Jason Snell, writing on Six Colors:
Over the years I’ve said numerous times that when it comes to battery life on iOS devices, Apple appears to have a target battery life in mind and builds its hardware—a balance of power-saving software, hardware efficiency, and battery capacity—to hit that number.
The iPhone and iPad’s battery life hasn’t changed too much over the years. It seems to be one of the constants that Apple still believes they got right with the first model. If they can get additional battery life in a new model they will, but not at the expense of an improved design or more powerful features.