I'm using an amazing android app called AutomateIt.
It's kind of a scheduler on steroids: it can execute actions based on triggers like time, position, connection status.
The latest release added support for "activity recognition": the phone should be able to tell if you're walking, driving, or standing still.
I defined a rule that turns on Bluetooth while I'm driving, so I can use my car hands free system.
It actually works, but at what price?
Here are the effect on a Samsung Galaxy S2 Plus.
This a night with an activity recognition rule defined: check the last hours in the graph: battery depletes at a rate of 1% every 4 hours.
Here is a night without any activity recognition: a 1% of battery juice lasts for 5 hours.
It seems that using activity recognition shorten you battery running time by 20%.