The latest leak to hit the web is that the phone will pack a power-house, that is a 3400mAh battery, in perspective, you typical Lumia 92x or 1020 have 2000mAh battery. But it is expected as the device will need all the juice it can get.
So After September 26th, what's next? Well, all indications say we should be looking at an early November general availability date. Which coincidentally is when the GDR3 update whould begin rolling out.