Battery was not holding a charge
For anyone who has opened a laptop, tablet or other small electronic device, this fix should be a snap. I had concerns after reading the horror stories but it was much easier than most laptops I've worked on. Just follow the instructions, be careful and most of all, be patient!
Skip removing the screen: just open the case (the suction cup worked for me even with a screen protector attached to the screen), remove the battery connector cover and FIRMLY but patiently work the battery out using the plastic tab attached to the phone (it WILL take some elbow-grease but just be FIRM and patient and you will hear the adhesive yield to your efforts!)
Reconnecting the battery is a bit of chore, especially if you haven't worked with small ribbon cables and MOBO connectors: just be gentle and seat it properly and you will hear the "click" when it is completely connected. Also, be sure you push the battery all the way down towards the bottom of the case so you don't get interference from the components at the top of the phone when you reseat the display panel. And, as has been previously stated, be SURE you get the top clips in first when you close the case!
I reset the phone after the replacement (HOME-POWER) and then did the discharge below 10% and recharge to 100% in order to recalibrate the battery. Don't know if it was necessary but I've been using the phone pretty regularly now for 2 days and still have 63% charge!