Third Rail System (for iPhone 4) review: Third Rail System (for iPhone 4)

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MSRP: $89.99

The Good The stylish and compact Third Rail Slim Case for iPhone 4 with Slide-on Smart Battery is the most flexible and innovative juice pack option to date. The pack's detachable batteries can also power other mobile devices and can be stacked together to increase the capacity.

The Bad The Third Rail case's included battery provides only enough power to charge an iPhone 4 up to about 65 percent. Also, it continues to discharge even when the phone's battery is already full. The package is rather bulky when used as a juice pack and the case needs to be removed completely when you want to use the phone with a docking station.

The Bottom Line The Third Rail Slim Case for iPhone 4 with Slide-on Smart Battery is arguably the best mobile juice pack on the market. It's especially useful if you also have other mobile devices.

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8.3 Overall

It can be a painful experience to let go of your favorite juice pack or protective case each time you change or upgrade your phone. And this is exactly what Third Rail, a start-up focusing on mobile power, intends to ease. Its very first product, the Slim Case for iPhone 4, is a lot more than just a simple juice pack and will likely become your first favorite that you won't ever have to completely part with.

While this Third Rail case fits only the iPhone 4, the included battery can work with almost any mobile device that charges using a standard USB cable--and it will also work with future, similar protective cases for other devices when made available by the company.

At the estimated cost of $90 for the whole package (a case and one battery), or $40 for just the case itself, or $60 for just the battery, Third Rail's new concept for a portable power pack makes an excellent investment for mobile users.

Design and features
As the lengthy name suggests, the Third Rail Slim Case for iPhone 4 with Slide-on Smart Battery has two components, the protective case and a detachable battery.

The case fits only the iPhone 4 and is one of the slimmest of the form-fitting variety. It's longer than other protective cases, however, as at the bottom it has to house a built-in 30-pin connector. The case has two parts, the main body and the top, which can be taken out easily. The two parts fit into the phone and each other firmly, but still leave enough room for another layer of protective film on the phone's bottom. This is a good thing as you will need to remove the case completely when you want to put the phone on a docking station, which might cause scratches on the back of the phone. Though we like the case's overall design, we wish we could access the phone's 30-pin port without having to take the case off. At the bottom of the case, there's a standard Micro-USB port to be used with an included USB cable for charging and syncing the phone with a computer. Next to the USB port, there's an on/off switch that sets the phone to draw power from the battery when need be.

The battery, which is thin and light, about the size of a small matchbox, is the most special part of the device. It's detachable and can be easily and firmly clipped on the back of the case, where there's a female port for a three-pin power connector. Once the battery is attached to the case, you've got yourself a juice pack, like many others, such as the Mophie Juice Pack. However, the Slim Case allows you to stack other batteries on top of the first one to increase the juice pack's capacity. Third Rail says that up to six batteries can be used together. In our trials with two batteries, they worked as intended.

Each battery has two ports on the bottom, a Micro-USB port for charging, labeled "in," and Third Rail's proprietary port, labeled "out," to be used with an included Third Rail USB adapter cable. On the other end this cable has a female USB port, like those found on the back of a computer. With this cable, the battery can be used as a backup power source for any mobile device that charges via USB cable, such as an iPod or smartphone. For ergonomic reasons and to avoid accidently unclipping the battery from the phone, we wish these ports were located on top of the battery instead.

Between the two ports there's a button that activates a power gauge with five little green LED lights that flash when the battery is being charged. The battery has a female power port on top and a three-pin male power connector on the bottom to connect to the case or another battery.

When batteries are stacked together, you only need use one of the USB ports, either on the case or on any of the batteries, to charge the whole system. You can even still sync at the same time if the port of the case is used. The batteries can be also charged just by themselves, without using the case. With Third Rail USB adapter cables, you can charge other mobile devices at the same time, too, via just one power source, such as a computer's USB port, an iPhone's USB power adapter, or a car charger. For those who travel with multiple devices, this flexible way of charging is really a great bonus.

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