As one of its daily deals, Groupon offers the MOTA 4,000mAh Power Block with Lightning Adapter in Green, Blue, or Pink for $29.99 with free shipping. That's $40 off list and the lowest total price we could find. It's compatible with most USB devices and features five different connectors (including Apple 30-pin and Lightning adapters), 4,000mAh rechargeable power capacity, and a built-in lithium-ion battery. Deal ends June 23.