Right now Anker is having a sale on both their 2nd Gen Astro E4 battery pack but also their PowerCore 13,000mAh battery pack. Both are on sale for $23.99, and you can grab them from the links below. Make sure to use the appropriate promo code as well. Both of these use Anker's PowerIQ Technology, which basically allows your phone to charge as fast as it can. It's smart enough to know whether your phone supports Quick Charge or not.
Anker PowerCore 13000 (13000mAh 2-Port 4A Portable Charger External Battery Power Bank - $23.99 with promo code T8D4HOME
Anker 2nd Gen Astro E4 13000mAh 3A High Capacity Fast Portable Charger - $23.99 with promo code TDHZW8HR