Check the manual or the battery door. Most external battery mods take one or two 18650 cells. Larger devices may take 21700, and a small number take 26650. If a mod accepts 21700 it will often take an 18650 with an adapter, but never force a cell that does not seat properly.
mAh is capacity, so it governs how long you vape between charges. The amp figure is the continuous discharge rate, which is how much current the cell can safely deliver. High wattage sub ohm vaping needs a higher amp rating; running a cell beyond its rating is unsafe.
Always in a case or sleeve, never loose. A bare cell touching keys or coins can short and vent. Stop using any cell whose plastic wrap is torn or whose casing is dented, and recycle it at a battery collection point rather than in household waste.











