Rumors about Apple's next iPhone continue to roll in, but one of the more persistent ones right now is that among the devices released for 2012 will be a bigger format iPhone sporting a larger screen. Given that there's an industry trend for bigger screens right now, this idea makes good sense. But Apple may choose to follow this route for totally different reason.
It's very simple: A bigger chassis for an iPhone with a larger screen than the iPhone 5's 4-inch display makes room for a larger capacity battery, and a bigger battery will deliver more talk time, more time to watch videos, more time to surf the web—and less time worrying about having to charge your device.
Apple has already pushed the iPhone's battery tech up a notch by slightly incrementing the voltage of the battery used in its iPhone 5. Almost as soon as the phone went on sale, iFixit tore one apart to find that compared to the iPhone 4S the iPhone 5's battery is rated as 3.8 volts and 1440 mAh versus 3.7 volts and 1432 mAh. The difference was enough to squeeze out another 225 hours of standby time, even though the battery is a similar physical size in each device. It's also worth noting that despite the fact each generation of iPhone has had more features and more powerful chips inside, each of which changes could draw more current from the battery, the iPhone's talk time and standby time have remained more or less the same or even improved slightly. Apple's done this by tweaking battery parameters and exercising amazing control over how the iPhone's circuitry and software work. This is the reason "full" multitasking isn't supported.
But peaking inside the iPhone 5 reveals a problem for Apple: It's absolutely crammed to the last few cubic millimeters with chips, antennas, circuit boards, sensors, and battery. We can assume that the iPhone 6 will have yet more powerful chips and features than the iPhone 5, and that Apple will exercise some of the same iron hard control over power consumption, but there's a limit to this sort of approach. Current hot rumors suggest the next iPhone will incorporate a fingerprint sensor and even NFC technology, at last, and both of these systems may really suck juice out of a battery. This is particularly true for NFC, and it may be one of the reasons Apple has held back from implementing an NFC payment system in the iPhone.
At some point Apple is going to either radically improve the battery technology it's using inside the iPhone, or it's simply going to have to use a bigger battery pack. But while there are continuing advances in battery technology, it really isn't developing fast enough to influence the next crop of smartphones.
Apple could achieve bigger battery capacity by shrinking the iPhone's circuitry, but you have to admit that the tiny sliver of motherboard inside an iPhone is already so dense and well designed that it's almost unrecognizable as a stereotype circuit board. A bigger battery could be achieved by using thinner screen tech or a modified case design, but Apple has already used the former trick to thin down the iPhone 5 from the iPhone 4S's depth and is unlikely to use the latter trick in case it makes for a thicker phone overall.
Which leaves us with one clear option: Make the entire iPhone bigger. It's true that a larger screen and its backlighting system may gulp down a little more power, but the bigger volume of the battery you could hide behind the screen's bigger area will more than compensate for this. And as a great demonstration of this, Apple rivals like HTC are already achieving fabulous battery life from their 5-inch-plus devices: HTC's new One phone, for example, can manage something like 17 hours of talk time from its 2,300 mAh battery.
Of course a bigger iPhone lets Apple tout a bigger screen, better viewability for movies and so on, and you can bet that if they do go down this route the chassis will be so tiny that it'll barely be bigger than the screen in order to keep the iPhone "portable." But the real reasons for the size will be more about chemistry.