Note of the day...
Always remember to take proper care of your external batteries. Batteries will conduct power with almost any metal including keys, coins etc. If any battery happens to come in contact with any of those materials, it will conduct power very fast and even get extremely hot. This happens because the battery is pushing out power and has nowhere to go which will lead to a venting battery and that my friends is never a good thing.
So be sure to have your batteries in your devices correctly and store them in a safe place (plastic cases are great).