Best iPhone 4 battery case: 5 we recommend

Like a good friend, your iPhone is there for you when you need it. Arrived in a new city and lost? Maps to the rescue! Want to find the time of your next train home? No problem. But unlike your best mate, the iPhone has a tendency to conk out at the worst moment if you've forgotten to charge its battery.

The solution is a battery pack. These come in all shapes, sizes, colours and prices. We've gone for the case-style power pack, because we feel they're the most convenient, portable and robust you can get. Since they're designed to fit snugly, they're phone-specific, so make sure you get the right model for your iPhone.

To help you choose, we've selected five for the iPhone 4 and another five for the iPhone 3G and 3GS, all available in the UK. There are other options that simply clip onto the bottom or your iPhone (or iPod or iPad, in some cases), or that you hook up to your dock connector with a lead.

These aren't as handy for travelling, but you can get some with awesome capacity – have a look at the Just Mobile Gum Plus Power Pack (£57). This little baby's 4400mAh battery charged our iPhone fully and still claimed to have a near-full tank.

We put each of the cases here through rigorous tests to discover how they perform in the real-world. The testing process was done on an eight-month-old iPhone 3GS and an iPhone 4 with three months on the clock. Both phones' batteries have run up a fair few charge cycles, so your results may differ from ours because your battery's in a different state of health.

But our results give a good indication of the relative performance of each case. We measured how much charge each pack could push into a dead iPhone, then timed how long it took to charge the pack and the phone fully – useful to know if you're off somewhere and want to go out with as much charge as possible.

Finally, we left the guided tour of Unreal's Epic Citadel running, which really mashes away at the processor and hence battery – a true acid test. Let's see how they fared.

For iPhone 4