Sure, it's just an iPhone battery case, but what marks this one out from the many others is the manufacturer: Apple. The US$99 iPhone 6s Smart Battery Case is the first sign that Apple is ready to acknowledge that its phones are too thin to contain adequate batteries. The case adds 80 percent to the battery life -- and about 100 percent to the thickness -- of an iPhone 6 or 6s. Unlike most third-party battery add-ons, the back of the case isn't flat, but has a bulge in the middle housing the battery.