I followed this guide, and the end result was that the Fitbit powers up, but the display only shows a half-filled bar. It's not the typical battery life bar, but a slimmer one that I've never seen before, and it's always half full, even if I charge overnight. Then, if I try to connect to it with my phone, it fails to connect, and then the Fitbit shows a little exclamation mark in the middle of it.

I've tried restarting it, but the behavior is the same when it restarts. I've also tried setting it up as a replacement band through my phone's app. The phone says it sees it, tries to connect to it, then fails.

This all tells me that

1) the bluetooth is functional, at least enough that other devices can see it, if not actually connect to it.

2) the new battery is connected, because the screen and the bluetooth are getting power.

So, anybody have any idea what happened, and how I can fix it? Did I short it out when I was soldering? Did I fry the board with an ESD? Did I loosen some other connection?