Underclocking CPU and using only AC with battery attached

Question

Is it possible to underclock the CPU and use only AC but with the battery attached to the MBP? If the magsafe connector is unplugged the MBP can switch instantly to use the battery.
There are many times when an underclocked MBP is enough, like when browsing the web, writting and listening to music. But using only AC with no battery is risking losing information.
And, if it's possible it can even be an option in the menubar (along with turning off-on WiFi and bluetooth). This way, if more power is needed (ie: running Corel's Painter X), it's a matter of just click underclocking-off.

Answer 1

A question remains: Why do you want to do this?
Do you want to maximise the life of your battery?
Do you want to develop sophisticated software that cares about power usage?
Do you care about the planet and want to minimise your ecological footprint?
As far as "an AC adapter that takes the place of the battery and can power the MBP with enough energy" - it is supplied with your computer and is the common or garden variety 85W powerbrick. Furthermore, when the AC is connected "the battery will never be used when not necessary" (only if the power demands are sufficiently high that the power adapter can't supply all the power required) - just as you desire.
Just to make it clear - when the AC is connected the computer draws its power from the AC first, regardless of whether the battery is in place or not.
I think you will achieve what you want by using the MBP normally, as designed, and not doing anything special. I think it is designed to do exactly what you seem to want.