Power Management

Matthew Saltzman mjs at clemson.edu
Tue Feb 17 20:19:08 UTC 2009

On Tue, 2009-02-17 at 02:32 +0000, Matthew Garrett wrote:
> On Mon, Feb 16, 2009 at 07:02:52PM -0500, brad longo wrote:
> > As I'm sure most of you know, leaving your laptop plugged in and 
> > charging with a full battery charge is harmful for the battery.  I have 
> > been trying to see if Fedora's power management tool has something built 
> > in so that when the battery reaches full charge, it will then discharge 
> > to lets say around 95% before beginning to charge again.  Friends of 
> > mine with the same laptop use such measures except they are running 
> > windows.  However, based on the fact I did not see any documentation 
> > about this, and that my battery charge does not appear to fluctuate at 
> > all once it becomes fully charged (according to the statistics), I'm 
> > guessing no such thing exists in Fedora.  Does anyone have any 
> > information as to whether this safety feature exists in Fedora, or 
> > whether some other measures exist instead?  Basically I'm just wondering 
> > if I need to periodically unplug my laptop to preserve the lifespan of 
> > the battery, which would be annoying.  Also if this is not a feature is 
> > anyone working on developing something like this for Fedora?
> Charging of the battery is generally under firmware rather than software 
> control. Laptops will typically stop charging at 100%, at which point 
> the battery will slowly self-discharge. When the battery hits some 
> threshold (typically somewhere between 95% and 97%) the firmware will 
> start charging again.
> What you're talking about is presumably an interface to modify that 
> threshold. This is device specific. The tp_smapi driver (which is not in 
> the kernel for exceedingly dull reasons) allows this to be configured on 
> Thinkpads. I don't believe that we know how to on any other systems.

My dual-boot ThinkPad T61 has a Windows power utility that sets the
level at which charging commences.  A recent update of that utility
reset the level to 85%.  So what happens is that the slow discharge when
suspended or used briefly on battery doesn't result in a recharge until
the level has dropped below 85%.  The utility suggests that this
lengthens battery life as compared to, say a 95% or 97% threshold.  I
guess the point is mainly to reduce the frequency of discharge/charge
                Matthew Saltzman

Clemson University Math Sciences
mjs AT clemson DOT edu

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