Same problem on 10 month old phone. It goes from 60% to nothing. If I plug in the charger it goes back to 60%. Very strange. I have reset the battery and used battery saving apps to try to sort this out. Battery is sealed so stuck at the moment. Might just need a new phone!
If you are not keen on Win 10 then restore it to 8.1 using recovery tool. I have fed up with this behaviour ( switching off over night and sudden switch off when we do some tasks or even get a call though it shows 60 % , 80 % battery).
I have reverted to 8.1 on yesterday again and using it , no such issues found so far but sadly I am not able sync my exchange account, it keeps throwing an error " we werent able to apply the security policy that's required by mail.xxxx.com . Conact your company's support person to fix the problem"
But it was working when i got this hand set Win 8.1 and also works if I upgrade to Win 10 ( No change to the policy). not sure how to fix this issue since I need this mobile for my mails