Comment by perennialreport

The button-top vs flat-top distinction trips a lot of people up here: many bike lights ship with protected button-top 18650s, and a bare flat-top cell is a few mm shorter, so it can rattle or lose contact over bumps even though it's nominally the same battery. The other gotcha is cells with welded solder tabs vs spring contacts - if the original pack is tab-welded, a drop-in replacement sometimes needs a shim to make reliable contact. Worth checking both before ordering the replacement cell.

The wider point stands though: a bike light is a battery, an LED, and a switch, and the battery dying first is the most repairable failure mode there is.