Hahnels duopal extra is an all in one mains free battery charger and a power bank with a whopping 5200mah capacity three times that of an iphone 5 battery the duopal may be used to simultaneously charge a pair of go pro hero4 batteries or to charge any usb compatible device such as your smartphone camera or tablet and if your phone or gadget run out of juice while youre on the go then the duopal can be used as a power bank powering them directly the unit takes 25 hours to charge a