A small insurance policy for your PV. These go between your battery and your spring. If there is a short the fuse will pop in the vape safe cutting off any voltage from battery. This can potentully save your PV, Battery, and your face.
If you have a mechanical mod or just want some extra protection - pick up a few. The price is a steal.
Use coupon code happyholidays to bring the cost down to $3.60