Karlmetz -- I was able to snag a Billet Box over the weekend myself, and have been wrestling with this issue. Listed below is the Q/A about batteries on the Billet Box Owners FB page which I found helpful:
====below text cut and pasted from the Billet Box Owners Facebook group ======
Q. What kind of batteries does The Billet Box use?
A. The Billet Box is designed to use 3.7 volt, AW IMR 16340 (Red) or AW ICR 123 (Black w/silver label), button-top batteries.
Q. How long do those batteries last in the Billet Box?
A. Short answer: until they die. Long answer: Most users report anywhere from 1/2 to a full day of vaping with a freshly charged pair of batteries and a 3.0 ohm cartomizer. Depending on your cartomizer resistance and how you vape, YMMV.
Q. How do the batteries die - do they peter out, cut off, or...?
A. Red AW IMR 16340's tend to lower in voltage near the end of the charge resulting in a noticable performance drop before they stop working.
The Black AW ICR 123's tend to keep their output and performance constant almost to the very end with only the last two or three vapes showing signs of final discharge.
Q. Can I use other brands or types of battery?
A. The Billet Box battery compartment is very specifically designed for the physical size of the AW IMR 16340 and ICR 123 batteries and has very tight tolerances. However, any 16340 or 123-type battery that is button top, no longer than 34.2 mm or larger than 16.6 mm in diameter should fit. If you MUST buy a different brand/model of battery, CHECK IT AGAINST THESE DIMENSIONS.
Q. Aren't 123-Type batteries lower voltage than 16340's?
A. Yes, No and Maybe. Broadly speaking, 123-type batteries have historically been used in 3.0 volt applications such as flashlights and photography equipment. However, while there are standards out there battery manufacturers are not really held to them and can make and label their batteries any way they want that is relatively safe for a particular purpose. As a result there are some (such as the AW ICR 123's) that are 3.7 volts.
The Billet Box will work with 3.0 volt batteries, but the upper voltage range will be limited to 4.8 volts (no matter how much higher you turn the dial). They can be used "in a pinch" but probably should not be used as your primary batteries.
LAST NOTE on batteries regarding size and voltage:
Think about this: 123-type and 16340 batteries should, generally speaking, all be about the same size. But manufacturers label similarly-sized batteries almost interchangablely and are free to muck about with actual specs and dimensions. A battery labeled "16340" does not necessarily have to be 16mm in diameter, 34mm in length and need not even be cylindrical. To add to the potential confusion, the IEC international standard name for this type of battery package is "CR17345".
Confused yet?
Certainly, one can experiment and all of the information needed to make an informed purchase is in the answers above. If you know what you're doing and comfortable "taking a chance" then have at it! Properly informed, you will most likely find something that fits and works well.
However, if you don't know what you're doing and find all this battery talk confusing ... do you really want to risk wasting time and money on something that will not fit or make the Billet Box vape it's very best?
Bottom line: The recommended AW IMR 16340 / ICR 123 batteries are ALWAYS a safe bet. Moving on.
Q. The batteries fit in there awfully tight. How do I get them out?
A. The recommended method of removing the batteries from the Billet Box is to remove the back plate and smack the battery compartment over the open palm of your hand. Done properly, at least one battery should drop out allowing easy removal of the remaining battery.
Prying them out with a tool is not recommended as it could damage the protective covering of the battery.
Some users recommend wrapping a bit of tape around the batteries to create a handle to pull them out. Be advised, there is not much clearance between the batteries and the back plate, so the tape must be relatively thin.
Q. Can I charge my batteries inside the Billet Box?
A. No. Batteries must be charged in a dedicated battery charger.
Q. What battery charger should I use?
A. Many users have recommended the NITECORE Intellicharger i2 or i4, but any charger capable of handling 3.7 volt 123/16340 batteries should work.