Recently, I’ve found an issue on my iPad Air (1). It drained battery so quickly when it slept.
I tried to reset (home + power), reset all settings, and disable almost every feature as suggested by those results from Google. — No, it did not work.
The last method is to delete everything and start as fresh. So, first, I tried to backup – and uh oh – it stopped backup for 3 days because my iCloud storage was full – AND THIS WAS THE PROBLEM.
It seemed like iCloud (perhaps since 8.4.1 ?) kept retrying to backup all the time and that was draining battery.
The issue could simply be resolved by making room enough for backup – either remove something or buy more storage. And that was it. No more battery drain.