Hi guys, i was also having this boot loop issue whilst having a battery percent that wouldn't rise or fall, after several attempts I found that simply tightening the screws on the battery ribbon protector stopped the boot loops but didn't fix the battery percentage issue.
After taking the iPhone 5 to a trusted repair shop which is the only place the phone has been repaired (screen connector fixed), they ask me when I had my battery replaced? Obviously I have never had the battery replaced :/ I asked why they thought this, they replied with, the battery has a small gold sticker on it, this isn't normally on apple batteries? Can someone confirm this for me, as if my battery was swapped to a knock off battery, it was clearly swapped out by the repair shop and could be the result of the battery percentage problem, the knock off batteries!