21700, 18650, 20700 and others simply refer to the physical size of the lithium ion #battery cell. For 18650, its an 18 mm diameter x 65 mm length. 21700 is 21 mm x 70 mm. While the explanation is 18650 vs 21700: Capacity, Performance & Applications Pkcell Vs Regular Vandy Vape Pulse V3 Mod, Pulse Iii, Squonk, Single 21700, Single 18650, Single Battery Australia Geek Bar Vape Pulse 18650 vs 18500 vs 21700 Batteries: Differences, Comparison & Selection Guide