you can either carry your battery in a
1. plastic case (usually fits 2 18650s)
2. silicone sleeve (fits a single 18650)
As for carrying keys, coins, etc. IMO if its a regulated device (a box mod with a screen and a chip regulating the voltage, etc) it should be OK assuming the battery door doesn't fail resulting in the battery falling out in your pocket somehow.
But I wouldn't do it b/c you run the risk of scratching up your mod and tank
