While the iPhone is rightly lauded for its power and versatility ironically this is also the reason for its biggest weakness - battery life.

Check a few too many emails, browse too many web pages or get too involved in a game, and you'll see the battery meter dropping faster than Gordon Brown's approval ratings.

This is where something like the MiLi Power Pack comes in. It's a sleeve, available in black or white, into which you slide your iPhone that thanks to its built-in 2,000mAh battery, it's designed to give your iPhone a serious injection of staying power.

The iPhone 3GS will slide neatly into the Power Pack, but it will accept the original iPhone too, though not as snugly.

With such a hefty battery, it’s no surprise that the MiLi weighs in at 79g itself, so you’re almost doubling the weight of your iPhone if you choose to carry it around.

That said, the chunky feel and curved back actually gives the iPhone some satisfying heft, and when holding its curves feel comfortable in the hand.

The downside though is that if you have an existing iPhone case, you have to remove the iPhone from it to place it into sleeve.

The MiLi covers the back and the bottom half of the iPhone but the top half remains exposed so you can get to the volume, headphone sockets and camera unimpeded, but if you drop it, it won’t offer much protection.

The base of the Power Pack contains sockets for mini USB in to charge the pack itself, and also a standard USB out, which enables you to plug anything else in and charge that too.

The other clever trick it does is let you sync to iTunes, it, so if you don’t have your iPhone cable to hand, but you do have any standard mini USB to USB cable, you can still sync your iPhone, which is handy.

There are four small blue LED lights on the front – press the power button that sits in the middle and it doesn’t turn anything on – it just shows you how much charge is left – one light indicates 25 per cent remaining, two fifty per cent, etc.

As soon as you slide in the iPhone it will draw power from the MiLi and start to charge. Plug in a power source, and sensibly the iPhone will start to charge first, and then the Pack, and from empty it will take about an hour to charge fully.

The Power Pack claims to pretty much double the battery life of your iPhone and after extended use that’s exactly what we found.

Yes, the MiLi will bulk up your iPhone, and it won’t fully protect the exterior, but if you want to put some heavy use in and get through the day or simply survive a plane journey on one charge, the MiLi Power Pack is a great solution and comes highly recommended.