About two weeks ago my Mom's iPhone started having issues charging. It wouldn't accept a charge most of the time, and when it did it was very little. Suddenly the iPhone started boot looping - it would boot up to the apple screen, wait about 3 seconds, and then reboot. Unplugged it wouldn't do anything - it wouldn't even show the "low charge" indicator - just a black screen, and as soon as it was plugged in it would start boot looping.
I realized the most likely cause was the battery due in part to it having issues taking a charge in the weeks prior, and the missing "low charge indicator". The fact that the iPhone was a brick unplugged and suddenly came to life being plugged in was a huge red flag to me.
I set about obtaining an iFixIt battery. A few days later the kit was in the mail.
The repair went very smoothly for me - I would say it only took about 20 minutes from start to finish. Thanks to the full set of tools inside the kit, I was able to complete the repair without a single additional tool - besides my iFixIt Anti-Static Project Tray :).
I don't have an iSclack, so opening the phone can be a little tricky. Just go slow and use consistent force when pulling up on the display to get inside the phone.
Personally, I really hate dealing with all the display cables, so I just left them in - I know it's not the right way and you risk damage to the phone, but in my case the adhesive on the battery had completely worn out, so I was able to remove and install the battery with practically zero force to the phone.
I would recommend checking to see if the battery is held down at all. If the adhesive on the battery has completely failed, I wouldn't bother with the display cables. Just hold the two parts of the phone very steady, keep as little strain on the display cables as possible, and swap batteries.
It's been several days now and my Mom's phone is working perfect again :).