General battery safety
Do not store or transport external batteries (18650, 21700 etc) without using a case. Carrying loose external batteries in your pocket or purse poses a major safety hazard, as it is possible that the positive and negative terminals of the batteries may be shorted by keys or other metallic objects. Shorted batteries will catch fire or explode.
Never leave batteries charging unattended. Do not charge when you go to sleep or leave the house. There is an inherent risk of fire with any lithium battery due to the nature of their chemistry.
If a battery is dropped, crushed, or shows any signs of damage, do not use it.
It is not safe to use external batteries (18650, 21700 etc) with any nicks, tears or holes in the battery casing.
Dual / triple battery MODs - Marry your batteries!
In any device that utilises multiple batteries it is critical that the batteries discharge at the same rate, otherwise one battery may be put under a greater load resulting in that battery operating outside of its safe limits. When a battery is overloaded there is a risk of venting, thermal runaway and fire.
The batteries you use in a multi-battery device must be the same make and model, purchased at the same time, and must only be used together (typically referred to as marrying your batteries). Charge all batteries to full capacity before first use.
Though most modern multi-battery devices offer balanced charging via micro USB, we still recommend an external charger. External chargers charge each battery independently which makes them both safer and faster.
When recharging married batteries in an external charger you should charge both batteries to full capacity before reuse.