Anker Astro Mini
The Anker Astro Mini is another excellent option for those who truly want a portable charger. The Astro Mini is about the size of a chapstick or tube of lipstick, and packs a 3,200 mAh battery.
It's extremely small and portable. Easy to charge up a few hours and toss in a purse or pocket, and then simply plug in your Galaxy S6 to fully charge it in no time. They claim it won't fully charge the Galaxy S5, which only has a 2,800 mAh battery, so we're not sure if the rating is wrong, or if the typical battery transfer loss is just playing into their advertising. Either way, if you want cheap and portable battery, get this.