SMOK and nearly all vape mod manufacturers design their mods so that the external batteries can be charged in-situ using a USB charging lead. I. for one would be, and have been perfectly happy to trust the designers of modern vape devices to make this way of charging, as safe as possible. I have a lovely 4 slot 18650 charger and use it a lot. I also regularly charge my Mods via USB and have had no problems in three years and dozens of mods.
Many vapers still don't trust mod manufacturers to produce a safe internal charger in their mods. I do, so you can choose to trust them or not.