I can at least vouch for this one: http://www.amazon.co.uk/PowerGen-8400mAh-External-Battery-output/dp/B0073F92OK/ref=sr_1_17?s=electronics&ie=UTF8&qid=1378895115&sr=1-17&keywords=external battery pack
I've had it for almost a year and it's been fantastic (I had the smaller version...