It's weird that rechargeable bike lights almost universally use soldered batteries. Conventional flashlights with onboard charging often use standardized cylindrical Li-ion cells that are field-replaceable by unscrewing the tailcap. They're usually 14500, 18350, 18650, or 21700 size, sometimes with a protection circuit stuck on the end (making it semi-standard, but easier to handle safely).

The light in the link uses an absolutely tiny 0.5Wh battery, which strikes me as insufficient for cycling. I use a light with a 21700 cell (20Wh).