Analysts and Apple fans have recently become enamored with the idea of an iWatch. Bruce Tognazzini, a respected ex-Apple employee has an excellent blog post in which he describes all sorts of smart uses/features for such a watch. As anticipation for such a device is building, so are the expectations for its features. Some have come to see it as essentially an iPhone with a small screen. I think those folks will be disappointed. Think about it: we're talking about a device that needs to be thinner than an iPhone and, at most, 4 square inches in size. There is no room for much of a battery! Add to that the fact that, historically, people don't expect to have to recharge their watches every day, and you begin to see how impossible a "mini-iPhone" iWatch is.
So the features of an iWatch must revolve around power: how to minimize consumption and/or how to provide more of it without taking the watch off.
What can be done to keep power consumption to a minimum? The two largest power hogs in a mobile device are its screen and the various radios driving communications (wifi, cellular, bluetooth). Screen power consumption can be minimized by using a technology used in e-Readers (and in the recent "Pebble" smart watch): e-Ink. With e-Ink, power is only needed when the screen needs to change. But the technology, by its nature, has two drawbacks: slow refresh rates (making video impossible) and lack of color. If Apple deems color and video playback indispensable, its option is OLED. Reducing power on the various radios is easy: simply limit communication to one low-power channel. I believe that is Bluetooth 4.0. Of course, if the iWatch doesn't have wifi built in, it will not be a "connected" standalone device. Apple's choice.
So, if the iWatch uses an e-Ink display and Bluetooth 4.0 for communication with an iPhone, what could such a device do? Well, it could tell time! :-) Also, like the Pebble, it could act as a remote display for all the phone's notifications. For notifications requiring action, the watch's microphone could be used to tell Siri (located on the phone) what to do. Similarly, the iWatch could display (or speak) results of any spoken Siri query. The built-in speakers and microphone could also be used to make or answer phone calls (like today's Bluetooth headsets). The bottom of the iWatch and/or the watchband could contain sensors that keep track of basic biometric info (e.g. pulse rate) - perhaps something similar to what Nike's Fuelband does - and send it to the phone occasionally (where an app can let you analyze the data over time). Finally, an e-ink based iWatch could display OCR codes from Passbook. When at a "favorite" store, the iWatch would automatically display the store's pass. No need to fish for the phone or wallet! Some analysts/users propose that the iWatch should have an NFC chip for payments. I think this might be a future addition to the watch - there aren't enough (any?) NFC-based cash registers out there to warrant the additional complexity (and power drain) in the watch. How long between charges? Well, we have a couple examples that give us a clue: the Pebble smart watch lasts 7 days. The Nike Fuelband is said to last 4 days. So Apple, with its engineering prowess and deep R&D pockets, should be able to produce a watch that lasts at least as long as the Pebble.
If Apple wants to endow its watch with a color display and the ability to carry on FaceTime video conversations or watch Youtube videos, e-Ink is out of the question - an OLED display becomes necessary (it uses less power than LCD and can be put on a flexible substrate to allow for a curved display). If the watch has a built-in accelerometer, intelligent power management software could turn off the display whenever its wearers' arm is hanging by his/her side and gets activated only by the touch of a finger or by an incoming notification. Perhaps, this way, the iWatch could still last 5-7 days between charges.
But no matter how hard Apple tries, an iWatch will have to be charged on at least a weekly basis. What can Apple do to make this as painless as possible? Some manufacturers have begun offering wireless charging pads (and Apple owns a patent on doing this over the distance of up to 1 meter) that might be useful here. If one didn't have to plug in yet another device and, instead, simply placed the watch, along with ones iPhone, on such a pad for charging at night, people might not mind! Heck, if they can pull off that feat, maybe an iWatch only really needs to last as long as the partner iPhone!