If you know that cases-cum-battery-packs for the iPhone exist, you've probably heard of Mophie, whose bulbous, iPhone-shaped lifelines might've been the (other) secret killer app at SXSW, where battery-slaughtering location apps ran amok. (Welcome to the future, btw!) I have one. It's blue and I use it with the top off because it is sexier* that way.
But it's not the best anymore. The best, according to The Wirecutter's research, is the PhoneSuit Elite. It's cheaper, smaller and will refill an iPhone's entire battery in two hours with 2100mAh of juice.
I am generally opposed to cases. (I say this as someone with 7 cracks in my phone, all told, so your potential skepticism is perhaps warranted. Anyway.) Jony Ive doesn't spend "months and months and months on a tiny detail" for you to shove his design in a case the size of a clown shoe with REALTREE® camo. Well designed gadgets are meant to be seen and touched, as they are.
Battery cases are an exception, because they prevent the following scenario: "Fuckfuckfuck my phone is about to die and I'm lost in the middle of Baltimore and why didn't I charge my phone before I leave oh hey is that Omar coming to save me AGHHHH." So a battery case could save your life. And I guess protect your phone for the two hours you're allowed to have it on while charging, since they are so very big.
The tragicomic part is that they probably won't be essential just at nerd lolololapaloozas anymore. As Farhad Manjoo points out, we're crossing the threshold where basically every new smartphone has 4G internet powers, whether or not people really want it, simply because of the tech spec arms race (which is how the whole 4G lie got started in the first place). While that speed actually is wonderful, it's like an internet adrenaline boost that works by socking your phone's battery directly in the junk. Seriously, Motorola turned around and almost immediately offered a second version of its "thinnest smartphone in the world," whose only feature is that it is fatter and has a giganto 3300mAh battery (the iPhone 4S's battery is just 1432mAh), so it lasts a whole day. Survival is a pricey novelty feature now. This is where your phone is going. Burns brighter, dies faster. That's just talking about 4G. Your phone does more things now, I bet, things that devour batteries like Foursquare Radar or (god) Highlight. And batteries aren't getting much better. The secret to the new iPad's amazing battery life? They made the battery bigger.
So if your next phone's got 4G next to it, plan to pick up a separate battery pack, a battery case, or live your life connected to a power outlet. You didn't want a computer in your pocket so you could go outside, did you? But maybe a silence would be nice, too.