Word of warning: batteries from AliExpress might not be the size it says on the tin.

Also: not all lights require soldering to change the battery. Some brands, like Fenix, offer lights where you can change the battery on the go.

I had a light from a lesser known Taiwanese brand - an INFINI Lava 500. At some point it stopped working, so I took it apart with my iFixit kit and inside was a 102660 battery (1700mAh instead of the advertised 2600mAh), so I bought a 102560 unit - 10mm thick, 25mm wide and 60mm long (supposedly).

It was a good few millimetres longer and didn't fit. What's worse - AliExpress stopped shipping batteries to my address, because due to safety regulations you can't ship batteries via airmail, it's just that one vendor who chanced it, so now I couldn't even buy a smaller replacement.

Local vendors didn't have anything nearly the same format, so I gave up and bought a LEZYNE LED Macro Drive 1300XL. Inside there are two 18650s wrapped in a pack that has a plug, so should they die, you can replace the battery without soldering.