You will see many removable battery numbers out there and it can be confusing as to what means what.
Common batteries are 18350, 18650, 14950 etc and attached to those numbers is a "mah" designation. The "mah" number is a capacity number and the higher the number the more capcity your battery has, (the longer it will last between charges) the mah has no bearing on if your battery will fit a device, that is where the battery number comes in.
Let's break down the most common battery for mods. An 18650 2000mah
The 2000mah is the battery capacity.
The 18650 numbers are simply measurements.
18 = Diameter in millimeters
65 = Length in millimeters
0 = Cylindrical
So an 18650 battery is 18 millimeters in diameter 65 millimeters in length and is a cylinder.
Easy Peasy :)
However: Always check the length of your mods for use with batteries, there are built in differences between them, for example an 18650 can actually be 66mm in length as they use the "65" as the base battery sizxe and often do not account from the button top, the circuit protection or other small changes. Be sure to always check with the manufacturer of the mod and measure our battery to make sure it will fit.