But what about your slightly old CPU that doesn't support those advanced power options? how you can you manually slowdown your CPU which resulting in a longer battery life??. The operating system can do very little if your CPU does not support those advanced built in features that can be accessed via the ACPI (advanced configuration power interface) standard.
For instance, I have an old Laptop which was made around 2005-2006. The Intel CPU does not have the ability to "switch" between different speed levels, so, no matter how low your battery level is (not quite true in my case.. explained below) ... the CPU will always run in its full throttle, thus "wasting" some of the battery energy.
But there is another way that we can manually slowdown the CPU by cutting out the FSB. Although I'm not quite sure, but most newer Intel CPU come with this disabled by default. Anyway if you purchase your Laptop via a major distributors such as HP, Dell, etc... then most of the time the CPU FSB could've been unlocked by default.
Although I'm not sure what's the case with never processors, but the ones that does not support most of those automatic CPU speed switching (Intel calls it Speed step... can't remember the AMD's version)... manufactures use the above mentioned FSB (front side bus = the speed your processor interact with your RAM) unlocked by default and without the operating system even knowing it... the BIOS automatically slow down the CPU according to your battery status by reducing the FSB speeds.
But for those of you who wants better control over your CPU ... since there's nothing much we can do about that BIOS method... then Rightmark CPU Clock Utility will come in handy. But remember, they haven't updated the utility since 2008... so if you have a never CPU that came after that time, then please don't use it.
Once setup correctly, Rightmark CPU utility lets you switch between different CPU speeds by simply clicking on your system tray + you can use it to adjust few of other things such as screen brightness, etc as well.
Although not just adjusting your CPU speed via the FSB... but Rightmark CPU utility have a lot of built in advanced features that can be used to "tweak" your CPU using many advanced power features, if your processor supports them.
They have an excellent manual.. so please read it first. Remember, this is not a free utility, but they do let you use it for some trial period (with few limitations) but for those of us who still use Windows XP from time to time, and looking for a dedicated app to tweak the CPU for achieving longer battery life... then this will come in handy.
You can download it from here. But please use it at your own risk.