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. -- Darac Marjal
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