If you have a mod, chances are you need to replace your batteries every few months. These are not your standard flashlight batteries, they are hold more energy and deliver it quicker than most household batteries. Vaping devices that don't have internal batteries usually use one of four sizes of batteries, 18650, 20700, 21700, or 26650. Make sure you get the right size for your device. If you are unsure of the size, look at your old battery for one of these numbers.
Tip: Always use the same two batteries together if your device requires two batteries and make sure the outer wrapping is not torn or removed.