My iPhone 6 was going down the tube; the battery was shot, and the cellular broadband controller was slowly degrading (a known issue, the solder joints fail). Eventually, the iPhone fell out of my pocket and broke the glass, a personal first for me, even after dropping it repeatedly on it's face to get the cellular to work, as well as accidentally dropping it 10ft onto a concrete floor and it coming out flawless. The broken glass wasn't an issue, but what broke the proverbial camel's back was when the digitizer decided it wanted to retire, causing the left half of the screen to go unresponsive--great. So, I powered up my old iPhone 5S that was laying dormant in a drawer, freshened it up with iOS 11 on a fresh restore, and after two days, the battery was still only holding 19% of it's original designed capacity, in other words, consumed (keep in mind though, it did have 1800 cycles).
After hopping on iFixit, ordering a battery, and impatiently waiting for it to arrive at my doorstep, I dove right in upon receiving the battery, adhesive strips, included tools (yay for more tools!) and iPhone liberation kit (I hate those pesky proprietary screws!). I've been inside too many electronics to count, but these miniature iPhones are a pretty compact design. Thankfully, iFixit had a perfectly detailed FREE online repair manual, and I was confident in myself, so the repair took all of 10 minutes from start to finish! I'm now enjoying the benefits of an iPhone 5S with a fresh batter on iOS 11, so it's most certainly lasting longer with the software improvements than when it first came out.
A couple things: (1) While it's tempting to remove the display for ease of replacement, if you're gentle and calm, just rubber band the display 90 degrees vertical onto the box the iPhone came in! (2) Those adhesive strips; The logic is, pull at a very low angle (25-30 degrees), as it allows the very thin-yet-tacky strips to stretch themselves out and slip between the battery and the rear of the case to become removed. And (3) CALIBRATE the battery! I cannot stress this enough as some people never do it and it ends up hurting the battery in the long run. I ran the battery down to 5%, and then proceeded to plug it in and let the phone charge uninterrupted (turned it off after plugging in) overnight. This ensured a healthy battery, and I'm perfectly pleased!