It should come set to do it by default, but:
For the 5660:
During boot up after your computer posts and shows CPU/RAM specs keep pushing "F2" when it says at the bottom of the screen to hit "F2" to enter setup. This will take you into the bios screen. Use the arrow keys to move left and right and highlight the tab "Advanced" then arrow down so that "Intel Developed Advanced Menu" is highlighted and press "Enter" to go into the sub menu for it. Now the second line should say "Throttle Duty rate" Arrow down to highlight it and use the "+" and "-" keys to change it's value. 50% means it will underclock by 50% and 0% will keep it from underclocking when on the batteries and of course there are inbetween settings. Once you've done that, push "F10" to save and exit.
For the 8886:
During boot up after your computer posts and shows CPU/RAM specs, keep pushing "Ctrl", "Alt", and "S" when it says to push them at the bottom to enter setup. This will take you into the bios screen. Use the arrow keys to highlight the menu on the top "Power"... you may have to push "Alt" to highlight the top menu so you can arrow over. Now arrow down and highlight "Low Power Saving" and hit enter. This will keep your CPU running 100% when on the batteries from my tests. Once you see the check mark next to the one you selected, arrow over to the "Exit" tab and arrow down to "Save and Exit" and hit "Enter".
Also, you can right click the desktop and go to properties/screensaver/power and set it for maximum battery if it's set in the bois to make the change.