Why? GearVR gets ~2 hours of usage that's more than enough for most VR play sessions (it's actually quite tiring). And GearVR also has to run Android and the rest of the cell phone crap and the cell phone form factor limits the battery size. You likely going to be easily be able to get 3-4 if not more hours of playtime from a pretty light battery by using just a slightly bigger battery than in an high end cell phone and while optimizing the hardware for VR rather than general purpose cell phone usage.