Anker Astro E1 Candy-Bar Sized Ultra Compact Portable Charger

I've always had a good experience with Anker products, so when I saw this 6700 mAh battery on sale for $14, I bought it. I have other batteries with a lot more capacity, but I wanted one that was more compact and lightweight, but still had enough juice to keep my phone alive for a full day of heavy use.

It doesn't include the short Lightning cable shown here. You probably already have a longer Lightning cable, but it you want a short one, Anker sells a 2-pack for $13.