I use my external Astro 3 to recharge my S4. The original charger requires 2 hrs 20 minutes to recharge from 10%. The Astro does it in only 1 hr 32 minutes. Designed to charge up to three smartphones or a tablet and two phones all at once, it's great to have such a compact solution to take with me.
Anker 2nd Gen. Astro3 12000mAh External Battery