Vaping, Electronic cigarettes

Go steady with the stacked cells. It's generally considered to be a bit dangerous, which is why no other device really offers that I suppose. Don't go below 7.4V. And only use 2 new batteries that have only been used doubled up together to avoid imbalances. Fully charged reading should be 8.4V (2 x 4.2V). If it's less, then one of your cells might be less charged than the other, meaning unregulated charging of the weakest cell, by the strongest cell.

gotcha. will avoid stacking.

but if the batteries are 18350 3.7V 900mAh's, isn't 3.4V minimum quite high? also, shouldn't fully charged be 7.4V? (2x3.7V)

I've ordered a pair of Sony IMR 18650 1600mAh

Also been reading up (well watching up) on variable Wattage. makes so much more sense, no?