periodic HD activity when running on battery

[tilman victor]

When running a laptop on battery power the HD is accessed every 5 seconds.
I thought the reason was the default commit interval of ext3, but mount says
with mains on (NO periodic access):
"... on / type ext3 (rw,errors=remount-ro,commit=0)"
with mains off (access every 5 sec):
"... on / type ext3 (rw,errors=remount-ro,commit=600)"

Funny enough, after writing something different to /etc/fstab,
the above "commit"-values get APPENDED to those settings, and
nothing else changes.

Any ideas?