21700 Mods sold by Evolution Vapes are battery mods that require external batteries/battery. The '21700' Refers to the size of the battery needed for the device. At Evolution Vapes we also sell the external Batteries themselves aswell as the needed external charger. The 21700 Batteries boast a bigger capacity than their 18650 counterparts, meaning you get longer battery life. At Evoltuion Vapes we stock both Single and Dual mods, allowing you to choose between a bulkier dual mod, or a more sleek single battery mod.