My first impulse is to compare Third Rail Mobility’s new Smart Battery and Slim Case to Mophie‘s Juice Pack, the best-known name in iPhone battery cases.Third Rail’s system does, indeed, provide an alternative way to extend an iPhone 4’s battery life. But this clever system goes way beyond that. I tried a review unit provided by the company.

The case itself looks pretty much like any nice two-piece snap-on black iPhone 4 case, except:

• The section below the dock connector is a bit taller than usual and sports a Micro USB connector;
• There’s an area on the back with electrical contacts.

The contacts on the case’s back are there because you can connect the battery there like a backpack. It’s a 1250 MaH unit, so it should come close to doubling the standard life of the iPhone’s own 1420 MaH battery. You can simply use the phone with the battery in place–it looks a little strange, but I found it comfortable enough in the hand–or attach the battery only when you need it, such as when the iPhone’s own battery is about to croak.

The Third Rail battery isn’t just an iPhone battery, though. It’s got a MicroUSB connector that accepts a bundled adapter with a full-sized USB connector on the other end, letting you plug in other products that can charge via USB–such as other phones, Bluetooth headsets, and wireless hotspots–either while the battery is attached to the iPhone case, or when it isn’t. (It’s not, however, designed to charge particularly power-hungry devices such as the iPad.) The batteries themselves can also be charged via USB. And they can be snapped to each other like Lego blocks–on or off the iPhone case–to create a bigger power pack with longer endurance.

For the most part, the setup worked well in my tests. (Exception: When I carried my iPhone in my pants pocket with the battery attached, the battery popped off a couple of times.) It’s important to note that Third Rail’s case blocks the iPhone’s dock connector and replaces it with Micro USB–possibly an advantage if you’re a Micro USB cable fan, but it could be an issue if you use iPhone add-ons that use the dock connector, such as the iPhone integration in my car. My understanding is that Apple doesn’t permit anyone to build devices with pass-through dock connectors, so Third Rail didn’t really have a choice but to do it this way.

A Third Rail case and battery will run you $89.99 ; a case alone is $39.99, and a single battery is $59.99. (Why might you want to buy a case without a battery? Well, for one thing, a household with multiple iPhones might share batteries. And when future iPhones come out, Third Rail plans to sell cases for them, too–letting you use the battery you buy today without having to start all over.) By contrast, Mophie’s Juice Pack Air lists for $79.99, or ten bucks less than Third Rail’s system.

If you have multiple devices you need to charge on the go, the premium for Third Rail would be worth it–especially if, like me, you’ve been known to  purchase and travel with both an iPhone battery and a separate external battery for charging other devices. (I use one with my Mi-Fi, since it can only go for a couple of hours on a charge otherwise.) But I’ll bet hardcore gadget hounds would appreciate a discounted combo pack with a case and two batteries. Currently, buying that configuration would cost an imposing $149.97.

Third Rail says it’s thinking about extending its offerings further–possibly with batteries in different capacities or cases for handsets other than iPhone 4. That would make a lot of sense: the richer the system gets, the more appealing it’ll be. And the Smart Battery and Slim Case are already worth a serious look if you carry multiple gizmos and are in the habit of draining their batteries in less than a day of use.

 
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Read more: Apple, Apple. iPhone, Batteries, Power, Smartphones