Calendar app of iPhone 15 Plus on iOS 18 draining battery

The hang/CPU utilization was so bad for me I’ve been unable to remove the Exchange account. Charging also stopped due to overheating. Calendar even hung when simply tapping OK to acknowledge notification of an expected appointment.

Update: this is the start of the 3rd day since trying the unpairing/re-pairing of my watch and the issue has not returned. I am still using my Exchange/Office 365 account calendar, and no more battery drain.

Just installed iOS 18.0.1 but the issue remains.

Same here. Battery drain 5 hours and 43 mins of the calendar running in the background. Have also upgraded to 16 pro max. Calendar also gets hung up when I try to add or change entries. It’s maddening.

I have been having the same issue, and I wanted to come on here to offer what (hopefully) worked for me. After looking around online and finding people who had similar issues in the past with other updates, I found a few posts to do with the calendar backup activity that were solved by unpairing your watch and pairing it again (I upgraded from a version 6 watch to a version 10 which is why I had to upgrade to ios 18). I know that sounds unrelated, but I have been willing to try almost anything since my battery was down to 30% by 9am the past 2 mornings with very limited usage. Almost all was calendar background activity. This morning, I unpaired my watch and paired it again. Right now it is 11:25 am and my battery is at 88%--I charge to 95%. If it doesn't hold out, I will respond to update. I hope this helps someone!

Turning off "push" for all my calendar accounts (settings > apps > calendar > calendar accounts) seems to have solved this problem for now. Both iPhone 15 Pro Max and Apple Watch Ultra 1 have been on for an hour and my phone is cool to the touch.

I also read in another one of these threads that someone on Apple phone support claimed that this issue is being worked on. Here's hoping that's true.

With my iPhone 15 on ios 18.0.1, I have the same problem with the calendar app draining my battery so fast. There is no option to stop the calendar app to be used in the background in settings!

Same here. Calendar app is using 98% of my battery daily, but its not actually getting updated with current info from exchange. Appointments and alerts are not appearing on phone as a result and getting missed.

Same here, iPhone 15 Pro Max, wake up to find the phone burning hot with the screen telling me that charging will resume when the phone has cooled down. Taking it off the charger, 30 minutes later without using it I see that the Calendar app has been responsible for 95% of all power use. I really hope this issue is on Apple’s radar.

FWIW, I tried a forced reboot of the phone (vol up, vol down, hold power until apple logo appears) and the issue came back just the same. Then I turned off my Apple Watch Ultra 1 and did the same phone reboot, and the issue is absent. I do have a work exchange account on my phone. It really does look like there’s some interaction with the watch and the phone causing this calendar app battery drain issue.

I should note that for me, restarting the phone will break the endless sync and get the phone to actually complete a sync and have current data, and stop the battery drain. However, if i try to send an email or edit my calendar it immediately stops working again. Also, it will eventually stop working on its own even if i dont open the apps. I now restart my phome more times in a single day than i used to do in a full year. Should also add im on a 16pro and on 18.0.1, but this was happening on my 14pro that i updated to ios 18 while waiting for my 16pro to arrive in the mail.

I too ran into this same issue when trying to upgrade my iPhone 13 to 16 Pro (iPhone 13 was/is running iOS 17.7.1 without issue). I ultimately had to revert back to my iPhone 13 due to the same exact "calendar/battery drain/iPhone over heating" issues.

I think I'm reading this post too late (already reverted back to my iPhone 13) as it never occurred to me that my iWatch could be causing this problem, but looking back it makes sense.

When I did the iPhone13-to-iPhone16 "data/settings" transfer I forgot to un-pair my iWatch (from my iPhone 13) prior (the thought never crossed my mind), and I do remember needing to re-pair the iWatch to my new iPhone 16 Pro.

I'm now wondering if this is somehow the root cause (not un-pairing my iWatch from my old iPhone 13 before performing the "data/settings" transfer to my new iPhone 16 Pro).

Makes me want to try it again. I'll keep you posted.

@edzfive - did this resolve your issue?

This was resolved for me on iOS 18.1. Very unpleasant bug, and a bit disappointing that 18.0 was released despite early reports about this.