The US$89.99 Third Rail system includes the iPhone 4 case and a single 'Smart Battery' bundled together, although both components will also be available separately ($39.99 for the case and $59.99 for additional batteries). The case is straightforward black, and it fits snugly on the iPhone 4 with a bit of effort. Alone, the case weighs 26 grams; that's scarcely more than the 20-gram Gumdrop plain plastic case seen in the 'family photo' above (the mophie is on the left, the Gumdrop in the middle, and the Third Rail on the right, battery disconnected). Once you put it on the iPhone, it feels like any other well-made slim case; the rubberized sides give it a bit of grip, and the wide control cutout on the left side accommodates both AT&T and Verizon models. At the bottom, there are audio vents similar to the ones on the juice pack, plus a Micro USB port for charging and a charge/idle switch.
The business side of the case is the back, where there are four slots and a three-contact charge connector lying flat. That's where the Smart Battery snaps on, locking into place quite firmly and providing power when needed. The battery is less than a centimeter deep and, surprisingly, doesn't make the phone uncomfortable to hold or knock it off balance. Once you connect it, flip the charge/idle switch from orange (bypass) to black (charge) and off you go.
The bottom of the battery features a charge indicator button, plus a standalone Micro USB charging port and a custom 'Out' port. Yes, that Out port means what you think it means: you can use the Smart Battery to charge other things, not just your iPhone. With the included adapter, you get a USB female port that you can attach to the charging cable for your Kindle, your Bluetooth headset, your GPS -- even your iPad, although it will trickle-charge rather than the full 10W quick-charge. That's the kind of flexibility that will let you leave a heavier/bulkier battery pack behind for short trips.
The battery pack's tricks don't end with USB power for other gadgets. The same locking tabs that connect it to the phone case allow you to stack up to six Third Rail Smart Batteries atop each other; plug in the bottom one and they'll all charge up eventually, which makes this an interesting solution for businesses that want to offer battery extenders for employees on the go.
If you connect multiple Smart Batteries without attaching the power cord, they'll negotiate amongst themselves to consolidate the available power and provide as many fully-charged batteries as possible (the 40 percent-charged battery will drain to charge the 75 percent-charged battery, for example). Third Rail says the batteries will be fully transferable to future case designs, so if you switch up to an iPhone 5 someday, you can keep your batteries for the new case.
When combined with the battery, the Third Rail system weighs only a hair less than the juice pack (65 grams vs. 68 grams), giving it a similar heft and pocket impact. The juice pack's curved lines and battery shroud make it theoretically less likely to snag on clothing, but the TR battery hugs the case quite closely -- I didn't have any problems with snaggage while I used it.
The charging mojo of the Third Rail system was substantial. I was able to take my in-use phone from 10 percent charge up to 60 percent in less than two hours while taking the Smart Battery from full charge down to zero. By comparison, the mophie juice pack's slightly larger battery makes it up to about 65-70 percent charge from 10 percent in about the same amount of time.
I noticed some quirkiness in the charging process when I tried to charge both my phone and the battery via the case's charge port; it seemed like the battery was charging first, which wouldn't be my preference. Third Rail told me that the prerelease battery firmware I have is updated in the production models, and that now the iPhone will be fully charged before the battery charges.
My week with the Third Rail system has made me a fan. It's simple to use, works well and delivers flexibility that up until now has been hard to find in the power boost accessory market. It's definitely more convenient and compact than the dock-dangle battery products, but it doesn't force you to carry the weight of the extra battery when you don't need it.
Is it better than the mophie juice pack air? If you always need the extra power, it's probably a wash; the convenience of the all-in-one juice pack balances out the extra charging features of the Third Rail. If you only need extended charging some of the time, I highly recommend giving the Third Rail a try.