Hi there, I've got a question about preserving the battery life of a
laptop battery. I don't mean getting the most out of a charge, but in
looking after its long-term capacity.

I use my laptop (IBM T40) 99% of the time plugged into the mains - and
I've heard that it can be really damaging to the battery to leave it
charged up at 100% all of the time. So I am currently storing the
battery outside of the laptop - what is the optimum charge I should
store it outside of the laptop at? The help file suggests less than
50%, and I've heard from other people that batteries should not be
stored with no charge - arghh! Can anyone help?! It's a Li-ion
battery, if that makes any difference.

Thanks,
Duncan.

Please don't reply to this email address personally - it's full of
junk

Li-ion batteries degrade regardless what you do, if your charger disconnects
at 100% charge , then it is safe to leav it in the chargger.

--
Dennis Savatski
"Duncan James Murray" <duncanjamesmurray@hotmail.com> wrote in message
news:992f1a28.0310081401.6446c065@posting.google.c om...

OK, here's a bit of technical info for you, sorry if it's a bit in depth but
I'm sure you will understand, all Li-Ion battery's are charged using
constant then constant voltage, this means (and we are talking about a
single cell here but exactly the same applies to a battery with multiple
cells in it, the voltage applied from the charger is just diffeent to suit)
if you have a cell rated at 4000mAh with a terminal voltage of 4.2 volts (at
max charge) then when charging the charging current is limited to 0.7c which
in this case would be 0.7 x 4000 = 2800mAh, the battery is charged at this
until it's terminal voltage reaches 4.2, at this point the charger changes
from constant current to constant voltage, this means the voltage never goes
any higher thna 4.2 and so the charging current starts to drop off, when the
charging current falls to 0.07c which in this case would be 280mAh the
battery is considered fully charged and charging ceases completely, unlike
NiCd and NiMh battery's that do not mind being trickle charged at low
currents for long periods this would be very damaging for Li-Ion cells so
the cell is not topped up again until the cell voltage drops to 4.0 volts,
so even if you have a flashing charge light (like on Dells) which means your
battery is being trickle charged, it's not, and as Li_Ion cells have a very
low self discharge rate I would think your battery only gets topped up once
a week at the most. I hope this explanation hasn't put you to sleep and that
you have gained some useful info from it.

Dave


"Duncan James Murray" <duncanjamesmurray@hotmail.com> wrote in message
news:992f1a28.0310081401.6446c065@posting.google.c om...

Thanks so much for your reply, Dave.

That does explain a lot - and I certainly didn't fall asleep! I take
it to mean that essentially, the Liion battery has a low discharge
rate, and charging occurs between 4.2V and 4.0V (for example) with a
periodicity of maybe 2 weeks. However, you didn't mention if this is
the optimum state to keep the battery in. I just looked up another
posting which told me that under no circumstances should I leave the
battery with no charge in it, yet the 'Access IBM' help file says I
shouldn't leave it fully charged, out of the laptop/charger!

Assuming I want to keep my battery in the best possible shape for the
next couple of years for when I will rarely need it, do I:

1.Store it outside of the laptop at 0% charge
2.Store it outside of the laptop at 50% charge
3.Store it outside of the laptop at 100% charge
4.Leave it in the laptop at 100% all the time
5.Leave it in the laptop, but periodically drain to 0% (even if mains
is available) and then charge to 100%

I think you are suggesting number 4. would be fine, but could you
clarify?

Thanks ever so much,

Duncan Murray.
p.s. please don't reply personally to this email.




"Dave" <dave@diw.d2g.com> wrote in message news:<bm38vv$i7o1v$1@ID-185896.news.uni-berlin.de>...

Hi Duncan,

I'm glad my explanation helped, although I can't say that leaving the laptop
battery in your machine 100% of the time is the best option, I leave mine in
this state for weeks and it only gets used on battery power rarely, my
battery is 2 years old and still seems to be fine. As for storage, all
battery's have a self discharge curve and on some it is steeper than others,
in simplistic terms Li-Ion battery management uses a very low value resistor
in series with the positive terminal (plus some other bits as well to
control charging etc) and by monitoring the voltage drop across this
resistor the microprocessor in the pack can tell the direction of current
travel (charge and discharge) and also how much, so if you charge it to 100%
and leave it out of the charger for a long period the pack may actually only
have 50% charge left in it (because of self discharge) but because the micro
inside hasn't seen any current flow out of the battery it still thinks its
at 100% so as soon as you start to use it the battery electronics can (not
always) get confused and so here starts the slippery slope to premature
failure, storing the battery between 50% and 70% means the micro will always
know it needs to charge the battery up when reconnected to a charger and so
should follow the charging formula I described before. I can say for certain
that I have seen packs fail this way as I have quite a number of Dell
Latitude battery's where you get a single green light flash on the battery
meter, I have taken them apart and although I could not convince the
electronics that all was well I have charged the cells up my self with a
charger I designed and built and continue to use them in a flash light, it's
one of those that should take 2 of the big 6 volt drycells that have spring
terminals on top, instead its full of Li-Ion cells and last about 30 hrs,
and its still lighter in weight than with the original batts in it even
though I put the charger in there as well. I'm off again, you ask a simple
question and a I write a book in reply, sorry, hope this clarifies it all
for you.

Dave.


"Duncan James Murray" <duncanjamesmurray@hotmail.com> wrote in message
news:992f1a28.0310101155.2b9e21e0@posting.google.c om...