I've had my iPhone 6 since launch day 3yrs ago. it's been great but ever since iOS 10.3.1/2 I feel like it killed my battery. coconut battery on my mac showed about 75-80% life left prior to the update. shortly after that it showed about 50% and would randomly shutoff around 30-40% left. About 2 months ago I needed to get a battery case so I got a Mophie case to at least let me get through the day without having to be connected all the time. within the past couple of weeks (maybe after iOS 11?) the battery capacity was around 10% from coconut battery and even my Mophie battery case wouldn't cut it all day. started dying even when it showed 80% left. I knew it was really time to get a new battery and was trying to wait it out until the iPhone X since that's the next iPhone i'm getting. I couldn’t wait any longer. It needed to be done NOW!
I contemplated purchasing the iFixit battery a couple of months ago as opposed to taking it to apple and let them install a new battery. I ended up watching multiple videos and seemed like an easy task. I’ve changed RAM and swapped HD to SSD’s on both mine and my wifes MBP in the past so the task didn’t seem to be that difficult. I ordered the kit and swapped it with ease. It was really easy. looking at previous videos, i warmed up the battery by using my wifes hair dryer. seemed to help with the tape coming off easier than i thought. only took about 5-10 minutes really.
any novice can complete this task and i highly recommend checking if your battery needs to be replaced. it's quite easy.