While many of the changes from Windows Phone 8.1 to Windows 10 Mobile have been impressive, unfortunately battery life has not been one of them. However Microsoft has been working to improve battery life especially in the latest builds. In a tweet from Gabe Aul, he notes that you should see better battery life in build 10572, and more on the way:
@jon5798 Yes, some bugs fixed which should result in some improvements. 10575 has some more fixes as well.
— Gabriel Aul (@GabeAul) October 20, 2015
Battery life, or lack thereof, especially related to Cortana, has been an issue, but hopefully we're on the way to better days, and longer battery life, ahead.