The battery was worn-out and no longer holding a charge.
Apple refused to ship the battery directly to me that I was PAYING FOR, to repair a phone that I OWN.
With a jeweler's magnifying headset, and tools provided by iFixit, 30 minutes later... I had a new battery installed.
The screws and parts are extremely microscopic! Don’t strain your eyes, or rely on your regular glasses to properly see what you’re doing. Buy yourself a simple magnifying headset, they’re pretty inexpensive and practical. Also, organize the screws/parts that you remove. I use an ice tray that I label.