When it comes to battery packs, the Cascade 7800 is by far the best one I have tested. It holds a massive amount of charge, enough to charge an iPhone 6s completely, at least three times. It charges with a Micro USB, so you don't have to worry about a specific cable or charger for it either. Call me overly dependent on my tech, but this has not left my side since I got it. With two usb ports, you can charge multiple devices at once. A small light indicates how much charge is left on the battery, and the Smart Charge tech will stop charging your device once it's full. It shows up fully charged, and is small enough to tuck neatly in your pocket with your phone while it charges. The 7800 mAh Cascade is available for $70, with smaller 5200 mAh and 2600 mAh models are available as well.