Upload Photos to iCloud Photo Library on iPhone only when plugged in?
Source:

Previous versions of iOS would only sync photos when the device was plugged-in and on wifi.

iOS 11 changed that.  Now photos will also sync when on cellular and when not plugged-in.  This means if you have a lot of photos, the first-time sync could kill your battery.  

There's no fix for this however there is a workaround according to this Apple discussion.  This user had both an iPhone and iPad and used the iPad to do an upload of several images first.  After that they turned iCloud for their iPhone back on.  I can't vouch for this method because I never tried it but it worked for several people on that discussion thread.


  Use iTunes to sync a minimal number of photos which automatically turned off iCloud Photo > Library. After that finished on the iPad I turned on iCloud Photo Library and agreed to 
  delete the photos I'd just synced and then it worked.