Hard to say ... I don't know which make and model you're
using, and how well it is supported in Linux. I don't know
whether you would be better off with acpi.
My TP R32 lasts for around 2 hours in Linux, and I can't
bear windows for that long ;) ...
One thing that may have an effect is that the machine
runs on 1.2 rather than 1.8 when on battery. I use ACPI,
btw, with apm I had weird things like repeating keys,
clock going wrong (fast, slow, randomly) ...