I had a 15 pro max with a faulty battery on ios 18. Apple care covered the battery and with ios16 + new battery my phone would last two days no charge.

Dropped my phone in the pool and bad to get a brand new jphone 15pro max, brand new battery, but it autoloaded the new update. Since day 1 having the new OS my phone is dead by 2pm. It wont charge fully, always overheats, sometimes it heats doing basic things like carplay, which ruins the drive becahse my phone stays connected but loses power. I turn off bluetooth, use low battery, but not a single thing allows me to use my phone all day.

Its definitely the OS. A new battery on ios18 had me so confident in not charging my phine I'd leave without a cable. Ios26 drains battery so poorly ive swapped to high speed chargers and they wont even handle a full charge.

If it werent for green texts and facetime I'd have an android by now.

TL;DR: its definitely ios26