[rlue]

Can you elaborate on or cite a source as to why this practice is incorrect? The Nitecore D4 battery charger supports recharging of this and other sizes of Li-ion batteries (in addition to NiMH), so I am skeptical that it is inherently dangerous.

[mafik]

I guess an exposed battery is more susceptible to short circuit. Imagine you have this in your pocket while in a heavy rain. Battery could short circuit due to water creating conductive connection between battery terminals. It would heat up quickly an maybe even start burning. Still, I prefer 18650 to these flat, lasagna-type cells which swell and can be pierced by any sharp object. Even though the latter has built-in protection and the former doesn't.