The USB plug should be a maximum of 5V / 2A. (Not a turbo charger)
In lay-man's terms, that means:
Use a normal small plug, like your iPhone or Samsung USB plug (Not a turbo charger, this can affect the battery life)
Do not use tablet or computer wall charging plugs.  These are intended for much larger devices and may cause overheating and/or destroy the battery.
Charge from too low voltage items, like computers, will not be sufficient