If you've got an iPhone you'll now that the battery is pretty awful, if you've got an iPhone 3GS then you'll know that it's even worse. So what are you to do? Well you could systematically go through the phone's settings and turn off all the features you've paid extra for; the location services, the wireless, and even the 3G connectivity.

Mophie have other plans, promising to give iPhone users a new lease of life and more importantly a phone that will last longer than 4 O'clock without the battery warnings.

In steps the Mophie juice pack air, a case for your iPhone that slots around the back just like any other case. The colours, white, black, and purple, (we got purple) aren't the most designer friendly, however the hidden gem is that built with-in the case is a battery that, if you are careful, can mean that your iPhone lasts around 3 days without needing to say hello to a power socket.

I though that would get your attention.

As we've said the case is simple in it's design, comes in a range of colours and slots into the docking connector. A USB cable out the bottom gives you connectivity to your Mac or PC so you can still dock it as well as providing charging capabilities for the battery itself.

Battery levels can be checked via four LED lights on the back at the press of a button and owners of a MacBook will be familiar with the design and concept.

The premise is simple. You charge the device, it then charges your iPhone throughout the day. When the Mophie juice pack air runs out, the iPhone resumes using its own battery, allowing you to go thin if you want.

In practice it works just as it sounds. First we tested the juice pack air with overly heavy use of the iPhone; turning on location services and Wi-Fi, and then going to town with calls, surfing the web, and Twitter. The result? It basically doubles the life of the phone and with the Mophie juice pack installed we managed to get through to the following morning without needing to worry about power.

On light use, i.e., how we currently use our iPhone 3GS in the office, we managed a whopping 3 days of battery, yes 3 days, although it's worth saying that this will drastically depend on how many calls you get and what you do.

In our 2 weeks of testing we not once had a battery warning half way through the day - or at a time in the bar where you really need to phone home to tell them you are going to be late.

So what's the catch? There is always a catch right? Well it's that for this need to have a phone that is capable of always being on, you have to live with the fact that the Mophie juice pack air will double the thickness of your iPhone.

Then there is the case design, not exactly mind blowingly hot or exciting.