The battery was at the end of it's useful life. Would go from full-charge to depleted in about 2 hours. Would no longer hold a charge.
The repair was easy. I followed the iFixit Battery Repair Guide. I'd previously replaced batteries in iPhone 5's, following the iFixit guides and had no problem at all.
The only difficulties encountered were removing the batteries themselves, due to the sticky strips securing the batteries. Battery removal requires two things: patience and a good hair blow dryer or a decent heat gun. Focus the hot air on the back of the phone, keep moving the hot air stream back and forth on the phone case, keep applying gentle pressure with a flexible plastic card or guitar pick, and Bob's your uncle. I got in a slight hurry removing the -6 battery and bent it, deforming the case and causing a small short circuit. The battery emitted a small wisp of smoke, I immediately stopped what I was doing, gently flattened out the battery (with a folded hand towel over it in case it detonated), took a deep calming breath and very gently and slowly continued. I got away with it this time. Patience is critical!
Yes I do. TAKE YOUR TIME! The guides are spot on. Follow them; they are easy enough. Also, TAKE YOUR TIME!