Just wondering, but does anybody else out there have an iOS device running iOS 8.4.1? Does your batter seem to drain at a constant rate to the point where it sometimes drops suddenly? I seem to be having this problem on my iPod Touch and never had it until very recently. I also have a friend who's on 8.4.1 and their iPhone 5c seems to have a larger problem than me with his phone's battery, and again it on;y started to occur with iOS 8.4.1.
Just thought I'd ask to make sure I'm not going crazy here. If anybody knows a fix (aside from having my device plugged in whenever possible), please let me know! Tahnks!
Can you kindly show us your battery usage? That's usually a good place to look at first to get an idea. It could very well be a rogue app.
I have devices running 8.4, 8.4.1, and 9 and none of them have any problems concerning battery. The one time I did have battery problems the culprit turned out be my signal – the signal is very poor around my area so the phone uses more power to establish a handshake with the nearest tower. And it has to establish this with a tower very frequently, so every desperate search for a signal costed me significant battery life.
Could you plug it through USB into your computer or mac and look at the amount of cycles and battery health (if you don't know how to do that I can give you a linky and a explanation) to see if it's 'cause of wear and tear.
My iPhone 4S's battery life only drains quickly when I'm using mobile data or wifi constantly. You should probably look into the apps that are running in the background and consider using less of those to preserve battery life.
It's either that, or it's probably a software issue in which I'm sure iOS9 should fix it by then.