I can't stress enough the importance of using plastic battery cases to carry your spare batteries in.
Any contact with metal (keys, coins, another battery, etc, possibly even static electricity) can complete the electrical circuit with a loose battery and cause it to hard short, and go into thermal runaway. Plastic battery cases prevent this type of catastrophe from happening. Use them, they're inexpensive.
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the above accident happened just last week. A novice vapor was carrying two spare batteries in a pocket of his bookbag. One of the batteries (aw 18490 imr) went into thermal runaway after allegedly coming into contact with some metal in the bookbag. What you see are the remains of that battery - the metal casing, and the inner guts of the battery which have mushroomed out of the case. Imagine the force of the venting gas that came from this battery. In hindsight, this person should have used a pastic battery case to protect his batteries in the bookbag from touching any metal.
ack!!!!!!!!! Holy spit!