Alleged Galaxy S6 battery pack snapped, confirming 2600 mAh capacity
Given that the Galaxy S6 Edge codename is speculated to be SM-925X, and, say, replacement batteries for the Galaxy S5 (SM-900) go on Amazon with models like EB-BG900BBC, there is a big possibility that these pieces you see below are Galaxy S6 or S6 Edge packs. Now, it's not entirely clear if those are replacement batteries, or the actual juicers that are ready to go into the retail handset (the guy who posted these just says "Galaxy S6 battery" as a comment), but, given their production date of 2/11, these are pretty fresh off the conveyor belts, so this might be the real deal.
Before you scoff at the 2600 mAh capacity, which is lower than on the S5, bear in mind that the Galaxy S6 will have a very frugal 14nm chipset. Moreover, we already have a handset out with the S6's rumored 5.1" Quad HD Super AMOLED display - the Galaxy S5 LTE-A - and its battery life tests showed that the panel is very power-sipping, too, performing on par with its 1080p sibling.
Thus, the Galaxy S6, together with the other software and hardware optimizations that Samsung placed within, should last at least as long, if not longer, than the Galaxy S5 on a charge, despite the 2600 mAh battery capacity. Moreover, there is speculation on a new type of polymer technology that Samsung is utilizing for the juicer, plus rapid wireless charging capabilities in the S6, so we wouldn't worry too much about the handset's endurance just yet.