Short answer - Open the start menu and type "Disk Defragmenter" in the
search bar. That will show you what the current defragmentation schedule is
set to.
Long(er) answer - Open the start menu and type "task Scheduler" in the
search bar. Navigate to Library> Microsoft> Windows> Defrag.
You will see two entries: ScheduledDefrag Corresponds to what you would set
using the Disk Defrag interface, and shows you exactly what conditions arise
when this task runs.
The other Task, ManualDefrag, should run at some point when your computer is
declared idle, which is some point after 15 minutes of inactivity, which is
why you sometimes see a flurry of disk activity when you're not using the
computer.
None of these tasks will run on battery power unless you specifically set
them so. In fact if they are running and you plug the computer out and
switch to battery power, they will cease.
I wouldn't worry too much about the HD blips and the logs popping. It is
most likely cached files being written to disk and events being fired and
the Master File Table being updated. It shouldn't really affect the idle
state of the computer.
If you are concerned about saving battery life when on battery power, you
can:
1) Set the power profile to "Power Saver" (Left Click the battery icon on
the system tray (by the clock)) and manually set it.
2) You can also lower the LCD brightness
3) Turn off the wireless radio if you aren't using it
4) Turn off all peripherals (or just don't use them) like the DVD, Sound
etc.
5) Run the minimum of applications, and avoid running disk intensive ones
like full screen games.
5) You could go into the power options (again!) navigate to the advanced
settings, and change some of the settings such as turning off the hard drive
after a lesser time. The default on my machine is 10 minutes on the balanced
and 5 minutes on the power saver. You could set it to a lower idle amount
like 2 minutes so after 2 mins idle time on your computer the hard drive
will turn itself off.