How to enable low power mode on AC?

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http://askubuntu.com – I am running Ubuntu 13.04 on a Dell D420 laptop. If I understand correctly, the system should automatically go into low power mode when on battery. I want to enable this mode while on AC. I would prefer not to install any other power management packages but use the scripts and tools that are already installed Ubuntu 13.04. How does one configure the system to always think that it is running on battery? (HowTos)