Boost Case is one of the most popular battery cases out there because it's thin and has the "world's highest capacity" 1,700 mAH battery inside. In other words, the battery's a little smaller and lighter but it still manages to double the battery life of your iPhone. The Boost Case Plus features a 2,200 mAH battery for $10 more but doesn't seem as popular with users as this model.
The newer Boost Case Hybrid model features a 1,900mAh detachable extended battery that you can remove when you don't need the extra power (it's also supposed to double the battery life of your iPhone).
The Eton Mobius NSP300B offers solar charging capabilities, but as you can see from the picture, it's on the bulky side. However, if you're looking for a solar-power charging option, it's one of the few battery cases that includes it.
Incipio touts its OffGrid case as weighing just 2 ounces and measuring 9.6 mm thick. While it's fairly affordable at $45, its 1,450 mAH battery is smaller (lower capacity) than some of its competitors'.
Note: This spring, Incipio will offer a new OffGrid case that has a removable battery, which will allow you to carry around a second (or third) battery and swap it in if necessary. We don't expect that product to be available till March, however.
The iBattz Mojo Removable features a removable 1,500 mAH battery that slips under the back cover. While it's fairly pricey at $80, it does come with two batteries, so you should come close to quadrupling your iPhone's battery life if you carry that second battery around with you.
Monoprice, a leader in value-priced accessories, offers its own version of the iPhone 4/4S extended battery for a mere $23.63. We can't say it gets huge points for style (not sure a lot of folks want to rock the Monoprice brand on their iPhones, but hey, to each his or her own). As its name implies, it has a 1800mAh battery.
It's unclear from the product page whether this case is compatible with both the 4 and 4S, but customer testimony seems to indicate it does (we have not tried this case).
As one customer says: "Although the product is described for the ATT version is does fit and work well with the Verizon 4S. The only button that is awkwardly placed for the case is the ringer mute button which is partially hid by the case but still accessible and functional." (It also appears to work with the Sprint and AT&T 4S with the same caveat).
One of the more affordable extended battery cases, the New Trent Rock case IMP220B ($34.95) has a 2200mAh battery.
The case is available on Amazon but there are two versions of it. One costs $34.95 and one (which says it's for the 4 and 4S) costs $44.95. It's unclear what the differences are so we've linked below to Newtrent.com, which offers the 4/4S case for $34.95.
See pricing for New Trent Rock case IMP220B from NewTrent.com.
While it's somewhat pricey at $69 (same as the Mophie), PhoneSuit Elite is billed as the "thinnest, fastest charging iPhone 4/4S battery case" and includes an integrated 2,100 mAH battery, which is a bit higher capacity than most competitor's integrated batteries. Comes in black and white.
The PowerSkin uses an Xpal battery (Energizer's Energi To Go case is also "powered by Xpal") and features a 2,000 mAH battery. We checked with the company and it's supposed to work with both the iPhone 4 and 4S.
Scosche says its 1,800 mAH won't quite double your battery life, but the kicker here, of course, is the built-in kickstand. If that's something you're looking for, that's what Scosche cases are known for. Drawback: Pricey at $80.
Third Rail Slim Case and Removable Rechargeable Smart Battery
The Third Rail case differentiates itself by offering a removable battery that attaches to the back of the cases and can be daisy-chained (you can connect up to six batteries).
The nice thing about the system is that if you move on to the iPhone 5, you can keep the detachable battery and use it with a new case (Third Rail says it's sticking with the same system for future battery cases). The one downside is that the batteries (1,250 mAH) aren't as high-capacity as those of some competitors. Also, the case and battery combo is fairly pricey at $90, and extra batteries will cost around $60.
But it's a great concept that allows you to keep your iPhone in a slimmer case and only attach the battery when you need to.