Dislaimer: I'm no expert - I didn't stay in a Holiday Inn Express. I'm just relating what I've learned from others. You should search ECF on the topic. There are MANY good threads which go into more detailed explanations. Having said that . . . . .
The designation of 18650 is an indication of the size and shape.
18 is the diameter - 18mm
65 is the length - 65 mm
0 means it is a cylinder form factor.
Although not noted in the designation, there are also button/flat tops as well a protected/unprotected.
Generally, if your PV is regulated, you can use unprotected batteries. The circuitry in the PV will provide some control if the battery gets wonky. If your PV is unregulated (ex. mechanical mod) you MUST research this thoroughly to know what you're getting into insofar as your battery choice is concerned.
Again, search ECF for the many threads on this area. A good start is:
http://www.e-cigarette-forum.com/forum/rebuildable-atomizer-systems/465483-sub-ohm-vaping-discussion-safety-battery-info-warnings.html
For PV's you want to use IMR high drain batteries. That is more important than the mah claimed by the "manufacturer". Sometimes the mah claims are accurate, sometime total fantasy.
I quoted "manufacturer" because most of the batteries are re-tasked batteries. Most have come from packs used for things such as laptops. How they are remade and by whom has a big effect on their performance and safety. From what I've been able to ascertain (see disclaimer) the AW, Panasonic and LG batteries are pretty much good-to-go. I've seen varying opinions. I've only used AW, so can't personally say.
As for mah . . . .
These are manufacturer claims. Sometime accurate, sometimes fantasy. I use them only as an indication of endurance, not a certainty, but only for a given "manufacturer". Just stay with the mainstream batteries that most of the veterans are using and you should be OK. If you are tempted to get one that is from a manufacture outside that group, research it before you buy/use it. Efast and Trustfire come to mind.
AW rates their unprotected 18650 at 2000mah. An 18650 being marketed 3400 probably isn't. Caveat Emptor.
