How to know the duration of battery usage in one session? I would like to know how long was my battery running since it's last charge. There is a built-in time-remaining counter but I would like a time-used counter.
Starting with Mavericks (OS X 10.9), this is available in Activity Monitor. Open Activity Monitor from the /Applications/Utilities folder, and go to the Energy tab. The center pane of the statistics at the bottom of the window includes the amount of time since you last unplugged the charger, which appears shortly after you switch to battery power.