In the quest for more battery life, smartphone users can either A) buy a spare removable battery, B) get an external battery pack that connects to the phone via a cable, C) put on a case that's also an external battery pack.

iPhone owners don't have Option A available to them, and Option B only works if you're going to carry a bag around. The Mophie Juice Pack Air pulls off the last idea in style.

Silver-effect paint along the sides mimics the iPhone 4's winning industrial design, but is admittedly a poor substitute. The back of the case is where the battery is, with edges nicely tapered so it feels secure in the hand, and not as thick as one might expect. There's a narrow cutout for the volume buttons, which is fine until you want to take a photo with the Vol+ button in iOS 5. Sister product the Juice Pack Plus solves this with proxy buttons on the case itself, but is also bulkier.

With the press of a button, the bottom edge lights up to tell you how much power the case still has. Fully charged, it'll put another 70% on your iPhone when switched on. That will get most people through about two days of use.

Where does the charging cable go if the bottom edge is covered? You have to use a microUSB cable (included) instead, which goes into a port on the side. It works just like a regular cable, passing power and data through to the phone.

The camera cutout is large enough that you never have to worry about glare when using the flash; a major annoyance on many lesser cases. A soft, rubberized back goes a long way towards making this feel right in the hand.

Apple requested a redesign of the case for a version sold exclusively through Apple Retail (above). It features an open top for easy access to the Sleep/Wake button and headphone jack (the original is closed, with cutouts), and a reversal of how the two parts of the case come apart. In the Apple edition, the bottom 20% pulls off, revealing the iPhone's original dock connector. In Mophie's design, the top edge comes off, meaning the case must be removed entirely to get at the connection port.

Both versions are alike in cost and performance, so it's a matter of preference.

There is something freeing about knowing that you can play that photorealistic game while waiting in line, take that hour-long call from your ex, snap a load of photos at the club and still have a working phone with which to book that last cab home at 3 in the morning. As normal people, we don't leave our houses every morning wondering if we'll make it back alive, but we do think that way about our smartphones. As they become ever more entwined with our lives, so must they promise to match our limits. Only then, will we reap the psychic benefits of 24/7 knowledge and ubiquitous communications.

Shop: Mophie Juice Pack Air for iPhone 4/4S (Original, Black)
Apple Edition (U.S. store) (Singapore store)