Hi Shawn, nice to hear from you and thanks for taking the time to help me out. I'm using 18650 batteries purchased from different sources, vape shops, Amazon and Vape Stores on line. I have 10 of them, the most recent purchased a couple of months ago, they charge fine and fully but the X-priv stops recognising their charge when only half used. I'm running at 46 watts. I took the device into a vape store today and they reckon the device is faulty, not the batteries. I'm trying to find the warranty for a replacement!
Of course, go for warranty replacement if possible. I fear you may have issues, though.
It's possible the shops sold you counterfeit batteries... make sure you buy your batteries from reputable sources such as IMRBatteries.com or Liionwholesale.com. Amazon, eBay, and even some vape shops (because of their distributors, not necessarily the shops themselves) are not to be trusted when it comes to batteries. Consider that a cheap 8 Amp battery suitable for a flashlight may only cost $3, and a printed battery wrap may cost $.10 where a quality battery may cost $6... many less than savory characters are happy to re-wrap the cheap battery and pass it off as a good one.
But, yeah, at 46 watts, a good set of 3000 mah, 20 Amp cells should last nearly all day (unless you take 217 x 4 second puffs in about 15 minutes).
The Android app VapeTool Pro (not available for Apple as far as I know due to Apples anti everything stance) has a battery life calculator. Worth checking out, I think.
As far as the mod is concerned... yeah, Smok has a bad reputation, but I have had Smok products last years! In fact, I think I've only ever had 2 out of the dozens I've owned die. That said, there are better devices out there. If battery life is paramount to you, take a look at the VandyVape Gaur-21, and get a couple sets of 21700 cells.
Good luck!