The battery life of the HTC One is rather bad and it doesn't help that you can't remove the battery. Enter Mophie. Mophie is the maker of popular cases for many of the popular smartphones available. These cases feature extended batteries to increase the battery life of devices, especially ones without removable batteries.
Mophie has released an HTC One compatible extended battery case to help with the poor battery life. This does mean you'll have to sacrifice some weight and size, but most will generally accept this in exchange for more battery life. With the case, the phone becomes 0.67 inches thick, 6.06 inches tall, and 2.88 inches wide.
However, you'll get about 100 percent more battery life thanks to the built-in 2,500mAh battery. The case will start shipping in mid-May and will set you back $99.95.