a 6V 18650 is only going to be two 18350 lifepo4 cells at 3.2V a piece, or two 18350 limn cells at 3.7V a piece with a diode attached (a la the blue tenergy 3V batteries) to bring the voltage down to 6V. voltage is a function of the battery chemistry, lifepo4 is 3.2, limn and regular li-ions are 3.6/ 3.7, nicd and nimh batts are 1.2V. even a 9V battery is just 8 nimh cells or 3 lifepo4 cells being run in series. you'll be multiplying voltage but dividing runtime and increasing the amperage demands. there's no way to get 5V without stacking batteries or electronically boosting the voltage of a given battery, this isn't likely to change. these four nimh batteries combined amount to 4.8V, but this is the unloaded voltage, before you put the atty on and fire it. the voltage under load is going to decrease even more and be sporadic at best if the batteries don't have the amperage required for the load. even a BE112, at 3.3ohms is going to demand 1.5 Amps to give 5V. a joye 510 or 306 will demand considerably more. needless to say these little batteries aren't providing 1.5 Amps, but whatever it is doing if it works for you then that's all that matters in the end. i was a 5V vaper, and swore by it, but 5V with a 510 atty is different from 5V with a BE112, which is different from a 5V device with tenergy lifepo4s, which is different from a 5V device with stacked CR2s, or a usb hub on your computer, or a 2Amp wall wart, etc. load impedence, amperage demand and availability, etc. all differentiate one 5V experience from the next. all of these factors are coming together to generate the power or watts, which is what you really want to match, and if a BE112 at 5V, or 7.5watts is your thing, then you can get better battery life and a fuller "5V" vaping experience by using a 1.7ohm LR BE112 and a single AW IMR16340, or the AW17670 in the GGTB, or the AW18650 in the GGTS....and no stacking required. if you want it a little warmer try a 1.5 ohm atty with the same setup, or get VHV attys like 5.2-5.6ohm and stack real IMRs. the AW IMR18350s should be out this month for the GGTS and the IMR16340s are great in the GGTB if you go that route. options galore, you gotta love vaping!