If you are looking for an external battery that packs a lot of charge inside a small case, the Microsoft Portable Power DC-21 external battery could be what you are looking for. Microsoft has now cut the price of the battery down by nearly half on its store site, from $49 to just $24.99.
The 6000 mAh battery is enough to charge most smartphones two times with a single charge. It can also hold its charge for up to a month if needed, and can be plugged in and charged while also charging a smartphone. The Microsoft Portable Power DC-21 is available in green, orange, and white. There's no word on how long this half-price sale will last.