How do I prevent Calendar from draining my iPhone battery? Recently, Calendar on my iPhone has started to cause my battery to drain rapidly by using 20%-40% of the phone's battery with "background activity". Calendar did not to this in the past, but the problem is now pretty consistent. 

How do I prevent Calendar from draining my iPhone battery?



(Note that disabling sync, or using low power mode are not answers.)
One of your Calendar\Mail\Contacts accounts is in an endless loop. Maybe a corrupted event or something of that sort.

What iOS are you using? I remember something of that sort in iOS 8, no idea if this was fixed yet.

First I would suggest removing one account at a time and check after a day of usage if Calendar is still going crazy. If it does, move on to the next account. Once you figured out which account is the problematic one, add it again and configure it as new. This should fix it.