Pretty sure it's a straight mechanical device, so some people will use 4.8v nimh cells that are the same size as your 18650... Never tried using those nimh cells before, but you need special nimh chargers to use them, and I would definitely talk to somebody who knows more about them before trying as I've never done it, or knew anybody who has, so couldn't tell you if there's anything special you need to do or look out for using a nimh battery instead of a normal lithium ion.
Otherwise, since it's mechanical, it will vape whatever your battery is outputting. If you stick to your higher quality IMR batteries (Panasonic or AW), you should be getting 4.2v or so fresh off the charger, and slowly your voltage will taper off as you use it. I also recommend the better quality batteries as it seems they don't drop the voltage under load as much as some of your cheaper batteries.
But if you stick to some lower res stuff, 4.2v is plenty, I'd personally recommend being in that 1.7-2.2 (Single Coil) range...