I am not using a computer with Windows 7 at this very moment, but you should be able to access your Power Options from the Control Panel.
Choose the "Balanced" power settings and click something like Advanced Settings which will open up another window with a lot of options for you to tweak. Scroll down near the bottom and you should see something for PCI-e and CPU settings. Expand them and you should see options to set the performance level when running plugged in and from the battery. Set the performance level for battery to 100%. That should ensure that laptop will run on battery as if it was plugged in to the AC outlet.
Just understand this will drain your battery a lot faster. If you were able to play a game running on the battery for 1.5 hours, then after making these changes it would probably drop down to 45 minutes - 60 minutes. Maybe less.
It's always a good idea to plug in the laptop to the AC outlet if you want to play games.