I was kindly sent a Pebble Time Steel as a possible replacement for my own Pebble Time watch - and though it was in great condition and it powered on when charging, as soon as the power was disconnected, the watch powered off.

I hunted down the right replacement lipo battery, the SP551923AE thankfully available on AliExpress. Heated the watch to melt the glue, opened and careful soldered the replacement battery onto the Pebble PCB (because I didn't want to remove the original battery connector and add it the new lipo battery - it seemed like a risky job).

A tiny job, but a very nice result.