Why you need battery cases for your iPhone:  The iPhone 6s has a 1715-mAh battery, the iPhone 6 has 1810 mAh, and the iPhone 5s has a 1560-mAh battery. Even though the iPhone 6s and 6 also have much larger power-hungry screens, Apple says you should expect the iPhone 6 to provide up to 14 hours of talk time (versus 10 with the 5s), 250 hours of standby time (the same as with the 5s), 10 hours of LTE browsing (the same), 11 hours of Wi-Fi browsing (versus 10), 11 hours of video playback (versus 10), or 50 hours of audio playback (versus 40). Whatever Apple says, you need extra battery power if you use your phone like crazy.

The best battery cases for iPhone 6, 6s, 6 Plus, and 6s Plus: Anker Ultra Slim Extended Battery Case (very affordable, best ratio of cost per charge, simple but efficient)

The second best battery cases for iPhone 6, 6s, 6 Plus, and 6s Plus: Anker Premium Extended Battery Case

The best battery cases for iPhone 6, 6s, 6 Plus, and 6s Plus: with an easily removable battery: Prong PWR Case for iPhone 6/6s

The best battery case upgrade for iPhone 6, 6s, 6 Plus, and 6s Plus: Apple iPhone 6s Smart Battery Case (costly because it is from Apple)

The best battery cases for iPhone 6 Plus only: Tylt Energi Sliding Power Case (for more power needs)