Power Management

Matthew Garrett mjg at redhat.com
Tue Feb 17 02:32:39 UTC 2009

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.

Matthew Garrett | mjg59 at srcf.ucam.org

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