For the most part, battery cases for the iPhone have maintained the same overall design: elongated sleds with case-like body coverage and significant bulges on their backs. Slimming battery cases down is not as easy as with other cases, since doing so involves reducing the capacity of the battery. The user is left with a device that’s doubled in thickness at all times, even when the iPhone’s fully charged. Third Rail Mobility recognizes this problem, and has taken a different approach with what it calls the Third Rail System for iPhone 4 ($90, aka Slim Case with Slide-on Smart Battery). Unlike most rivals, this system is modular, composed of a slim case that can be used completely independently, and a battery that snaps into place when needed.

Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

Third Rail Mobility describes the battery system as an all-in-one solution, combining the features of a traditional protective case with a battery, external charger, and universal USB charger. The Third Rail System is composed of two main pieces, each available separately—Slim Case sells for $40 and Smart Battery sells for $60. Also packed in with the full System are a Micro USB to USB charging cable and a special USB adapter. Slim Case is a thin, light slider-style case; the dimensions are not that much different from a traditional stand alone case and it adds only 0.09 inches of thickness. At the bottom of the case is a male Dock Connector. The electronic components create a little bit of a “chin” at the bottom of the iPhone 4, about a quarter of an inch long. On the underside of that extended area are a switch for controlling the flow from the battery and a charging port.


Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

Slim Case separates at the top; the whole thing slides on and off easily, and stays securely put. The level of protection provided by the case is pretty good, though some users might find it to be a little much for daily iPhone use. There are precise openings for the Sleep/Wake button, noise-canceling mic, headphone port, and rear camera; the headphone and mic holes are very small, so only tiny headphone connectors of Apple plug-like size will fit. Along the left side there is one large hole for the silent switch and volume buttons; this allows the case to be compatible with both GSM and CDMA based iPhone 4s. Two rectangular openings directly underneath the phone’s bottom edge allow the microphone and speaker to breathe, but the Dock Connector is always filled by the case; you need to remove the whole case to connect other accessories. The case’s edge is flush with the iPhone 4’s glass face, providing no protection there, and no screen film is included in the package.


Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

Taking up the majority of the back of Slim Case is a spot to attach the Smart Battery, bordered by a thin strip of glossy plastic. There are three contacts, as well as holes to slide the battery’s clips into. Smart Battery is a little bit thicker than the stainless steel antenna of the iPhone 4, so when it is attached to the case the total thickness is a bit less than double the thickness of the phone itself—combined, Slim Case and Smart Battery have a thickness of 0.73 inches at maximum; other portions of the case’s back remain as thin as ever. The battery has a capacity of 1250mAh, which is 80% of the iPhone 4’s battery capacity. Because some energy is lost in the transfer, Third Rail Mobility claims that Smart Battery will provide about 60% to 70% of a full iPhone 4 charge. Each battery has a series of five LED lights to indicate the current charge level. While the company has not announced any further products at this point, it would certainly be possible for it to release a higher-capacity battery down the line.


Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

Another unique feature of the Third Rail System is that the Smart Batteries can also be detached to serve as external battery packs for USB devices. Each has an output port for use with the included USB adapter, enabling you to connect whatever USB cable you have for your other device. While the battery does not have enough wattage to charge an iPad, it can certainly power up other USB devices such as Bluetooth headsets, cameras, and even iPods. Those devices can even be connected to Smart Battery as it is connected to Slim Case. The iPhone 4, Smart Battery, and USB device will all charge, with priority going to the iPhone 4. Also interesting: the batteries are stackable. Up to six can be connected at once, making this solution good for someone who may be traveling extensively without access to power, assuming that he or she wants to go this route rather than just buying a super-large battery pack. The latter option would probably be more affordable and practical.


Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

In our testing, a fully charged Smart Battery was able to provide a depleted iPhone 4 with a 58% charge in one hour and 38 minutes before running out, going from 19% to 77%—just a hair under what the company set as a base-level expectation. This was on a powered-on 3G/Wi-Fi-connected iPhone 4 with the screen turned off, no calls coming in, and no audio playing. Such testing conditions are quite strenuous, so the result was as expected. That outcome is better than the closest capacity competitor, Kensington’s PowerGuard, a thicker 1200mAh case that only provided a 41% recharge in our testing. Mophie’s Juice Pack Air, on the other hand, features a 1500mAh battery and was able to add 70% battery life. It is worth noting that Juice Pack Air is just a bit slimmer at 0.68 inches, but the battery is not removable. Mophie’s larger option, Juice Pack Plus, has a battery that weighs in at 2000mAh and provided a 91% charge. PowerGuard sells for $60 while Juice Pack Air goes for $80 and Juice Pack Plus retails at $100. By comparison, Third Rail System falls inbetween the Mophie options in price, but not in power; you pay a bit more per mAh than the other three options. For competitive reasons, the set would have ideally been priced $10 or $20 lower. On the other hand, Third Rail’s design has the significant benefit of the detachable battery.


Review: Third Rail Mobility Slim Case for iPhone 4 with Slide-on Smart Battery

Overall, we were quite impressed with the Third Rail System. While it doesn’t provide as much raw power for the dollar as its competitors, it still performed well in our testing: Slim Case makes a nice standalone case that all but heavy accessory users will enjoy having around, and while attaching the Smart Battery certainly adds bulk, it is not terribly obtrusive—it’s still very pocketable. The fact that the batteries can also be used as standalone chargers for USB devices is a big plus, as is the fact that they are stackable. Different users will reach varying conclusions as to whether a thicker, non-detachable battery pack or a thinner, detachable solution is superior for their needs, but Third Rail Mobility definitely has something neat on its hands here. From a rating standpoint, the Third Rail System falls on the edge of our B+ and B ratings due largely to pricing, tipping into the B+ category because it has succeeded at doing something different and valuable with this design. We’re actually excited to see what other products the company introduces that interact with Slim Case, and hopefully there will be versions of the case and battery for other Apple devices. In the meantime, Third Rail System is worthy of recommending as a very good solution, particularly if you can find it at a discounted price.

Our Rating


Company and Price

Company: Third Rail Mobility


Model: Third Rail System

Price: $90

Compatible: iPhone 4

Nick Guy

Nick Guy was an Accessories Editor at iLounge. He had reviewed thousands of iPhone, iPod and iPad accessories; provided visitors comprehensive evaluations of products on the industry-leading independent authority.