Many electronical mods have a build in battery but there are also others that the battery comes separately.
Because there are many kinds and brands of Mods there are also many kinds of batteries.
Although there are many kinds, there is one in particular
that is used more than others.
That is the 18650 battery which a size classification of lithium-ion.
Furthermore it can hold a lot of energy. That energy can be from 2000 to 3400mah in modern cells.
Most people don’t know this but the names of the batteries are actually the diamentios.
Therefore the 18650 is actually 18mm by 65mm.
When is in your hand, it probably seems like your holding an AA battery.
Because the shape is almost the same it often gets confusing.