Otterbox has developed the Defender: an iPhone 4 case that not only protects the Apple device but also doubles its battery life.Shown off at this year's Consumer Electronics Show, the Otterbox Defender doesn't look like it's got a 1450mAh li-ion battery built within, but that's exactly what it's hiding inside. And as it matches the iPhone 4's battery power it's said to double up the life per charge.Designed for the iPhone 4 and 4S, the lack of a Defender for the iPhone 5 or other phones does seem to be a bit behind the curve, but Otterbox will introduce new models further down the line. Besides, that iPhone 4 battery is surely only a shadow of its former self a few years into its contract, so the Defender could be exactly what you need.There's also an Otterbox app that can cleverly show the percentage of charge left in both the case and the iPhone itself, or the display can be adjusted to show hours remaining. Cool stuff.The Defender also has front-facing LED indicators to show how much charge is left in increments of 10 per cent. Problem is, the case we saw had spent all its charge, so we're not entirely sure how bright the LED display is or whether it can be switched off separately.The Defender has to be charged separately from the iPhone itself, which is a bit of a shame as we'd prefer just one cable to juice up both devices at the same time.It may bulk up the iPhone 4 and add a bit of weight, but for the likes of weekend festivals where electricity is like gold dust, the Defender ought to be a very shrewd investment. It's set to hit the shops in February at a price of $130, with UK pricing yet to be confirmed.