For a limited time Amazon has the Yubi Power YP250ABLK 2500mAh Ultra Compact Lipstick Size Portable Power Bank Backup External Battery Charger (black) for only $12.99, a savings of $17 off the normal price of $30.

This power bank has gotten 4.3 out of 5 stars from over 250 reviewers on Amazon. It'll hold one full charge to an iPhone in a pinch.