Manage Power Profile of Aspire one ao751h, problem with battery alimentation

Hi, I'm Marco, I'm from Italy and I try to take the best from my old netbook.
My problem is a change of performance from AC supply to Battery supply even if I set the Windows Performance in : Always active.
In details, when I put the AC supply , fan start and run till I take out the AC supply. And for example, in You Tube video playing , even if I overclock Atom z520 to 1555.1 Mhz , still lag if run with battery and doesn't lag if I put AC supply, at the same clock. Another one strange thing is that even if brightness is 100%, when I unplug the AC there's a very little bright change to minor brightness... fan stop run (and run only when strictly necessary for a little time) and video start to lag.
Now, I think that maybe the cause it's the battery that in seven years lost its performance, with AIDA64 I see that when I unplug AC the tension's getting a little bit lower, sometimes under 11 volt, but it's a little difference.
Also I think that's the ACPI control of Windows that manage the power AC control and set istruction like this. I try to uninstall the ACPI drivers but Windows re-install all of the drivers automatically.
I also think it's cause I use Windows xp SP3 and If I install windows 7 with a uptaded ACPI control, there's a difference or anyway I have more chance to change power option.
Finally I think I may unlock advanced bios to manage power control directly from it.. but now I not understand how can I unlock the bios.
Only for my curiosity.. There's a trick to make my pc believe that's AC supplying even if it use the battery?
Sorry for my poor english , I hope that you could give me advice . Bye MARCO