Here is a great deal on a reusable mod fuse. What is a mod fuse? This goes between your battery and the spring on your mod to create a stop-gap encase of a short. Instead of the battery going critical, damaging or you mod, and or ruining whatever your vaping - this will trigger a cut off. These are reusable which makes this an even greater value.
This is a must for people stacking batteries or using a mechanical mod without short circuit protection. $8.25 is a cheap price to pay for some extra added protection. I own a few and use them everyday.