I recently resolved this problem with my Acer Aspire One netbook. I spent many, many hours googling and reading forums to little avail. I hope this post will help others avoid that. (I also have this problem on my Acer Aspire 6930 but have not been able to apply the fix yet. See below.)
On my netbook I use a distro called Peppermint Linux. It is based on Ubuntu Lucid. The kernel I have is 2.6.32
The main symptom I had was with Gnome Power Manager. When I boot my netbook on battery GPM shows it as on AC and doesn't detect the battery. Also GPM is supposed to have a tab that says On Battery Power
so that you can have different power options then when on AC but it wasn't there.
If I queried the battery from a terminal GPM would register the battery and dim the display:
(But the "On Battery Power" tab was still gone.)
In dmesg there was a entry like this:
ACPI: Battery Slot [BAT0] (battery absent)
This was obviously what was causing GPM to have problems.
(I hope I put enough keywords in here for other people with the problem.)
The fix for me:
Update the BIOS.
The netbook I bought in August had v1.15 of the BIOS. The latest one offered by Acer was v1.25. The BIOS updates available from Acer are .exe files. I happen to be dual booting my netbook so was able to easily update it.
Now in dmesg it says
ACPI: Battery Slot [BAT0] (battery present).
GPM works perfectly. It now has a tab for On AC Power
and On Battery Power
On a side note, my Acer Aspire 6930 does not dual boot. So I am trying to find a way to update the BIOS. There is supposed to be a couple ways to do it. I tried using FreeDOS but it will not boot properly. Its not as much a priority since it generally stays on my desk, plugged in. When I get that updated I will let you know what happens.