[TriLUG] Thinkpad Battery Charging Thresholds

[Joseph Mack NA3T]

On Tue, 15 Feb 2011, Joel James Adamson wrote:

> After a while of just unplugging when I got near 80 %, the battery
> remarkably stopped charging by itself.  I mean that I could plug it in
> and if it were below 30 %, it would charge up to near 80 % (79.7 % IIRC)
> and stop charging.  Also, if it was above 30 %, I could plug it in and
> it wouldn't charge at all, but then if it dipped below 30 %, it would
> charge as described above.  Problem solved, right?
.
.

> I know that .. get to it through Thinkvantage in Windows.
.
.
> [2]  http://www.thinkwiki.org/wiki/Tp_smapi

This sent me off on some exploration. To confirm Joel's 
write up...

Fig 2 of

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

shows that if the cell voltage is 4.2V or less, the battery 
life is greatly extended, presumably past when you'll want 
to chuck the laptop.

Table 2 of

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

shows that 4.2V corresponds to 85% charge. Using 
Thinkvantage under Windows (dual boot), I set the battery to 
not start charging till the battery is below 50% and to stop 
charging at 80%. Here's my laptop going through two 
charge/discharge cycles.

http://www.austintek.com/battery/battery-day.x60.gif

At 19:30 I unplug the charger. At 20:10, after the battery 
drops below 50%, I plug in the charger and the battery 
charges to 80% and stops charging.

At 00:00 through 02:15 I unplug and replug the charger 
several times. Untill the battery drops below 50%, plugging 
in the charger only takes over powering the laptop, but 
doesn't charge the battery (battery charge doesn't change).

At 02:15 the battery has dropped to 45% and plugging in the 
charger recharges the battery to 80% again.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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Homepage http://www.austintek.com/ It's GNU/Linux!