I've been using 18650's and single CR123's on 3.7 with the i3, but I ordered a couple CR123 3.0V batteries to try out my i6. The 6V with the i6 is good, but I've found that I really like using the i7 at 6V. Great vapor and throat hit, and not so needy on the juice. It's basically a notch up from freshly charged 3.7V batteries on an i3, but you don't have to take 10 second drags to get good vapor. I don't know what voltage equivalent using an i7 at 6V is, but I'm really loving that setup on the GGTS with AFS.