If you are looking for a compact and lightweight backup battery that you can use on the go, check out  the Xtorm FS101 Fuel Bank 2X.
The FS101 Fuel Bank has a 5000 mAh battery with a maximum 2.1 Amps input that is powerful enough to charge a smartphone twice over, though you can also use it to charge a digital camera, game controller, wireless headphones and other USB devices.
The FS101 5000 mAh battery comes already pre-charged so it works right out of the box. Simply plug it into any USB device and the battery will automatically start and stop charging when the USB device is fully charged (That means no need to worry about charge overload).
On the Xtorm FS101 Fuel Bank battery, you get a power button, LED power indicators and a standard USB and micro USB ports. The standard USB port is a 5V USB OUT, while the micro USB port is a 5V USB IN. The battery exterior is rubberized so it has a nice feel to it when gripping in your hands.

Charging time for a standard USB device is around 90 minutes. Charging the 5000 mAh battery can be done via a laptop/ PC or AC main adapter. Charging through a laptop and mains takes 7 and 5 hours respectively.