What can cause "Android OS" process to use high percentage of battery?
I believe, there is at least one bug related to "Android OS" and battery drain. I'll outline how to reproduce the issue: (Tested with Motorola Flipout, Blur_Version.0.28.9_MB511.Retail.en.DE (stock ROM the phone was sold with), Android 2.1.)


Start with a fully charged phone.
Download the Audiobooks (free) app and launch it.
Choose a book from the free library and listen.
Send Audiobooks to the background, e.g. by clicking home button.
Go back to the Audiobooks app by the same operation you used to start it, e.g. click the icon in the app drawer. (This is the only way I am aware of to get the app in the foreground again.)
Do whatever it takes to stop the audiobook playback. (There is a "shutdown player" button, but I never get to use it, because the player just stops/disappears before I can navigate to the player screen.)
Leave the phone idle. After 8 hours the battery will be almost flat (&lt;15%). Android's battery usage monitor attributes 33% of the batter consumption to "Android OS". The remainder is split between "cell standby", "phone idle".
Shut down the phone. Switch it on. Recharge.
Leave phone idle.
After 8 hours, check the battery status (>50%). "Android OS" does not show up as draining battery.


Whether this is a bug in the Audiobooks player or Android or both is not clear. However, this does not appear to be normal behaviour.