Just to add, charging batteries in an external charger allows you to monitor battery temperature by touch. Charging can create heat. Batteries do not like heat and can react quite violently. You can't easily monitor battery temperature by touching a mod's surface when charging by USB, that's very unreliable.
Always charge batteries on a
fire resistant surface; never charge on a bed, couch, or while sleeping.
Most battery explosions occur during charging.
E-Cigarette Explosions: Comprehensive List
There have been 50 major incidents this year (2016) alone and that's just the ones which make the news. They also go on to say...
"It is interesting to note that the nature of e-cigarette explosions has changed over the years. The FEMA document cited above suggests that approximately 80 percent of e-cigarette explosions happen during charging."
THE GOOD, THE BAD, AND THE UGLY
The good:
Smoke detector batteries were charged and smoke detector worked.
Mod was being charged on a flame resistant surface.
Owner was present in same room as the charger so that he could react immediately to the danger.
The bad:
Owner was charging battery while asleep.
Mod manufacturer was SMOK. That's like using a battery with the word "fire" in it (Trustfire, Ultra Fire, Sure Fire).
And the ugly:
Had the smoke alarm not gone off, the owner and his family might have become trapped in their home by the time they discovered their house was on fire.