Micky,
The "power switch" on the PRS-505 does nothing other than to blank the screen and lock the buttons. It sounds, from your description, as if your power switch is faulty, but this should in no way affect the battery life of the Reader. With moderate usage, you should get around two weeks battery life from a battery charge.
You can, of course, do a full shut-down from the menu. This really does turn the device off, but if the power switch on your machine is indeed faulty, there is a danger that you may not be able to turn it back on again if you do that.
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