 15:35:29 -0000
"Journey" <journey@xxxxxxxx> wrote in message
news:76p2q3p5i0retavutdip8gpt98gs57d8d2@xxxxxxxxxx
On Wed, 30 Jan 2008 18:06:05 -0000, "PDR"
<peter.rieden@xxxxxxxxxxxxxxxxxxxxx> wrote:
The
lithium cells just need to be fully charged (usually around 90 minutes
from
empty, but the charging systems vary so check your laptop instructions)
and
then used. The laptop's battery management circuits will ensure the cells
aren't over-discharged, and lithium cells don't really care about
partial-vs-deep discharging, so the use pattern doesn't really affect
them.
I recommend charging the battery in the laptop overnight before you
use it. That way you are ensured of a full charge. I wouldn't only
charge it for 90 minutes or even a few hours.
That advice only ever applied to nickel cells and is inappropriate for
Lithium cells due to the way they are charged. Nickel cells are charged at a
constant current until a particular change in the voltage is seen (the so
called "Voltage Kick" or "Delta Peak"), which occurs at around 95% capacity.
Up to this point they can safely be charged at very high rates - in modern
computers this might be anything from 1C-3C ("1C" being the rate which
wiould fully charge the pack in one hour, "3C" being the rate that would do
it in 20mins etc). Beyond this point the charging rate must be reduced to a
much lower level, because over-charging woulddamage the cells (often
explosively). So the battery charging circuit looks for the delta-peak and
then switches the charge rate to something very small - less than C/10 (the
rate that charges in 10 hours) to "top off" the remainder of the capacity
and then to condition the cells.
Now when a pack is brand-new the cells require a "forming charge" to get the
best out of them. This is done by charging at the low rate for at least 10
hours after the delta-peak point. This isn't essential, but it does increase
the usable capacity of the pack by about 10-15%. So when you get a brand new
laptop with nickel (especially NiMH, but less so with Ni-Cad) cells it's a
good idea to initially charge it for 10-12 hours to "form" the cells. After
that things change - all over-charging generally damages the cells even when
done at low rates, so it's not a good idea to leave a laptop permanently
plugged into the mains. Doing so wrecks the batteries to the point where
after anythiong from a few months to a year the pack capacity will have
reduced by 30-70%. According to some sources this is the origin of the "cell
memory" syndrome, which those same sources describe as a myth, but I'm not
going to get dragged into that one. Tghe important things to remember are
that the charging rates are controlled automatically by dedicated battery
management circuits (so you don't get involved in the details) and that
almost no modern laptops have Nickel batteries - I've only mentioned them in
detail because they are the origin of some of the misunderstandings about
later technologies.
OK, so what about Lithium cells? These are charged rather differently
because they are subject to a couple of limitations. Firstly the batteries
are damaged if they are subjected to a voltage above a certain value (for
lithium-polymer cells this is 4.2v per cell of the pack). Secondly they must
not be charged with a current greater than a certain value (typically 1C).
So the charging circuits for lithium cells initially charge at a constant
current (just like nickel cells) at the 1C rate, but they only do this until
the voltage required to do so reaches the voltage limit. At this point the
charger switches to charging at constant VOLTAGE (like for lead-acid cells)
and the current steadily drops. When the current has dropped to a small
value (typically C/20) the charger actually switches OFF to prevent
over-charging. The Constant current phase of the charge takes the cells up
to around 70% capacity and takes around 40 minutes. The following constant
voltage phase typically takes another 40-60 minutes, and after that the
charger is completely switched off, so keeping the mains connected after
that does nothing to the batteries at all. All of this is done by a
dedicated Li-battery supervisory circuit (you'll find them listed as ICs in
electrical component catalogues) rather than in software, so it's the same
for all systems.
As far as battery life, I have heard that deep-discharging of lithium ion
batteries shortens their life.
Lithiums have a further limitation - they must never be fully discharged,
because this destroys the plates. In the case of early Lithium Polymer cells
they must never be discharged lower than 3volts per cell, and for later
cells this is sometimes listed as 3.5 or even 3.6v/cell. But this isn't a
problem because the same dedicated Li-battery supervisory circuit flags the
voltage to the system (in this case our laptop) so that it can warn, prepare
for shutdown and then disconnect the power before this voltage is reached.
So it's perfectly safe to "deep discharge" your laptop because the point the
*laptop* thinks of as "empty" is actually above the battery's low-voltage
warning point.
A final thought - we all heard about the lithium battery fires which some
manufacturers suffered a couple of years ago. These were caused largely by
the computer manufacturers trying to be too clever. SOme of the very latest
type of lithium-polymer cells were capable of being charged at 2C or even
3C, but ONLY duringthe constant-current part of the charge, after which the
current was determined by the voltage limit (as described above). Obviously
being able to suggest that the laptop would recharge in 20-30 minutes was an
attractive catalogue claim, so [it is alleged that] some computer
manufacturers set their charging systems to charge at 3-4C and played with
the voltage thresholds used to further speed the charging. The results were
a little spectacular, and so they appear to have stopped doing it...
HTH,
PDR
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