Like a good friend, your 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 . But unlike your best mate, the iPhone has a tendency to conk out at the worst moment if you’ve forgotten to charge its .

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 , in some ), 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

TAP02.group.battery8 420 90 Best iPhone 4 battery case: 5 we recommend

Dexim Supercharged Leather Power Case – £60

TAP02.group.battery6 420 90 Best iPhone 4 battery case: 5 we recommend

Mophie Juice Pack Air – iPhone 4 – £70

TAP02.group.battery4 420 90 Best iPhone 4 battery case: 5 we recommend

Mili Power Spring 4 – £47

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Exogear Exolife – £60

TAP02.group.battery3 420 90 Best iPhone 4 battery case: 5 we recommend

Logic3 PowerSleeve for iPhone 4 – £30

For iPhone 3G/3GS

TAP02.group.battery10 420 90 Best iPhone 4 battery case: 5 we recommend

Logic3 Power Case – £30

TAP02.group.battery1 420 90 Best iPhone 4 battery case: 5 we recommend

7dayshop iPower External Battery Case – £11

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Dexim BluePack S4 – £25

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Mili Power Spring Case for iPhone 3G/3GS – £43

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Mophie Juice Pack Air – iPhone 3G & 3GS – £40

5 best iPhone 4 battery cases

Test 1 – Battery power

We plugged the full battery pack into a dead iPhone and left it to charge with the phone on, but the display off. The battery percentage reading was taken from the iPhone as soon as the battery case was empty. The high-capacity Dexim came out top; the Exogear was surprisingly low.

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Test 2 – Battery charge time

We plugged a dead battery and dead iPhone into a 500mA USB port on our Mac and timed how long it took for both to charge fully. Don’t hold your breath, because it ain’t quick. The Mophie did well but the Logic3 was fastest. It’s not the whole story though – look at test three.

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Test 3 – Extra power

With the pack and iPhone charged, we left the guided tour of Unreal’s Epic Citadel running with sound muted, and timed how much extra time we got with the battery compared to the iPhone on its own. This is where the differences really showed up.

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Test 4 – Design and Features

We like Dexim’s flip-case design for the screen protection it offers, though it’s the bulkiest of the ones on test, doesn’t grip the phone well and the flip clip isn’t particularly elegant.

The Mili’s one-piece spring clip, which hooks over the top of the iPhone, is a nice idea that just doesn’t work, because it kept coming loose in our pocket, and the Exogear’s clip-on ring is functional but flimsy. The case is mighty thin, though.

Our favourites are the Logic3 and the Mophie: the former because of its one-piece design; the latter because it’s so solid, looks the part and has a nice rubber back.

All five have a power switch, so you can decide when to use the charge in the battery to boost your iPhone. With the Mili, it won’t start charging until you press its On button, but you can’t then stop it without removing the iPhone – less elegant than the others.

And all but the Dexim have a strip of lights that show you how much juice is in the battery. The Dexim does have a light to tell you if it’s charging (orange) or full (green), but the light colour changes depending on your angle of view. Frus-tra-ting.

ip4 test4 420 90 Best iPhone 4 battery case: 5 we recommend

And the best iPhone 4 battery case is… Mophie Juice Pack Air £70

The Mophie’s performance and design panache make it a worthy winner This test has brought out a surprising trait in us – we’ve started to sympathise with the X Factor judges. When faced with two completely different choices, each with its own distinct strengths, how do you pick between them?

Unlike Messrs Cowell et al., we were picking between the top two rather than the bottom pair, and we had longer than they did to deliberate which was to be our victor. But it was tough.

While we liked the phenomenal capacity and added use time of the Dexim, its design lacks finesse, and since you’ve got a stylish and well-designed phone, you need a case to match.

For the same reason, we didn’t feel the Mili and Exogear packs were quite up to scratch, despite the former’s impressive additional use time. And so we were left with our final-two conundrum.

As we said in the design test, we were impressed by the Logic3 and Mophie in equal measure, and felt both were a worthy complement to the iPhone’s sleek looks. In terms of how much juice they pumped into the phone, they were practically identical, and they were the two quickest-charging packs we tested.

So it came down to whether the Mophie’s more-than-double additional use time justified its more-than-double price tag. Let’s first say that we do think £70 is too expensive and that at £30, the Logic3 is an attractive option if you’re on a tight budget. But at the end of the day, if you’re going to shell out on a battery case, it’s because you’re the sort of heavy iPhone user who regularly finds your battery running out, so the more additional use time you get, the better.

And while the Mophie isn’t top of the table in this sense, it offers the best overall balance of great design and good performance. It protects your phone well, is solidly built but still thin, oozes class and gives you plenty of extra time to enjoy all that’s great about your iPhone.

5 best iPhone 3G/3GS battery cases

Test 1 – Battery power

We plugged the full battery pack into a dead iPhone and left it to charge with the phone on, but the display off. The battery percentage reading was taken from the iPhone as soon as the battery case was empty. The 7dayshop iPower, despite being good on paper, performed poorly.

ip3 test1 420 90 Best iPhone 4 battery case: 5 we recommend

Test 2 – Battery charge time

We plugged a dead battery and dead iPhone into a 500mA USB port on our Mac and timed how long it took for both to charge fully. Don’t hold your breath, because it ain’t quick, though we were impressed by the Mili’s time and, to a slightly lesser extent, the Mophie’s.

ip3 test2 420 90 Best iPhone 4 battery case: 5 we recommend

Test 3 – Extra power

With the pack and iPhone charged, we left the guided tour of Unreal’s Epic Citadel running with sound muted, and timed how much extra time we got with the battery compared to the iPhone on its own. As you can see, it was largely much of a muchness.

ip3 test3 420 90 Best iPhone 4 battery case: 5 we recommend

Test 4 – Design and Features

You will notice that the Logic3 and Dexim leather cases are virtually identical, the only significant differences being the type of leather coating and the fact that the Dexim has a velcro clasp while the Logic3 has a magnet. We preferred the latter.

But both these cases are bulkier than the Mili and Mophie ones. The Mili clip-on design that didn’t work with the iPhone 4 grips the 3G/3GS much better: it also leaves the iPhone’s sidemounted buttons more accessible. The Mophie looks similar to the Mili but the top is a separate slide-on piece.

The Mili, 7dayshop iPower and Mophie packs have a strip of lights to show you the charge level, and we like the Mophie’s best because the button’s easier to press than the Mili. The Logic3 and Dexim ones only have a status light that doesn’t show you how much juice they still have in the tank.

All five have switches so that you can decide when to use the juice in the pack, although the only way to cut off power with the Mili is to remove the phone. One nice feature of the 7dayshop iPower is how it’ll charge other USB devices.

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And the best iPhone 3G/3GS battery case is… Mili Power Spring £43

In a close-run contest, the convenience of speedy charging is worth the extra cash We’ve agonised long and hard about this one. Let’s first explain why we didn’t go for the 7dayshop iPower case.

Yes, it only costs about a tenner, and from a pounds-per-additional-minute-of-use perspective, it’s great value for money. But then if you’re investing in a battery pack, why do the half-hearted thing of getting one that delivered so much less extra use time compared to the others? If you’re gonna do something, do it properly!

So our attention turns to the others. In terms of how much charge they gave our iPhone and the additional use time they provided, there’s nothing really to choose between them. If we were interested purely in these two things, the Logic3 case would be a good bet (we preferred its magnet clasp to the Dexim’s velcro). We like the flip-case style despite the extra bulk, and its modest £30 pricetag is reasonable.

But even with this, we can’t help feeling you’d be better off shelling out an additional £13 for the Mili, and here’s why. Granted, you don’t get any additional capacity or video time at the top-end of the price scale, but the Mili and Mophie products have three distinct advantages.

First, there’s the charge level indicator, which we find incredibly useful, and the fact that they’re much lighter. But more importantly, look at those charge times. They’re both significantly less than the Logic3 and Dexim efforts. And we like that, because if you’ve only got a short time to get your phone charged before you go out, speed matters and these guys deliver.

While some of the minutiae on the Mophie are better implemented (the on/off switch and the charge level button, for example), the Mili’s quicker charge time, coupled with the handier one-piece design and the way the iPhone’s buttons were easier to press with the case on are what swung us in its favour.

120a636a8a00 200.jpg Best iPhone 4 battery case: 5 we recommend

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Buying Guide: Best iPhone 4 battery case: 5 we recommend