I too was facing extreme battery drain on the first day of buying 2a. Even with latest 2.5.5 OS same battery drain issue alongwith heating issue.
Both these issues got resolved once I manually powered off the device and switched it on after some time.
It seems after the automatic update of OS to latest version phone “Restarts” but to resolve the drain and heating issue one need to manually Power Off and On.
Hope this information would help.