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On my Google Nexus 7 I noticed that it would consume around 1% of battery each hour that it was just being idle. I suspected that battery-usage during idle time could be improved. So the next night (7 hours) I put my Nexus into airplane mode, and the next morning the battery had only used 1% for those 7 hours, which proved that there was room for improvement.
Next I disabled bluetooth and set wi-fi, to be disabled when the screen is locked (changed "Keep Wi-Fi on during sleep" from "Always" to "Only when plugged in"). After a fews tests of 7 hours idle the battery was down 2 ~ 3% each time. But then I noticed in the battery graph, that the wi-fi still showed quite a lot of activity. This probably explains the difference in battery consumption with airplane-mode.
I had expected the wi-fi to be completely disabled with this setting, and the wi-fi bar in the battery graph to be almost completely black. To see the difference I manually disabled the wi-fi and then the bar is indeed completely black (the right block in the image): enter image description here

I would expect the wi-fi to be completely off when the screen is locked, but looking at this question: Turning on Wi-Fi on demand that is apparently not how Android works. So now I'm wondering what the wi-fi is doing in this setting? Why does Android keep the wi-fi 'partly' enabled? If Android would still let apps do their syncing etc on a regular basis I would understand this, but this is not the case (Notifications don't come in & it even seems to cut off streaming etc), so it seems to only waste battery?

I first suspected a misbehaving app, but I checked with Wake lock detector and Network log but don't see any suspicious apps. I guess I could use JuiceDefender or a similar app to completely disable the wi-fi when the screen is locked (and still let it sync from time to time), but that's not really the point of this question.

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