Save yourself some money and get this alternative from amazon. Unlike the other battery packs that people listed here, this is actually the same battery pack, and I have owned it for over a year and can vouch for it.
http://www.amazon com/gp/product/B00JM59JPG
This drop is just a slightly redesigned shell with 3,000mAh more, but it costs more than twice as much (plus you have to pay for shipping on massdrop). Functionally, this is the exact same thing as the power bank I listed above, only with the obvious slight difference in total battery storage.
cheeseandcerealI bought that KMASHI branded 10k pack a couple of weeks ago for $9 on Amazon and it is really pretty great. It charged my Note 4 about three and a half times before it conked out, and even though it isn't Qualcomm fast charge certified it still charged my phone faster than most normal wall outlet chargers. Great value for what you pay!
whackoI got the $9 deal on that Kmashi one, too. It's basically the one in this drop with a different name on it. It's been really good, but there's no way I would have paid $30 for it.
http://www.amazon com/gp/product/B00JM59JPG
This drop is just a slightly redesigned shell with 3,000mAh more, but it costs more than twice as much (plus you have to pay for shipping on massdrop). Functionally, this is the exact same thing as the power bank I listed above, only with the obvious slight difference in total battery storage.