You asked: "Is it for better battery life, stronger vape?" Yes.
The "
mods" are battery housings containing larger generic
batteries with either higher mAh* and/or greater voltage.
* mAh: milliamp hours, loosely translated: puffs per charge.
There are basically two categories: 3.7V and 5+V:
- The batts in 3.7V mods range from 360 mAh (ridulously low for a mod IMO) to 3000 mAh (two days of vaping). Another advantage of high mAh batteries is that they stay at maximum voltage (~4.2V longer). You may have noticed that a freshly charged battery delivers a stronger vaping experience. A 3000 mAh battery will sustain that for a long time.
- Most 5+V mods contain two 3.0V batteries (protected RCR123A or unprotected CR2) totalling 6.0V. A 5V mod (e.g., Prodigy) includes a resister, which damps that back to 5V: what many regard as the "sweet spot." All other things being equal, the higher the voltage, the more powerful the experience (e.g., vapor and throat hit). Flavor may be enhanced or worsened by higher voltage; it varies. These batteries typically are 750 mAh: so great power and pretty good puffs per charge.
- Some mods can operate at many voltages, depending on the batteries you put in them: 3.7V up to 7.4V (two 3.7V batts). And a couple have automatic or semi-automatic juice injection.
So
that's why "some go to those big ones" despite their greater size and cost. Speaking of which...
Mods vary greatly in size and shape. Some are quite slender, others are boxes, others look like rocket ships. Prices range from $50 to $125 for most of the 3.7V and from $75 to $160 for most of the 5+V mods, although a fully decked out GG Telescopic Storm with AFS (Automatic Feeding System) will run you close to $300.
The best overview (in English) I've seen of most mods is
this article in E-cig News. New mods come out every other week so some of the latest and greatest are missing.
There is a more complete listing
here, but, unless you speak Finnish, you'll need to use Google Translate. The translation works pretty well ... although there are some, quite enjoyable, gaffs, e.g., 'atomizer' becomes 'atomic soybean'.
