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.
While the case itself is very slim and compact, the whole package becomes rather bulky when a battery is clipped on, though you can still use the phone like normal. This gets worse when you add more than one battery. However, as the battery or batteries can be removed from the case easily, you can take it or them off when making call, to be replaced when you're done. Note, however, that you want to turn the charging off when the phone's battery is full. In our trials, the battery continued to discharge even when the internal battery of the device being powered had already been fully charged.
The best thing about this design is the fact that the battery, which is the most expensive part, can be reused as you change to another phone. In this case, just use the battery as a backup power source as described above, or wait till Third Rail releases a similar supporting case for the new device and clip the battery on. This also means that when the battery deteriorates, as batteries do over time, you won't need to replace the case, just the battery itself.
The Third Rail Slim Case for iPhone 4 with Slide-on Smart Battery performed as expected in our trials. Each battery added from 4 to 6 hours to the iPhone 4's usage in our trials, depending on how the phone was used. This is about two-thirds of the phone's built-in battery life, just as claimed by Third Rail.
The battery did better with other devices, however; it was able to provide a full charge and still have some juice left. In our trials, we found one battery could refill an iPod Touch's internal battery in about 3 hours. The battery itself took about 1 hour to be fully charged.
The case looks good and feels sturdy and offers a better grip on the phone. It actually makes it easy to use the phone's Home button with one hand, without risking dropping it. We noticed that the system seemed to respond slowly, however. For example, it would take a few seconds for the phone to start drawing power from the battery after the switch was moved to the "on" position. It was the same for charging. This is probably because the "smart" battery requires time to measure and determine what to do. While this is not a big deal, it would make those who are used to regular juice packs feel like the device is finicky at first.
Overall, after a few days of trying, we found that the juice pack, especially the battery, delivered what Third Rail claims for the device. This will be an even better deal when the company releases more cases for other devices.
With a stylish and flexible design, the Third Rail Slim Case for iPhone 4 with Slide-on Smart Battery, though not perfect, is by far the best juice pack for mobile devices on the market. You may have to change the protective case but you for sure can keep the battery with you as you move on to other devices.