Really wish I knew more about that Val. Been stacking batteries in for instance flashlights all my life, no problems. But I've seen several posts about the danger of stacking batteries in PV's and I sure don't want my GGTS blowing up in my mouth. My favorite setup for several months was 2 of the 3.0v RCR123A stacked in my GGTS with drip mode Ody on top and a .9 Ohm dual coil. Loved the throat hit and believe it or not the juice flavor was excellent also. But on occasion I would get a small "mini blast" with a puff of air blown back into my mouth. This scared me after I read several posts about the danger of stacking batteries so I changed to just using 1 of the IMR 18650 3.7v batteries with the Ody still using the .9 Ohm dual coil. I also had developed a nagging cough I think was caused by the intense throat hit. Anyhow I don't like the 3.7v setup as well. Throat hit and vapor flavor is much less intense with the 3.7v setup but my cough seems to be getting a little better and I haven't had any more of the little "mini blasts" and feel a little safer. But I don't like this setup as well, plus now I have all these 3.0v and 3.7v batteries that I'm afraid to use. Would love to hear some comments from knowledgeable people about how safe, or unsafe stacking batteries is especially at high voltages. Thanks!!!