A few days ago, my FitBit Charge 5 started continually prompting me with the Add Card notification many times a day. Then the screen went dark, and I couldn't get it to register, and the battery drained very quickly - from 50% to 0% in just a couple of hours. I tried a reset, no improvement. I charged the battery fully, and the next day it seemed fine. Then the same thing happened again. Yesterday I charged it at midday, and an hour later the battery was dead. Then I charged it fully overnight, and within a few minutes this AM it was dead again. I have it charging right now, and I can hear the "Add Card" notifications occurring several times a minute while it's plugged in.
