There are a few different applications/daemons you should install on your system: acpid (the daemon that will control your hardware states), and a monitoring program. I personally use wmacpi. There are lots to choose from in ACPI packages and related software.
Do not let apmd and acpid run at the same time unless you REALLY know what you are doing. Debian will not make sure only one is running at a time. You will have to check.
APM will try to put your system into S3. On the 2.4.x (and before) series kernels this will probably cause your machine to lock up. S3 is not supported until at least 2.5.x. Even the patch will not provide support for S3 in the 2.4.x series kernels. I completely removed APM support from the kernel, and removed the apmd daemon from my system. On Debian this was as easy as apt-get remove apmd.
You can also look in each of the ACPI files individually for
information about your system. Look in the /proc/acpi directory
for various things of importance. If I want to check my battery I read the
following file like this: cat
/proc/acpi/battery/BAT0/state. The output is as follows:
present: yes capacity state: ok charging state: discharging <-- running off battery present rate: unknown remaining capacity: 3920 mAh <-- watch this number present voltage: 14800 mV
present: yes capacity state: ok charging state: discharging present rate: unknown remaining capacity: 3840 mAh <-- capacity getting smaller present voltage: 14800 mV
present: yes capacity state: ok charging state: charging <-- AC adapter plugged in present rate: unknown remaining capacity: 3840 mAh present voltage: 14800 mV
If I want information about my battery in general I check it out like
this: cat /proc/acpi/battery/BAT0/info
present: yes design capacity: 3920 mAh last full capacity: 3920 mAh battery technology: rechargeable design voltage: 14800 mV design capacity warning: 30 mAh design capacity low: 20 mAh capacity granularity 1: 10 mAh capacity granularity 2: 3470 mAh model number: Bat0 serial number: battery type: Lion OEM info: Acer
For more information on each of these files, please read http://acpi.sourceforge.net/documentation/index.html.