Groupon offers the Mophie Juice Pack Boost 2,000mAh Battery for iPhone / iPod , model no. 2036_JPU-BOOST-2-RED, for $19.99 with free shipping. That's $5 under last month's mention and the lowest total price we could find by $10. This 2,000mAh battery pack purports to charge your device twice as fast as conventional batteries. It features a retractable 30-Pin connector, built-in carabiner keychain, and micro USB cable. Deal ends March 13.