[ltp] battery full capacity dropped significantly

[Michael Karcher]

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Am Donnerstag, den 21.02.2008, 13:21 +0200 schrieb Andriy Khavryuchenko:
> Tonight I've forgot to plug my T42 (with 9-cell battery) to the power
> and left it open.  Of course, battery discharged completely :(
>=20
> This morning I've found that 'last full capacity' as reported by acpi
> dropped from about ~70W down to ~57W:
>
> Is this an electronic issue and that could be reset somehow or that's
> the cells where drained so much that their maximum capacity suffered?

Your cells did not suffer that much because of this single drain, but it
is also not just an electronic issue. Your cells have probably degraded
that much since the last calibration of the 'last full capacity', but
that might be years ago.

The background of the story is that the battery electronics does not
really know how much charge is left in the battery, but it can do quite
accurate accounting of how much charge went in during charge and how
much has been drained. If the cells feel full, the "remaining capacity"
value is automatically set to the "last full capacity" value to prevent
accumulation of errors, as it is (kind of) known, how much charge the
battery holds when it feels full. With age, heat and charging, your
cells loose capacity. The electronic does not know about it, and still
reports 70W as capacity of the pack, unless it is once discharge until
the pack "feels empty". If the battery "feels empty" and a charging
cycle is startet and it will be measured, how much charge can be put
into it until it "feels full". This is now stored as new "last full
capacity".

What happened to your battery is mostly that the real capacity has been
remeasured after the deep discharge and this value is now reported.
Usually, doing another cycle increases the reported value a bit, but
this is *not* *recommended* for lithium ion battery packs, as every deep
cycle stresses the cells quite much. The cause of this phenomenon is
that a battery that has not been discharged for a long time "feels
empty" with more residual, unaccessible charge left than a battery that
has been used lately, as the internal resistance is lowered by using,
and "feels empty" means that the cell voltage is below a certain
threshold *with* the load of the laptop. If the internal resistance
increases, the voltage at the cells gets lower for the same load and
charge state.

Regards,
  Michael Karcher

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