As the iPhone has gotten more powerful and capable, I find myself using it for more and more things. It is now my navigation system, my primary point-and-shoot camera and stays in pretty much constant use as I travel for itineraries and more. That’s why I love external battery packs like the Mophie Juicepack, as they afford me the ability to use my phone as heavily as I want and not worry about it going dead.

One of the biggest downsides to these battery packs is that they are bulky and a bit heavier than I’d like, especially because the iPhone 4 is so light on its own. That’s just one of the reasons that I think that the design of the Third Rail battery case system for iPhone is so brilliant.

The Third Rail case itself isn’t much larger than any other iPhone 4 case, aside from a thin extra chin on the bottom side to accommodate a charging port and an on/off switch. The battery portions of this case are actually separate, in the form of 1250mAh Smart Batteries. These are thin extra batteries that you can attach to the back of the case as needed.

IMG 5255 520x346 The Third Rail battery case for iPhone is incredibly clever and useful

This means that you can rock the regular case until you need extra power and slap a battery on the back to get more juice. But the cleverness of this case doesn’t stop there. You can also stack multiple batteries on the back for more juice, adding to the life of your battery.

And that is just the tip of the iceberg, as when you stack multiple batteries together, either on or off the case, they will automatically charge the battery at the bottom of the stack. This means that you can simply stack up a bunch of partially used batteries and come away with a couple of fully charged ones.

IMG 5259 520x346 The Third Rail battery case for iPhone is incredibly clever and useful

In addition to the trickle down charging, you also get a small micro-USB to USB adapter with each battery. This means that you can charge any device off of these batteries that takes USB, not just the iPhone.

If you’ve got a bunch of devices that need charging at once and you’re stuck in a hotel room, you can even create a charging station for them by stacking all of the batteries on the case, plugging the case in and using dongles to attach your devices. So cool.

IMG 5257 520x346 The Third Rail battery case for iPhone is incredibly clever and useful

In addition to being really cleverly designed, the Third Rail system also really works. I got about 55-60% additional charge out of each battery that I used on a recent marathon Disneyland trip. I kept the additional batteries packed away until I needed them and just rocked the phone in the regular case until I ran out of juice.

The design of the unit is pretty great all around, with great details that exhibit care in its design. Every battery uses spring-loaded pins to make its contact instead of flat tines, which reduces the chances of bending or loss of connection. The batteries lock on smoothly and solidly.

IMG 5258 520x346 The Third Rail battery case for iPhone is incredibly clever and useful

If I had one gripe it’s that the notches for stacking the batteries are a bit sharp and could be smoothed off to prevent it from scratching your palm. Pretty slight issue overall though.

If you’re interested in an iPhone battery pack, but have been resisting because you feel that you don’t want the extra bulk and weight, then the Third Rail system is an excellent one to check out. Third Rail have gone way above and beyond with all of the cool features of this system and they are still very competitively price compared to other offerings.

The Third Rail System, including one battery, for iPhone 4 is $89.99 and extra batteries are $59.99.