[svlug] question - my laptop won't charge to 100% even though plugged in and using AC power

John Conover conover at rahul.net
Mon May 4 21:47:31 PDT 2009


On Sun, May 03, 2009 at 10:52:03PM -0700, Chris Miller wrote:
> On Sun, May 3, 2009 at 9:56 PM, Brian J. Tarricone <bjt23 at cornell.edu> wrote:
> > On Sun, 03 May 2009 18:47:08 -0700 Alan Denney wrote:
> >> "Need"?  Maybe not, but it's a darn good thing to have in place  if
> >> the power goes out in the middle of an unsaved session, even if just
> >> becomes some idjit kicked the plug out or stepped on the power switch
> >> for the power strip.
> >
> > True, but leaving Li ion batteries plugged in all the time is one of
> > the main things that reduces their life (it's basically the increased
> > temperature that does it).
> >
> >> It's probably at least partially salvageable by repeated drawing
> >> downs and full recharges.
> >
> > Nope, not in the least.  That may have been true of NiCad and possibly
> > NiMH batteries, but you'll get the exact opposite out of Li ion
> > batteries.  Repeated full discharges + full charges are also what
> > reduces their life.  Li ion batteries don't have the "memory" that
> > other types do that can sometimes be fixed by "refreshing" them in this
> > manner.  Doing full discharge/charge cycles will actually make the
> > battery worse.
> 
> I'd love to believe you, but personal experience with Li Ion batteries
> would suggest otherwise... Unless I'm getting stiffed and they're
> giving me NiCad while telling me it's Li Ion.

Kind'a getting away from the Linux stuff, but:

If the battery consists of cells in series, then the battery should
never be fully discharged, ever, (usually, in laptops and cellphones,
the battery monitor circuit will not permit this to happen; it shuts
down the device when the battery voltage is down by one cell voltage,
except for 911 stuff.) However, it is very difficult to design a
circuit to measure the charge capacity of Li-ion batteries because of
the V-I discharge characteristics and their dependence on battery
temperature, (as opposed to ambient temperature-which most use.)

The cells in a multicell battery have slightly different charge
capacities, (manufacturing variance,) and one of the series batteries
will completely discharge before the others-at that point, it will
forward bias, (and become a plating process, plating the metal ions
out of the electrolyte,) and will be irreparably damaged.

NiCad batteries have a memory issue, (i.e., not charging past a
voltage that the battery was setting at for a long time without
current being drawn from the battery,) and so does NiMH, (but far less
so than NiCad,) so totally discharging NiMH will be of little help.

The NiCad industrial units, (emergency building lighting, etc.,) that
advise(d) total discharge have a switch that discharges each cell,
individually to avoid forward biasing of the weakest cell.

    John

BTW, a standard automobile battery can be totally discharged three
times, at which time a cell-the weakest-is lost, and at 10 Volts, the
car will not start below 32F, (its about 7 or 8 Volts cranking.)
That's why when the battery is totally discharged 3 times in the
summer, the first cold day, it won't start the car. Three total
discharge cycles is considered the life of a standard battery, (about
once every 1-2 years.)

-- 

John Conover, conover at rahul.net, http://www.johncon.com/




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