To help with the on going battle to monitor your Windows Phone 8 battery performance, enter the Windows Phone 8 app myBattery.
myBattery will chart your batteries life cycle, note the percentage of power left and an estimated time left on the current charge. Live tile support includes the multiple sized tiles with the medium and small tile showing just the percentage left and the large tile showing both the percentage and time remaining.
Additional features include customizable colors for the battery meter and chart, charts cover not only daily use but overall use history and notifications in the lockscreen.
There are some limitations due to the Windows Phone 8 APIs. Live tiles and lock screen are updated approximately every 30 minutes and can only show the battery percentage up to 99%. The icons on the lockscreen and live tile won't change.
For now, you can't export the data but the developer is working on an update to backup your battery charts to SkyDrive. Which will be a nice and handy feature to have.
There is a trial version of myBattery that is limited to eight uses with the full version running $.99. You can find myBattery here at the Windows Phone Store.