On Fri, Feb 17, 2012 at 01:10:31PM +0100, Lorenzo Sutton wrote:
I am running XFCE 4.8 on debian wheezy on my laptop and since about
two weeks the xfce Power Manager gets the battery charge percentage
wrong, the most critical problem being that the machine shuts off
without any previous warning.
I wonder where the problem might be.
Indeed this battery is getting old (3 years now) and less
efficient, but I can't explain why suddenly Xfce Power Manager is
getting it so wrong given that it was working like a charm (even
giving a pretty accurate esteem of remaining time). I imagine Power
Manager is relying on some lower level (software, kernel?)features,
maybe in the kernel?
I believe this is a common failing with batteries. I might be wrong
here, but as they age, the discharge profile of a battery changes such
that the monitoring hardware tends to over-estimate the remaining
capacity. This typically manifests as normal discharging down to, say,
10 or 15%, followed by a sudden step to 0%.
Now, most power profiles are set up to warn of low battery at, say 10%
and treat 5% as critical. If the battery capacity suddenly drops past
the warning level into the critical level, the system has no choice but
to take emergency measures.
As for where this information comes from: XFCE Power Manager will query
the ACPI daemon which is running in the background. That will talk to
the kernel's ACPI subsystem will, in turn, will talk to the ACPI
implementation in the BIOS. That, ultimately, is what decides what the
battery level is. Only the BIOS really knows what the battery charge
currently is, what a 'full charge' is and what 'zero charge' is. It MAY
be possible to re-teach the BIOS about the current charge profile of the
battery, but it's generally just easier to increase the warning level in
Power Manager.