The Asus Zenfone AR features a 3,300 mAh battery, which in my opinion is pretty bad considering that this phone comes with Daydream and Google Tango features, which lead consumers who intend to buy this phone for AR and VR gaming, to charge it very frequently and it will definitely impose limited continuous gameplay sessions.
I wish the phone had the same 5000mAh battery of the ZenFone 3 Zoom. If the phone had a small form factor I could understand the use of a relatively small battery, but it has a 5.7-inch display and it's quit a large phone. I assume that the reason for that is due to the Tango hardware components that limited the size of the battery Asus can put inside this phone's chassis. The good news is that the Zenfone AR supports Quick Charge 3.0, so although it's not the latest QC 4.0, it's still allows phone owners to quickly charge the phone for another playing session.
I also think that there won't be a custom attachable battery pack for it due to its very unique curvy and textured back and the fact that it has a very large protrusion at the back for its TriCam setup. I also think that due to the field of view of the lenses, it might not even be possible, but I am not 100% sure about it.
So overall I see the battery capacity as disadvantage and we can clearly tell that there is definitely an urgent need for new battery technologies that can hold much higher charge in a small sized battery. There are technologies like that out there, but they still didn't make their first commercial appearance, and maybe we'll see one coming in 2017. Devices like the Tango-enabled ZenFone AR really needs it.