MiLi is no stranger to iPhone battery packs. So it should come as no surprise (it doesn’t to us), that today they released the MiLi Powerspring 4. It’s a 1600 mAh backup battery case for the iPhone 4. It should double your battery life, that is depending on what you’re doing. In our experience MiLi’s batteries have a checkered past in terms of power output. Total charge time, according to spec, is 4 hours, which seems a bit high, but at least you won’t be tethered to a wall while rejuicing your phone.