External battery mods: These are slightly more technical, and other factors come into play. Firstly, they come in various configurations; Single, dual, triple and quad battery. The more batteries, the more power, or the longer the battery life.
The fact that you can remove the batteries is helpful to most vapers. You can buy an external battery charger and charge up several batteries at one time. This is great if you are worried about running out of battery. If you do, simply pop out the dead batteries and throw in your fully charged ones. The only issue to consider (and a very important issue at that!) is battery safety. You must ALWAYS keep your batteries in a plastic case or specialized battery case. Batteries can be very dangerous if they come in contact with metal, if they do, they can vent (or explode).