iPhone 6 Battery Decreasing After Battery Replacement

Hello!

So about a month ago I had my iPhone battery replaced by a friend of mine. There a few hiccups during it, with the main portion being that the battery connector kept coming loose so my phone would lose battery in like 15 minutes. Those issues were since resolved.

My iPhone battery was working flawlessly, holding charge just like the time I first got my phone. However sometimes, the battery connector would come loose again, so I would have to open up the iPhone, unscrew the metal plate and push the battery connector back in.

Now, I've recently ran into an issue where my battery is draining within 3-4 hours, and sometimes I can watch the battery percentage decrease. I've reset the phone and restored from backup but the issue persists. I have also done several battery calibrations. One thing I should mention, is when I check out the battery page in settings, I cannot see battery usage, it is stuck at "Battery information will be available after using iPhone for a few minutes". The phone has also started to randomly die around 20-30% and will sometimes get stuck in a boot loop until it properly dies.

I am iOS 10.3.3. What's wrong with my phone? Should I upgrade to iOS 11? Is my software acting faulty? Or is it a bigger issue with my hardware? Coconut battery shows the battery as having 100% of its charge.