EasyAcc® - PB12000A - 12000mAh USB External Battery Output 3.5A 4 USB Powerbank Portable Charger External Backup Battery for iPhone 5S, 5C, 4S; iPad Air, Mini, 3 (Apple adapter not included); Samsung Galaxy Tab 2, Tab 3, S4, S3, Note 2 3; Google Nexus7, 5.4; Google Glass; HTC One, Bluetooth headsets, Android / Windows Smartphones, Colour: White (Accessory)
After 1 year of use, it has not lost its charge. OK, this might not be the lightest, smallest, thinnest but it does exactly what he's supposed to do.