The problem with the iPhone is that it makes itself so indispensable, and is such a pleasure to use, that the battery has been known to run out before the day is over. (The battery capacity itself isn't especially poor; it's just that you use the thing so much.)

Rather than try to find a plug for a mid-afternoon recharge, we've taken to carrying extra battery packs with us when we know we're heading off on business. We were big fans of Mophie's first battery sled – a kind of cover that you slid the iPhone into – so were keen to see what this new version brought.

It's not as capacious as the original – and still available – Juice Pack; it's rated at 1200mAh, while the original packed in 1800mAh. It is, though, slimmer, and rather than being about three quarters of the length of the iPhone – leaving the top of the iPhone sticking out, unprotected – it cocoons your entire iPhone in plastic. The case splits apart for fitting.

Battery performance is good. Its claim of almost doubling the iPhone 3G battery life is pretty much bang on, and it has a switch to turn charging on and off.

But we must confess to being disappointed with the build quality. It's not just that our first sample shattered – this just happened with an early batch, apparently; Mophie will replace for free if you're affected – but that joins, seams and finishing seem a little rough.

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