I have successfully used the PowerGen Mobile Juice Pack 5200mAh with my Nexus 7
Amazon.com: PowerGen Mobile Juice Pack 5200mAh External Battery Pack High Capacity Power Bank Charger 1A output for Apple iPad 2, iPhone 4 4s 3Gs 3G, iPod Touch / Samsung Galaxy S3 S S2 S II, Advance, Galaxy Nexus...